UNESCO-NIGERIA TECHNICAL & VOCATIONAL EDUCATION REVITALISATION PROJECT-PHASE II
NATIONAL DIPLOMA IN ELECTRICAL ENGINEERING TECHNOLOGY
ELECTRONICS (I) COURSE CODE: EEC 124
YEAR I- SEMESTER II THEORY Version 1: December 2008
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Table of Contents Chapter 1: Concept of Semiconductors Materials Week 1 ................................................................................................................... 1 1.1 Matter. 1.2 Fundament of Electricity Week 2 .................................................................................................................... 7 1.3 Fermi Energy Levels 1.4 Electrons Energy Levels 1.5 Conductors, Insulators and Semiconductors 1.6 Conduction in Metals 1.7 Conduction in a Gas Week 3 .....................................................................................................................15 1.8 Intrinsic Semiconductor 1.9 Extrinsic Conductor 1.10 Advanche Breakdown Week 4......................................................................................................................19 1.11 Electrical Force and Potential 1.12 Diodes Chapter 2: Characteristics of a PN Junction / Zener Diode: . Week 5 .....................................................................................................................21 2.1 Zener Diodes 2.2 How we use semiconductor Diode Week 6......................................................................................................................24 2.3 Voltage Stabilization and Reference 2.4 Voltage Shifting 2.5 General Characteristics Week 7......................................................................................................................27 2.6 The Transistor 2.7 The Bipolar Transistors 2.8 Junction Transistors Chapter 3: Application of Bipolar Junction Transistors: .................................................... ...... Week 8......................................................................................................................29 3.1 Testing a BJT Transistor 3.2 Testing with a Multimeter 3.3 Field Effect Transistors Week 9.....................................................................................................................31 3.4 Basic Transistor Operation 3.5 Transistor Currents
Week 10....................................................................................................................35 3.6 Semiconductor Diodes 3.7 The PN Junction 3.8 The Electric Field at a PN Junction 3.9 Forward and Reverse Bias 3.10 Transistor Currents and Voltages 3.11 Collector Characteristic Curves 3.12 Cut-off 3.13 Saturation 3.14 DC Load Line 3.15 Maximum Transistor Ratings Week 11...................................................................................................................41 3.16 DC Bias 3.17 Waveform Distortion 3.18 Voltage-Divider Biased PNP Transistor 3.19 Base Bias 3.20 Emitter Bias 3.21 Collector-feedback Bias 3.22 Q-point Stability Over Temperature Chapter 4: Basic Structure & Application of the Thyristor: Week 12.....................................................................................................................45 4.1 Basic Structure of the Thyristor 4.2 Forward-Breakover Voltage 4.3 Switching Current Week 13....................................................................................................................47 4.4 Working Principles of the Thyristor 4.5 Holding Current 4.6 Switching Current Week 14....................................................................................................................49 4.7 The Silicon - Controlled Rectifier ( SCR) 4.8 SCR Equivalent Circuit 4.9 Turning the SCR On 4.10 On-Off Control of Current 4.11 Half-wave Power Control 4.12 Lighting System for Power Interruptions 4.13 An Over-voltage protection Circuit 4.14 The Diac Week 15....................................................................................................................52 4.15 Advances of the Thyristor
Concept of Semiconductors Materials
Week 1
1.1 MATTER All matter is made of atoms, an atom has two parts, nucleus and the electron cloud (see fig 1.1.) Electrons are the smallest and lightest particles. They carry a negative electric charge. The electron cloud contains one or more electrons which are moving at high speed around the nucleus. Each electron carries the same amount of negative charge. The nucleus consists of one or more particles. The particles in the nucleus are of two kinds. Protons:
A proton is about 1840 times more massive than an electron. It carries a positive
electric charge that is equal in size but opposite in sign to the charge carried by an electron. Therefore all protons carry the same amount of positive charge. Neutrons: A neutron has about the same mass as a proton but carries no electric charge. Modern physics has shown that there are other kinds of participle. It also tells us that protons, neutrons, electrons and the other particles may really be made of even smaller particles. To understand electronics we need to know about protons, neutrons and electrons, but not about the other kinds of particle.
Fig. 1.1 The Structure of an Atom 1
Concept of Semiconductors Materials
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1.2 Fundamental of Electricity 1.2.1 Electron and Protons All solids, liquids and gases are principally made up of two basic types of participles known as electrons and protons. The electron is the smaller of the two; the proton is 1840 times more massive than the electron. The electron carries a negative electrical charge. The proton carries an equal and opposite positive charge. When a material is in an uncharged state, it contains as many protons as it does electrons. However, if we remove some electrons from the material, the net positive charge on it exceeds the remaining negative charge so that the material exhibits a net positive charge. This phenomenon can be experienced by anyone wearing clothes manufactured from man-made fiber; while the garment is being worn, some electrons transfer to the wearer, and the static charge built up in this way may cause the wearer to experience an electrical shock during removal of the garment.
1.2.2 Basic Atomic Structure All atoms have broadly the same type of structure, with the heavier protons forming the nucleus, around which the electrons orbit (see fig 1.2). The electrons orbit in distinct layers or shells. The radius of the orbit depends on the balance between two forces: the mechanical outward force on the electron due to its motion and the inward electrostatic pull between the positive charge on the nucleus and the negative charge on the electron. The shell in which an electron finds itself depends on its energy; a high –energy electron orbits in a shell further away from the nucleus than does a low-energy electron.
Fig. 1.2 Structure of Hydrogen, Helium and Silicon Atoms 2
Concept of Semiconductors Materials
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Scientist have lettered the shells alphabetically, beginning with the K-shell 9which is the shell nearest to the nucleus). Each shell can also be given a number (K = 1, L = 2, M = 3, etc) and it has been shown that the maximum number of electrons which can orbit in any shell is 2n2, where n is the “number” of the shell. The maximum number of electrons which may orbit in shells K to N is given in table 1.1 below. Shell
Shell number
Maximum number of electrons in orbit
K
1
2
L
2
8
M
3
18
N
4
32
Examples of atomic structure are shown in figure 1.2 above. In the hydrogen atom, the K –shell contains only one electron, and the shell is said to be an incomplete shell. Like hydrogen, neon has only one shell (the K-shell) but, since it contains two electrons. It is described as a full shell. Silicon with fourteen protons has fourteen electrons in orbit, which completely fill the K-shell and L-shall and partially fill the M-shell. In a complex structure like silicon, the electrons in orbit, which completely fill the K-shell and L-shell and partially fill the M-shell. In a complex structure like silicon, the electrons in the inner shells are tightly bound to the nucleus due to the electrostatic force involved. Electrons farthest away from the nucleus (those in the M-shell in silicon) can be detached from the atom more easily and are said to be loosely bound. The gaps between the shells are regions where electrons cannot orbit, and are described as forbidden energy gaps.
It is the electrons in the outermost shell which are of particular interest electrical and electronic engineers. Since these dictate many properties of the substance. The outermost shell is known as the valence shell and the electrons in this shell are known as valence electrons. 3
Concept of Semiconductors Materials
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1.2.3 Atomic Bonds When atoms combine, they do so by attempting to empty the outer shell by losing electrons, or by attempting to fill the outer shell by gaining electrons, or alternatively they share electrons with other atoms in order to give the appearance of a full shell. The latter method is of particular interest to electronic engineers, since this is the way in which some of the most useful semiconductor materials bond together. In the sharing process, each valence electron forms an orbit around two atoms including the parent atom and one other atom, forming what is known as a covalent bond between the atoms.
1.2.4 Ionization and Excitation Since an individual atom contains as many electrons as it does protons. It is electrically neutral in its normal state. However, the addition of an electron gives it a net negative charge; and the removal of an electron gives it a net positive charge. The process of adding or removing electrons was first mentioned in section 1.2.1 when an atom carries either a negative a positive charge it is known as an ion and the process of producing this charge is known as ionization. When an electron receives energy from an external source, such as heat or light, the extra energy allows it to move to a higher orbit. This process is known as excitation. Similarly, when an electron gives up energy, it falls from a higher orbit to a lower one. This loss of energy from the electron may appear in the form of heat or light; an example of the latter occurs in the light-emitting diode (led).
1.2.5 Holes and Electrons When an atom losses an electron, the electrical charge balance is upset and the atom takes on a net positive charge. This positive charge is described as an electronic hole, and can be regarded as the absence of an electron where one would normally be found. Thus a hole is regarded as a positive charge carrier, much as an electron is a negative charge carrier. 4
Concept of Semiconductors Materials
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1.2.6 Current and Charge When the electrical circuit between a generator and an electrical load such as a lamp or heater is complete, electric current flows round the circuit, Electric current is simply the movement of electrical charge carrier (such as electronics) around the circuit. Consider now the movement of electrons; when the circuit is complete, the electrons are attracted from the negative pole of the supply and flow via the load to the positive pole of the supply. Supply, electronic cannot “accumulate” at any point in the circuit; they must complete the return path inside the generator to return to the negative pole. Current (symbol I) has the unit of the ampere (symbol A). Certain devices such as cells and capacitors have the ability to store a quantity of electricity. Electrical quantity or electrical charge (symbol Q) is the capacity of a piece of electrical apparatus to store (or to discharge for that matter) a certain current for a given length of time. For example, the storage capacity of an accumulator is stated as a certain number of ampere-hours (unit symbol ALh). Thus an accumulator with a storage capacity of 40 Ah can discharge electricity at the rate of 1A for 40 hours or 2A for 20 hours. However, the hour is long period of time, and we normally specify quantity or charge in ampere second or coulombs (unit symbol C). Thus.
Quantity = Current x
time Or in symbols
Q = I t coulombs (C)
egn. ……….. (1.1)
Where the current is in amperes and time in seconds. An electron is a charge carrier whose electrical charge is E = - 1.6 x 10-19C That is, a current of 1 A flows in a circuit when 1/1.6 x 10-19 = 6.25 x 1018 electrons Pass through each point in the circuit in one second. Strictly speaking we should talk of “rate of the flow of charge” rather than current flow, but the latter is more conventional.
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1.2.7 Worked Example Example 1.1 If a current of 3 A flows in a circuit for 120 ms, calculate the quantity of electricity which is involved. Solution, I = 3 A; t = 120 ms = 120 x 10-3 s = 0.12s From eqn. (1.1) Q = I t = 3 x 0.12 = 0.36 C (ans) Example 1.2 If a charge of 8C moves past a given point in a circuit in 0.2s. Calculate the current in the circuit. Solution, Q = 8 C: t = 0.2s. I = Q/t = 8/0.2 = 40 A (ans)
Example 1.3 If an insulated conductor is charged to 3, C, how many additional electrons has it acquired? Solution, Q = 3C: negative charge on an electron = 1.6 x 1019C. Number of electrons = 3/(1.6 x 10-19) = 1.875 x 1019 electrons (ans)
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Concept of Semiconductors Materials
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1.3 Fermi Energy Levels The outer electrons of a single atom can jump from one energy level to another as they receive or give off quantities of energy. Unexcited electrons are in the valance band. Excited electrons are in one of the conduction bands. Only one such band is shown in fig 1.3 but atoms of all elements have several higher conduction bands. Electrons gaining energy jump from a conduction band to one of even higher energy. If they drop from one conduction band to one at a lower level, they lose energy as they do so.
Fig. 1.3 Electron Energy Levels: (a) in Non-Conductors; (b) and (c) in Conductors; (d) in ntype Semiconductors; (e) in p-type Semiconductors
In a block of conducting material we can imagine many millions of atoms with electrons continually gaining or losing energy. They are continuously jumping up or down from one band to another. If the atoms are mainly unexcited, most electrons will be in the valence band and a few in the lower conduction bands. As the atoms become more excited – by heating them for example – we find fewer electrons in the valence band, most in the lower conduction bands and a few in the higher conduction bands. The greater the average energy of the electrons, the more we find the higher conduction bands. We need some way of saying where most of the electrons are and some way of saying what their average energy is. We can show where electrons are by adding to the energy level diagrams, as in Fig. 1.3
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Concept of Semiconductors Materials
Fig. 1.4 The Femi Level: (a) in a Conductor; (b) in a Non-Conductor;
Week 2
(c) in a Semiconductor,
EF is the Femi Level.
1.4 Electrons Energy levels It is found that the electrons of an atom can each have one of only a few fixed amounts of energy. As an example take the atom of lithium, which has three electrons. The two electrons of the inner shell each have about the same amount of energy and they have less energy than the single electron in the outer shell (Fig.1.5)
Fig. 1.5 Electron Energy Levels in Lithium; (a) in a single unexcited (ground state) atom; (b) in a single excited atom; (c) in a bar of Lithium – if all the atoms are in the ground state, their electrons are in the bands.
In an unexcited atom the outer electron has it least energy (its ground state). If the outer electron is given extra energy it becomes excited. It takes in exactly the right amount of energy to take it up to the next energy level, the excitation level.
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Later it may give out this same exact amount of energy, become unexcited again and return to its ground state. It can also take in greater amount of energy and go to one or higher excitation levels not shown in the drawings. The description above applied to a single atom or, for example, to an atom which is a long way from other atoms in a gas. In a solid, the electrons (especially the outer ones) can gain or receive energy from all the atoms around them. This means that their energy can be that of their energy level plus or minus small amount, owing to the action of surrounding atoms. Instead of sharp energy levels, we have energy bands. Electrons in the lower bands are permanently part of the electron could. Electrons of the outer shell which are in the valence band do not have enough to become free electrons. Given extra energy, they become excited and jump into the conduction band. Then they can leave the atoms and becomes charge carriers. Energy can be supplied from other atoms, by an increase of temperature or by an electric field.
In a non-conductor there is a large energy gap between the valence and conduction bands . It would require a temperature of thousands of degrees to make the electrons jump into the conduction band. Such a temperature would destroy the insulator! In a conductor the energy gap is small or the valence and conduction bands overlap . This makes it easy for electrons to gain enough energy to leave the atom and become free. In a conductor there are always plenty of free electrons ready to carry charge in a filed. The energy gaps of pure semiconductors such as silicon and germanium are much less than those of non-conductors but more than those of conductors. When we dope such a semiconductor with phosphorus or some other pentavalent element we provide electrons that have higher valance energy than that of the pure semiconductor.
An electron can have any amount of energy within the levels of the bands. It can have as little as the lowest energy of the lowest band (the inner electrons) or as much as the highest energy of the highest band. Most of the electrons spend most of their time in low energy states. If we were searching among the atoms trying to find an electron of low energy, we should be almost certain to find one. We can say that the probability of findings such an electron is 1. 9
Concept of Semiconductors Materials
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A probability of a means ‘certain to find one’. (For example we might say that the probability that the sun will rise tomorrow is 1.) Next we might look for an electron with energy equal to that of the top of the valence bands. We would not be certain to find such an electron, for the electrons might all have lower energies than this. But there might be, say a one-in-five chance of finding one – this is a probability of 1/5 or 0.2. At the extreme we could look for an electron with the highest level of the highest conduction band. The probability of findings such an electron is very low indeed. It is so low that it is almost impossibility. The probability is very close to zero. (The chance of having a snowstorm on a tropical beach is another event with practically zero probability). As we go from the lowest levels to the highest levels, the probability of finding an electron with a given amount of energy decrease from 1 to 0. At some level between the lowest and highest the probability must be 0.5. This is called the Fermi level. The Fermi levels are marked on Fig. 1.5. In conductor the level lies in the conduction band. This means that the probability is high that some electrons will be in the conduction band at all time. In non-conductor the Fermi level is in the valence band. This means that it is very unlikely (probability close to 0) that any conduction electrons are ever present. In semiconductors the Fermi level lies between the conduction and valence bands. No electrons can have that exact amount of energy, since it is between bands, but some have more and some have less. Doping a semiconductor to make it into n-type material raises the Fermi level. This is because the doping provides more electrons at the higher doping to make a p-type material lowers the Fermi level.
1.5 Conductors, Insulators and Semiconductors 2.3.1 Conductor A conductor is a material through which free electrons will flow easily. In such materials, the valence electrons can be easily dislodged from their parent atoms by applying electromotive force. As stated in section 1.2 earlier, most elements with one, two or three valence electrons are example elements with one, two or three valence electrons are example of good conductors of electricity. Copper, Silver and aluminum are example of good conductors of electricity. 10
Concept of Semiconductors Materials
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2.3.2 Insulator Elements whose valences orbits are almost filled-up with valence electrons hardly give up (their valence) electrons instead they prefer to fill up their valance orbit with electrons; such materials have very few free electrons which may not be able to support current flow. They are therefore poor conductors of electricity. They are better known as insulation or dielectrics. Plastic, rubber and paper are examples of insulate conductor so that the current which they (conduct) carry will not leak off or pass through unwanted conductor materials. It is matter of technical interest to note that al insulating materials (insulators) will break down and conductor current if a sufficiently high voltage is applied across them. In some elements or compounds the electrons are not easily excited. They are strongly held by the nucleus and cannot become free. If there are no charge carriers there can be no electric current. Such materials are called non-conductors or insulators. Examples are glass, mica and many kinds or plastics. It is possible to charge these materials.
2.3.3 Semiconductor Semiconductors are materials with intermediate characteristics between good conductor and good insulators, some of these materials normally have four valence electrons. Carbon, silicon, Silicon and germanium are example of semiconductors. Silicon and germanium are used in the manufacture of diodes, transistors etc. Carbon is used in the production of resistors and some sliding contacts such as motor brushes. A resistor can be considered a material that supports the flow of electric current better than an insulator but not a well a conductor.
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Concept of Semiconductors Materials
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1.6 Conduction in Metals Metals have a crystalline structure. The atoms of the metal area in regular rows and columns. They are held in their places by the strong forces that exist between an atom and its neighbors. We say that the atoms are in a lattice. There is space between the atoms. The electrons in the electron could of each atom have different amounts of energy. Those in the inner shells have the highest energy. They are attracted strongly by the nuclease. The outer electrons have less energy and are not so strongly attracted. The outer electrons are also affected by the atoms around them. Sometimes they gain energy, sometimes they lose it. The outer electrons have the same amount of energy an average, but at any time some electrons have more than the average and some have less than the average. Those electrons that have the most energy are said to be excited. They may have enough energy to escape from the attraction of the nucleus. In that case they leave the electron cloud and become ‘free electrons’ move the spaces between the atoms. When an electron has escaped from an atom, the atom becomes a action. After a time this atom may attract one of the free electrons which has lost some of its energy. The electrons in the outer shells of metals atoms are very easily excited to become free electrons. Always some electrons are gaining energy and becoming free. Always, some are losing energy and returning to the electron cloud of an atom. At any instant there are free electrons moving around in the lattice Fig. 1.6 (a). The electrons move in all directions, so it cannot be said that there is an electric current. However, if the piece of metal is placed in an electric field all the free electrons will tend to move in the same direction.
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Concept of Semiconductors Materials
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Fig. 1.6 Part of the lattice of a metal showing free electrons moving in the space between the atoms: (a) not in an electric field; (b) in an electric field (an electric current).
In fig 1.6(b) we see what happens when a piece of metal is connected between the terminals of a cell. The e.m.f of the cell makes a field. The electrons are repelled from the negative terminal and attracted towards the positive terminal. We have an electric current. Some electrons will leave this piece of metal and pass into the metal of the positive terminal. Electrons from the negative terminal pass into the piece of metal. As the electrons flow, negative electric charge is carried from the negative terminal of the cells to the positive terminal. We say that electrons are charge carriers. They flow from a negative polarity to a positive polarity. There are the three conditions for an electric current. (a)
There must be charge carriers
(b)
The charge carriers are free to move.
(c)
There must be an e.m.f. to make them move
If any one of these conditions is absent, there is no current. Some metal, such as silver and copper, are very good conductors of electricity.
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Their atoms give up free electrons very easily so there are plenty of charge carriers. Also, the lattice allows the electrons to pass without much difficulty. In other metals there are fewer free electrons and the lattice offers greater resistance to the flow of electrons. These metals have greater resistance than silver and copper. The resistance of metals is also affected by temperature. As the temperature increases all the atoms in the lattice gain energy and vibrate strongly. This interferes with the motion of the electrons through the lattice.
1.7 CONDUCTION IN A GAS An example of this is the neon discharge tube (Fig. 1.7) the tube contains a few free electrons from the metal atoms of the electrodes. When the p.d between the electrodes is increases, these electrons begin to move. They hit atoms of neon and knock electrons from them forming neon cautions. The number of moving electrons increases and even more neon ions are formed. There are two kinds of charge carrier, electrons and cautions. They are free to move. There is an e.m.f between the electrodes, so a current flows. The neon ions are being bombarded by electrons and receive a lot of energy. They become very excited and this make them give off light.
Fig. 1.7 Flow of charge carriers through a gas in a discharge tube. 14
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1.8 Intrinsic Semiconductor When an electron breaks free from an atom and thus becomes available for a current it leaves the atom with a net positive charge and a vacancy for an electron. This vacancy is called a hole. It represents a location into which an electron from another atom can easily move. However, such a movement would mean that there was a hole in another atom. An electron movement has led to a hole movement. In pure germanium or silicon, semiconductors, there are as many holes as there are free electronics. Thus when a potential difference is applied to such a semiconductor there is a current due to the movement of the free electrons, each hole being created by an electron becoming free. Such semiconductors are called intrinsic semiconductors.
In an intrinsic semiconductor the current is due in equal parts to electron and hole movement. The introduction of impurities into silicon or germanium can have a great effect on their resistivity, the deliberate introduction of impurities being known as doping. When small amount of phosphorus, arsenic or antimony are added to silicon or germanium the materials ends up with more electrons than holes. This is because these materials have easily detached electrons. Such impurities are known as donors, because they donate electrons. The materials are known as n-type the n being because the majority carriers of current are electrons which are negatively charged particles. When small amounts of aluminum, gallium or iridium are added to germanium or silicon the materials ends up with more holes than free electrons. This is because these atoms supply sites which can be occupied by electrons from the germanium or silicon. Such impurities are known as acceptors, because they accept electrons. The materials are known as p-type, the p being because the majority carrier of current are holes which behave like positively charged particles (under a potential difference the holes move in the opposite direction to electrons). .
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1.9 Extrinsic Semiconductor The material produced by doping is known as an extrinsic semi-conductor because the impurity introduces charge carriers extra to the intrinsic one. In such materials there is a majority charge carrier and holes and electrons do not contribute equally to a current If we have crystal of pure silicon, it is possible to replace a few of the atoms in the lattice by atoms of some other element. If we use the right element, we can do this without upsetting the regular arrangement of atoms in the lattice. Fig. 1.9. To make the diagram simple, it is drawn in only two dimensions. Silicon had four electrons in its outer shell, it is quadrivalent. The complete shell would have eight electrons. An atom of silicon shares its four outer electrons with its four neighbors’ so that, for part of the time at least, it has the full number of eight electrons in its outer shell. This sharing holds the atoms together by what is called a covalent bond. The electrons are strongly held in the bond so they cannot easily escape and become free.
In fig. 1.9 we see what happens when an atom of phosphorus replaces one of the silicon atoms. Phosphorus is pentavalent. It has five electrons in its outer shell. Four of these form covalent bonds with neighboring silicon atoms. This leaves an electron to spare. The spare electron is weakly held by the atom. It can easily escape to become a free electron. By adding phosphorus to the silicon crystal we have provided a supply of free to act as charge carriers, Electricity can easily pass through the crystal. The crystal has been made into a conductor.
The adding of small amounts of impurity (such as phosphorus) to crystals or pure semiconductors is called doping. In the example silicon was doped with phosphorus. We could have doped it with other pentravelent atoms instead, such as arsenic or antimony. Germanium, another semiconductor can be doped in the same way, with the same effect.
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Because the charge carriers to do come from the semiconductor itself, they are called extrinsic charge carriers and conduction by these carriers is called extrinsic conduction. These carry a negative charge, so materials doped in this way is called an n-type semiconductor. Boron is trivalent, with only three electrons in its outer shell. It shares these electrons with its neighbours but, at most it has only seven electrons for its outer shell. This leaves on vacancy. We call this vacancy a hole. If a free electron is available, it will be attracted to the atom to fill this hole.
Let use see what happens when a hole is filled with an atom. The hole attracts an electron an electron from further along the bar. This electron leaves a hole at atom B. The holes at B attract an electron from atom C. In turn, this hole is filled by an electron from atom D. Each electron had moved a short distance the bar and has been captured. The hole has moved a long distance from A to B to C to D (fig. 1.10). The electrons move along the bar from negative to positive, but the hole moves in the opposite direction, from positive to negative.
Think of a hole as positive charge carrier shows a bar of this types of materials connected in a circuit. At the positive end of the bar, electrons are attracted away from the atoms, leaving holes. The holes travel along the bar. At the negative end the are filled with electrons. Since this material is doped to provide plenty of holes it conducts electricity well. Holes are positive charge carriers so a material doped in this way is called a p-type semiconductor. The p stands for positive. Other doping elements used for making p-type semiconductors are aluminum and indium. The idea of a ‘hole’ is used as a convenient way of thinking about conduction in-type semiconductors. It reminds us that whenever an electron escape it leaves behind a hole increases of reverse e.m.f gives little increases of leakage current. This is because most of the minority carriers are being used - few more are available, so current cannot increase much more. There is a saturated reverse current.
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1.10 AVALANCHE BREAKDOWN As we increase the reverse–bias e.m.f the minority carrier of the leakage current gain more energy. If the reverse e.m.f is high enough, they gain so much energy that they are able to set electrons free from the atoms in the lattice. This makes more electron-hole pairs, and conduction increases. The new carriers also gain high energy and can set more electrons free. More electrons produce even more electrons, increasing like an avalanche of snow down a mountain–side. Conduction increases very rapidly and a big reverse current begins to flow. If the diode is not built to carry this amount of current, it will probably be destroyed. Avalanche breakdown happens at a fixed reverse current begins to flow. If the diode is not built to carry this amount of current, it will probably be destroyed. Avalanche breakdown happens at a fixed reverse e.m.f which may be between 5V and 100V, depending on the type of diode. Some types of diode are made that break down at a fairly low but definite e.m.f. These avalanche diodes can pass a reverse current safely if it is not too big. Such diodes are often called Zener diodes, though the way the Zener diode works is rather different.
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1.11 Electrical Force and Potential If there are two objects or atoms or particles which both have the same charge they are repelled from each other. If they are free to move, an electrical force makes them move apart. If they have opposite charges, they are attracted to one another. We sum this up as follows: Like Charge Repel Unlike Charge Attract If fig 4.1 there is a positively charged object, it might be a caption, or it might be a plastic ball that is positively charged because some of its atoms are captions. Suppose that this object is fixed in position. If another positively charged object is placed at A, it is repelled. It moves along the line from A to B and beyond. It moves because of the electric force between the two objects. The line it moves along is a line of force. A negative charged object placed at C moves towards the object, along another line of force. The lines of force around the charged object make up an electric field. We drawn arrows on the lines of force to show which way a positive charge moves.
In Fig. 4.2 we have a positively charged body B in the filed of a fixed positively charged body A. To move B towards A, we have to use force. This means that we have to do work on B against the electrical force. If we push B until it gets to B’ B gains energy from the work we do. This gives it potential energy. If we stop pushing B and release it, B will be accelerated away from A along the line of force. It loses some of its potential energy and gains an equal amount of kinetic energy. To move any charge object from some distant point to point B’ we have to do work. If the object is negatively charged, we so work as we pull on the object to stop it from rushing all the way to A. When the object reaches B’ it has a certain amount of potential energy. This equal the amount of energy we use to bring it to B’.
An n-type semiconductor is one with mobile negative-charge carriers (electrons) in its structure. Current flow in semiconductors is largely due to the movement of what are known as majority-charge carriers in that material; in n-type materials electrons are the majoritycharge carriers; in n-type materials, positive –charge carrier (holes) are the minority –charge carriers.
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Week 4
P-type semiconductors A p-type semiconductor is one with mobile positive –charge carrier (holes) in its structure. In p-type materials, current flow is largely due to the movement of holes (which are in this case the majority –charge carriers) while a small proportion of current flow is due to electrons (which are in this case the minority –charge carriers).
1.12 Diodes Diode Characteristics A rectifier is a two –terminal device (diode) that offers low resistance to current flow in one direction and a very high resistance to current flow in the reverse direction. A diode has two electrodes: an anode and a cathode. It offers low resistance to current flow when the anode negative with respect to the cathode; in this mode it is said to be reverse – biased. The characteristics of both ideal and practical diodes are shown in fig 4.3. AN ideal diode (characteristics shown in bold line in the figure) offers no resistance to current flow in the forward –biased mode (first quadrant) and infinite resistance to flow in the reverse-biased mode (third quadrant). A practical diode offers a small resistance to current flow in the forward-biased mode, when forward conduction takes place. In the reverse-biased mode a leakage current of small value passes through the diode, in which case it is said to operate in its reverse-blocking mode; as the reverse-bias voltage is increase, a point is reached at which the current through the diode increases rapidly, from which point it is said to operate in its reverse breakdown- mode.
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Characteristics of a PN Junction/ Zener Diode
Week 5
2.1 Zener diodes Certain types of diode, known as Zener diodes, are operated in their reverse-breakdown mode. The diode is not damaged provided that the rating of the device is not exceeded. For example , the maximum „reverse‟ current through a 1 W, 10 V Zener diode should not exceed 0.1 A. A feature of Zener diodes is that, when operated in the reverse –breakdown mode, the voltage across them does not alter significantly over a fairly wide current range. Application of Zener diodes include voltage –reference sources, meter protection and bias-voltage supplies.
2.2 How we use semiconductor diodes One of the main uses of diodes is in rectifiers. These are circuits that change alternating current into direct current. They are important in power supplies. Power diodes are often used here; these are large diodes which used in the detector stage of a radio receiver. Diodes used in these circuits carry only small currents. They need to be able to act very quickly as the direction of the current changes, so the junction must be small. A very narrow junction is made by touching the end of a fine metal wire against the semiconductor. Zener diodes (either true Zener diodes or the avalanche diodes) break down at a definite reverse e.m.f. This property is made use of in circuits that make stable the voltage of a power supply.
An understanding of diode behavior is directly transferable to the analysis of more complicated devices, such as the transistor, so a detailed study at this point is justified. We assume that the diode consists of a single crystal with an abrupt change from p – to n-type at the junction in the y-z plane; only variations with respect to x are significant. 21
Characteristics of a PN Junction/ Zener Diode
Week 5
Circles represent immobile ions with their net charges the separate + and – signs represent mobile hole and electors. We imagine that a junction is created instantaneously; because of the density gradient at the junction, hole diffuse from the p-type material to the right across the junction and recombine with some of the many free electrons in the n-type materials. Similarly, electrons diffuse from the n-type material to the left across the junction and recombine. The diffusion of hole leaves uncovered bound negative charge on the left of the junction, and the diffusion of electrons leaves uncovered bound positive charge on the right. To avoid repeating every statement, we call holes in the p-region and electrons in the n-region majority carriers. Thus the result of the diffusion of majority carriers is the uncovering of bound charge in the depletion or transition region and the creation of an internal electric field. Electrons in the p-region and holes in the n-region are called minority carriers. Under the action of the electric field, minority carriers drift across the junction. With two type of carriers (reduced from four by the use of the terms “majority”) and two different conduction mechanisms (drift and diffusion).
Under open-circuit conditions the net current is zero; the tendency of majority carriers to diffuse as a result of the density gradient is just balanced by the tendency of minority carrier to drift across the junction as a result of the electric field. The height of the potential barrier and the magnitude of Vo is determined by the necessity for equilibrium; for not net current, the diffusion component must be just equal to the drift component. The equilibrium value Vo is also called the contact potential for the junction. This potential typically a few tenth of a volt, cannot be used to cause external current flow. Connecting an external conductor across the terminals creates two new contact potentials which just cancel that at the junction.
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Characteristics of a PN Junction/ Zener Diode
Week 5
Forward Bias: The diffusion component of current is very sensitive to the barrier height: only those majority carriers having kinetic energies in excess of eVo diffuse across the junction. (the probability of a carrier possessing sufficient energy can be expressed in term of e-eVo/kT) If equilibrium is upset by a decrease in the junction potential to Vo – V. the probability of majority carriers possessing sufficient energy to pass the potential barrier is greatly increases. The result is a net flow of carriers. Holes from p-type material cross the junction and are injected into n-type material where they represent minority carriers, electrons move in the reverse direction.
In other words, a reduction in the potential barrier encourages carriers to flow from regions where they are in the majority to regions where they are in the minority. If the junction potential is reduced by the application of an external biasing voltage. (The small voltage drop across the diode body at moderate currents is neglected in this discussion). Under forward bias, the current is sustained by the continuous generation of majority carriers and recombination of the injected minority carriers.
An analogy may be helpful. In a vertical column of gas, molecules drift downward under the action of gravity, but diffuse upward due to their random thermal energy. A pressure difference exists between top and bottom, but there is no net flow. The pressure difference cannot be used to causes any external flow.
Reverse Bias: If the junction potential is raised to Vo + by application of a reverse bias the probability of majority carriers possessing sufficient energy to pass the potential barrier is greatly decreased. The net diffusion of majority carriers across the junction and therefore the minority carrier injection current is reduced to practically zero by a reverse bias of a tenth of a volt or so very small reverse current doe flow. The minority carriers thermally generated in the material adjacent to the transition region drift in the direction to the electric field. Since all minority carriers appearing at the edge of the transition region are forced across by the field (hole “roll down” a potential hill; electrons “roll up”) this reverse saturation current Is depends only on the rate of thermal generation and is independent of the barrier height. (Actually Is reaches its maximum value or “saturates” at a low value of reverse bias).
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Characteristics of a PN Junction/ Zener Diode
Week 6
2.3 Voltage Stabilization and reference A simple method of providing a stabilized d.c voltage is given in figure 2.1 Assuming the supply voltage is approximately constant, then R must satisfy the condition.
Fig. 2.1
Where Izmax is the maximum value of the current Iz which can safely be carried by the diode. This is specified by the power by the power dissipation Izmax Vz, and it is this which set the lower limit for R. An upper limit to R is set by the requirement that the current Iz must exceed the value Izmin required to maintain operation of the diode in the Zener region. This ensures that the voltage Vz is maintained across R1. Hence Where IL = Vz/RL. and VZmin and Vz are not strictly constant as the depend on operating conditions. For higher values of R, the voltage divider made up by R and RL would give a situation where (RL/(R+RL)] VA < VZ and the diode would not be in the Zener region and thereby not providing its reference voltage The actual reference voltage provided by a Zener diode depends on the diode current and temperature. The temperature coefficient is zero in the neighborhood of V z = 5 volts, being negative below this value and positive above. Typical temperature coefficient is zero in the neighborhood of Vz = 5 volts, being negative below this value and positive above. Typical temperature coefficients (1/Vz) Vz/T)I are in the range 10-4 to 10-3/oC. For a normal voltage of 10V, a change of current over the full permitted range might produce a voltage change of about 0.1V. When a Zener diode is being used to provide a reference voltage it is important, therefore to maintain it at constant current by some specific means. 24
Characteristics of a PN Junction/ Zener Diode
Week 6
2.4 Voltage Shifting In this context the diode is used as a coupling element in a circuit, usually in place of resistor. Consider figure 2.2 in which a source generates a voltage across a load R1 and it is required to transmit this to the resistor R2. This is the sort of situation met in a d.c amplifier and it could be dealt with by means of a network such as P and R2. This network has usually to be chosen so that it does not present a significant load to the source.
Fig. 2.2
(R + R2)) >> R1 and preferably such that the signal across R2 is not drastically attenuated by the divider R and R2 while giving the required d.c level at A in the circuit. R can be replaced by a Zener diode to satisfy these requirements. The diode has negligible a.c impendence and so the signal is unattenuated at A, and the power dissipation is small. It maintains a fixed d.c voltage between points B and A and so by correct choice of Zener voltage, A can be at the required d.c potential. It is, of course, necessary for (R + R2) to satisfy the conditions quoted in the previous section so that the Zener voltage is always maintained.
2.5 GENERAL CHARACTERISTICS There are several different practical forms of the diode semiconductor junction; point contact; thermionic, metal –oxide barrier, and a variety of circuit symbols are used to represent them. We shal use the symbol shown in figure 2.3 to represent any type of diode except the Zener diode.
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Characteristics of a PN Junction/ Zener Diode
Week 6
Fig. 2.3
The ideal diode has the property of being unidirectional in the sense that a voltage applied with given polarity will cause flow of current with negligible resistance, a voltage of opposite polarity will give no current whatever. The can be summarized as follows. Using the sign convention illustrated in which Va and Vk are the potential of anode and cathode respectively the voltage across the diode is Vd+Vak = Va – Vk, and Id is the forward current through the diode, the Forward bias Id > 0 if Va > Vk (ON condition) Reverse bias Is = 0 if Va Vk (OFF condition)
It should be emphasized that the second of these relations applies only to an ideal diode; in a practical device a small but finite current known as the reserve leakage current flow when the diode is reverse biased. In general, we can regard Id = f (Vd), where the function of f usually takes a fairly simple form . It is important to note the scale in the figure which give an indication of the order of – magnitude of the current and voltage for a typical diode usable as an a.c rectifier. It will be useful on occasions to use an extra approximation to the diode characteristic.
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Characteristics of a PN Junction/ Zener Diode
Week 7
2.6The transistor is a solid state semiconductor device which can be used for amplification, switching, voltage stabilization, signal modulation and many other functions. It acts as a variable valve which, based on its input current (BJT) or input voltage (FET), allows a precise amount of current to flow through it from the circuit's voltage supply.
Fig. 3.1 Bipolar junction transistor (BJT)
Typical Transistors
2.7 A bipolar junction transistor (BJT) is a type of transistor, an amplifying or switching device constructed of doped semiconductors that employs both types of charge carriers: electrons and holes. The BJT is a three layer sandwich of differently doped sections, either N-type|P-type|N-type (NPN transistors) or P-type|N-type|P-type (PNP transistors). The center layer is called the base of the transistor and made from lightly doped, high resistivity material. By varying the current into the base terminal, the current allowed to flow between the emitter and a third terminal known as the collector (which are both heavily doped and hence low resistivity regions) can be varied. effect can be used to amplify the input current. BJTs be thought of as current-controlled current sources and usually characterized as current amplifiers. Early transistors were made from germanium but most modern BJTs are made from silicon.
The is
This can are
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Characteristics of a PN Junction/ Zener Diode
Week 7
2.8 JUNCTION TRANSISTORS A junction transistor consists of a thin piece of one type of semiconductor material between two thicker
Fig. 3.2 Symbols of An NPN and PNP Transistor Layers of the opposite type. For example, if the middle layer is p-type, the outside layers must be ntype. Such a transistor is an NPN transistor. One of the outside layers is called the emitter, and the other is known as the collector. The middle layer is the base. The places where the emitter joins the base and the base joins the collector are called junctions.
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Application of Bipolar Junction Transistors
Week 8
3.1 Testing a BJT transistor Transistors can be damaged by heat when soldering or by misuse in a circuit. If you suspect that a transistor may be damaged there are two easy ways to test it:
3.2 Testing with a multimeter Use a multimeter or a simple tester (battery, resistor and LED) to check each pair of leads for conduction. Set a digital multimeter to diode test and an analogue multimeter to a low resistance range.
Fig. 3.3 Testing of an NPN Transistor Test each pair of leads both ways (six tests in total):
The base-emitter (BE) junction should behave like a diode and conduct one way only. The base-collector (BC) junction should behave like a diode and conduct one way only. The collector-emitter (CE) should not conduct either way.
The diagram shows how the junctions behave in an NPN transistor. The diodes are reversed in a PNP transistor but the same test procedure can be used.
3.3 FIELD EFFECT TRANSISTORS A field effect transistor has only two layers of semiconductor material, one on top of the other. Electricity flows through one of the layers, called the channel. A voltage connected to the other layer, called the gate, interferes with the current flowing in the channel. Thus, the voltage connected to the gate controls the strength of the current in the channel. There are two basic varieties of field effect transistors-the junction field effect transistor(JFET) and the metal oxide semiconductor field effect transistor (MOSFET). Most of the transistors contained in today's integrated circuits are MOSFETS's
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Application of Bipolar Junction Transistors
Week 8
Fig. 3.4 Diagram of a FIELD EFFECT TRANSISTOR
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Application of Bipolar Junction Transistors
Week 9
3.4 BASIC TRANSISTOR OPERATION In order for the transistor to operate properly as an amplifier, the two pn junctions must be correctly biased with external dc voltages. In this section, we use the Ilpn transistor for illustration. The operation of the pl1J7 is the same as for the I1PI1 except that the roles of the electrons and holes, the bias voltage polarities, and the current directions are all reversed.
Fig. 3.5 An NPN and PNP Transistors To illustrate transistor action. let's examine what happens inside the npn transistor. The forward bias from base to emitter narrows the BE depletion region, and the reverse bias from base to collector widens the BC depletion region, as depicted in Figure below.The heavily doped /Hype emitter region is teeming with conduction-band (free) electrons that easily diffuse through the forward-biased BE junction into the p-type base region where they become minority caniers,just as in a forward-biased diode. The base region is lightly doped and very thin so that it has a limited number of holes. Thus, only a small percentage of all the electrons flowing through the BE junction can combine with the available holes in the base. These relatively few recombined electrons flow out of the base lead as valence electrons. forming the small base electron current, as shown below.
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Application of Bipolar Junction Transistors
Week 9
Fig. 3.6 An Internal Structure of a Transistor Most of the electrons flowing from the emitter into the thin. lightly doped base region do not recombine hut diffuse into the BC depletion region. Once in this region they are pulled through the reverse-biased BC junction by the electric field set up by the force of attraction between the positive and negative ions. Actually. you can think of the electrons as being pulled across the reverse-biased BC junction by the attraction of the collector supply voltage. The electrons now move through the collector region, out through the collector lead, and into the positive terminals of the collector voltage source. This forms the collector electron current, as shown in Figurebelow.. The collector current is much larger than the base current. This is the reason transistors exhibit current gain.
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Application of Bipolar Junction Transistors
Week 9
3.5 Transistor Currents The directions of the currents in an npn transistor and its schematic symbol are as shown in Figure 4-5(a); those for a pnp transistor are shown in Figure 4-5(b). Notice that the an-ow on the emitter of the transistor symbols points in the direction of conventional current.
Fig. 3.7 These diagrams show that the emitter current (IE) is the sum of the collector current (Ie) and the base Current (IB), expressed as follows: IE = Ie + I B As mentioned before, I B is very small compared to IE or Ic. The capital-letter subscripts indictate dc values.
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Application of Bipolar Junction Transistors
Week 9
when a transistor is connected to dc bias voltages types. VBB forward-biases the base-emitter junction and Vcc reverse-biases the base-collector junction. Although in this chapter we are using battery symbols to represent the bias voltages, in practice the voltages are often derived from a dc power supply. For example. Vee is normally taken directly from the power supply output and VBB (which is smaller) can be produced with a voltage divider. Bias circuits are examined thoroughly.
Fig. 3.8 Biasing of an NPN and PNP Transistor
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Application of Bipolar Junction Transistors
Week 10
3.6 SEMICONDUCTOR DIODES A semiconductor diode has the same action as the thermionic diode that was described. It has two terminals, called cathode and anode. Current can flow easily from the anode to the cathode but not in the reverse direction. Although the semiconductor diode has the same action as the thermionic diode, it is made in a very different way and the way it works is completely different.
3.7 The PN Junction Fig 3.9(a) shows the stages in the making of a pn junction. Silica of silicon is put in a furnace where there is boron vapor. Some of the boron diffuses into the silicon. Boron is a trivalent element. It change the outer type layer we have a pn-junction. Another way of making a pn junction is shown in fig. 3.9 (b). A disc of indium (trivalent) is placed on silica of n-type germanium. The germanium is placed in a furnace. Some of the indium passes into the germanium, making it p-type germanium. As before, we have layer of n-type germanium in contact with a layer of p-type germanium. This is another pn –junction.
3.8 The Electric Field at a PN- Junction The electrons from the n-type layer can drift across the junction into the p-type layer. There they fill the holes. Holes can also drift across into the n-type layer and combine with electrons there. This loss of electrons from the n-type layer leaves some of its atoms with a positive charge. These positively charged atoms, or ions, are fixed in their positions in the lattice of the crystal, so they cannot move. In the p-type layer we find fixed negatively charged ions caused by the loss of hole from this layer. Fig 3.10(a) shows the result. The layers of ions on each side of the junction make an electric field across the junction. Having an electric field across the junction is like having an electric cell connected there. Although there is really no cell there, we can imagine one. We call it the in-built cell , or the virtual cell (fig. 3.10(b). The e.m.f of then in-built cell is about 0.6V in a silicon junction and about 0.2 V in a germanium junction. The field due to the in-built cell acts to stop any more electronics or holes from crossing the junction. Electrons trying to cross from the n-type layer are repelled by the negative charge in the p-type layer. Holes trying to cross from the p-type layer are repelled by the positive charge in then n-type layer. 35
Application of Bipolar Junction Transistors
Week 10
This means that there are no charge carriers in the regions on either side of the junction. We can this depletion region. Because of the depletion region, no current can flow across, the junction.
Another way of looking at this is to think of the potential across the junction Fig 3.10(c). As we cross the junction form the p-type layer to the n- type layer there is a rise of potential. We could say that there is a potential hill. This hill is steep and in the wrong direction to allow electrons or holes passes it.
3.9 Forward and Reverse Bias If we connection a battery to the junction, as in fig. 3.11(a), a current flows across the junction. The battery has an e.m.f of 6V. This battery is connected so that its e.m.f is opposed to the e.m.f of the in-built cell. It reduces the effect of the in-built cell and makes the depletion layer less wide. We now have a strong current of electrons in the n-type layer and a strong current of holes in the p-type layer. Holes are filled by electrons at the junction and more holes created where the p-type materials is joined to the external wire. The potential hill is not as high as before so that holes and electrons can flow freely across the junction (Fig. 3.11(b). A junction connected like this to an external source of e.m.f (such as the battery) is said to be forward biased. We can sum it up in this way.
If the battery is connected the other way round, we have reviser bias fig. 3.11(c). Now the e.m.f.s of the battery and the in-built cell are in the same direction. Ore electrons and holes are repelled from the depletion region. The depletion region becomes wider. The potential hill is higher (Fig. 3.11(d)) so it is even more difficult for a current to flow across the junction. 36
Application of Bipolar Junction Transistors
Week 10
From the description above we can see that the pn junction lets current flow in one direction, but not in the other. It behaves in the same way as a thermionic diode. It can be used for the same purpose. Fig 3.12 shows some typical semiconductor diodes. They are made from silicon or germanium. The terminal of then-type region is called the cathode and the other is the anode. We use the names that we use for the thermionic diode, even though the structure and action of the two kinds of diode are so different. Current and Voltage Analysis Consider the basic transistor bias circuit configuration in Fig.10.5. Three transistor dc Currents and three dc voltages can be identified. I B : dc base current IE: dc emitter current Ie: dc collector current V BE : dc voltage at base with respect to emitter V CB : dc voltage at collector with respect to base V CE : dc voltage at collector with respect to emitter VBB forward-biases the base-emitter junction, and Vee reverse-biases the base-collector Junction. When the base-emitter junction is forward-biased, it is like a forward-biased diode and has a nominal forward voltage drop of V BE == 0.7 V
3.10 Transistor currents and voltages. Although in an actual transistor VBE can be as high as 0.9 V and is dependent on current. We Will use 0.7 V Throughout this text in order to simplify the analysis of the basic concepts. Since the emitter is at ground (0V), by Kirchhoff's voltage law, the voltage across R B is V RB = V BB - V BE Also, by Ohm's law, V RB = IBR B
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Application of Bipolar Junction Transistors
Week 10
3.11 Collector Characteristic Curves Using a circuit like that shown in Figure 3.11(a), you can generate a set of collector characteristic curves that show how the collector Current 10 varies with the collector-toemitter voltage, VCE ' for specified values of base current, lB' Notice in the circuit diagram that bothVBB and Vcc are variable sources of voltage. Assume that VBB is set to produce a certain value of IB and Vcc is zero. For this condition, both the base-emitter junction and the base-collector junction are forward-biased because the base is at approximately 0.7 V while the emitter and the collector are at 0 V. The base Current is through the base-emitter junction because ofthe low impedance path to ground and, therefore, Ie is zero. When both junctions are forward-biased, the transistor is in the saturation region of its operation. As Vcc is increased, VCE increases gradually as the collector current increases. This is indicated by the portion of the characteristic curve between points A and B in Figure 3.11(b).Ic increases as Vcc is increased because VCE remains less than 0.7 V due to the forward- biased base-collector junction. Ideally, when VCE exceeds 0.7V, the base-collector junction becomes reverse-biased and the transistor goes into the active or linear region of its operation. Once the base-collector junction is reverse-biased, Ic levels off and remains essentially constant for a given value of I B as VCE continues to increase. Actually, Ic increases very slightly as VCE increases due to widening of the base-collector depletion region. This results in fewer holes for recombination in the base region which effectively causes a slight increase.
For this portion of the characteristic curve, the value of Ic is determined only by the relationship expressed as Ic = (3 Dc IB ).When VCE reaches a sufficiently high voltage, the reverse-biased base-collector junction goes into breakdown; and the collector current increases rapidly.
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Application of Bipolar Junction Transistors
Week 10
A transistor should never be operated in this breakdown region. When IB = 0, the transistor is in the cutoff region although there is a very small collector leakage Current as indicated. The amount of collector leakage Current for IB = 0 is exaggerated on the graph for illustration.
3.12 Cut-off When IB = 0, the transistor is in the cutoff region of its operation. This is shown in Figure 3.13 with the base lead open. Resulting in a base current of zero. Under this condition, there is a very small amount of collector leakage current. ICEO' due mainly to thermally produced carriers. Because ICEO is extremely small. it will usually be neglected in circuit analysis so that VCE = Vcc . In cutoff, both the base-emitter and the base- collector junctions are reverse-biased.
3.13 Saturation When the base-emitter junction becomes forward-biased and the base current is increased, the collector current also increases (lc = (3DcIB) and VCE decreases as a result of more drop across the collector resistor (VCE = V cc - IcRc). This is illustrated in Figure 10.8. When VCE reaches its saturation value. VCRsat), the base-collector junction becomes forward-biased and Ic can increase no further even with a continued increase in lB' At the point of saturation, the relation Ic = (3 Dc IB is no longer valid. VCEIsat) for a transistor occurs some where below the knee of the collector curves, and it is usually only a few tenths of a volt for silicon transistors.
3.14 DC load line Cutoff and saturation can be illustrated in relation to the collector characteristic curves by the use of a load line. Figure 3.14 shows a de load line drawn on a family of curves connecting the cutoff point and the saturation point. 39
Application of Bipolar Junction Transistors
Week 10
The bottom of the load line is at ideal cut-off where Ic = 0 and VCE = Vcc. The top of the load line is at saturation where Ic = IC (sat) and VCE = VCE (sat)- In between cutoff and saturation along the load line is the active region of the transistor's operation.
3.15 Maximum Transistor Ratings A transistor, like any other electronic device, has limitations on its operation. These limitations are stated in the form of maximum ratings and are normally specified on the manufacturer's data sheet. Typically, maximum ratings are given for collector-to-base voltage. Collector-to-emitter voltage. Emitter-to-base voltage. Collector current, and power dissipation. The product of VCE and Ic must not exceed the maximum power dissipation. Both VCE and Ic cannot be maximum at the same time. If VCE is maximum. Ic can be calculated as Ic. For any given transistor, a maximum power dissipation curve can be plotted on the collector characteristic curves, as shown in Figure 3.15(a). These values are tabulated in Figure 3.15(b). Assume PO (max) is 500 mW, VCE(max) is 20 V, and IC(max) is 50 mA. The curve shows that this particular transistor cannot be operated in the shaded portion of the graph. IC(max) is the limiting rating between points A and B, (POrmax) is the limiting rating between points Band C, and VCE(max) is the limiting rating between points C and D.
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Application of Bipolar Junction Transistors
Week 11
3.16 DC Bias Bias establishes the dc operating point for proper linear operation of an amplifier. If an amplifier is not biased with correct de voltages on the input and output. It can go into saturation or cutoff when an input signal is applied. Figure 3.16 shows the effects of proper and Improper of biasing of an inverting amplifier. In part (a), the output signal is an amplified replica of the input signal except that it is inverted, which means that it is 1800 out of phase with the input. The output signal swings equally above and below the de bias level of the output, VDC(out). Improper biasing can cause distortion in the output signal, as illustrated in Parts (b) and (c). Part (b) illustrates limiting of the positive portion of the output voltage rc- as a result of a Q-point (dc operating point) being too close to cutoff. Part (c) shows limiting of the negative portion of the output voltage as a result of a de operating point being too close to saturation. Actually, there is a small voltage (VCEsat)) across the transistor, and (Icsat) is slightly less than 45.5 mA, as indicated in Figure 3.17. Note that Kirchhoff's voltage law ap plied around the collector loop gives Vcc – IcRc – Vce = 0 This result in a straight line equation for the load line of the form y = nIX + b as follows: Ic = - (1/Rc) VCE + Vcc/Rc Where - (1/Rc) is the slope and Vcc/Rcis the y-axis intercept point.
The region along the load line including all points between saturation and cutoff is generally known as the linear region of the transistor's operation. As long as ( the transistor is operated in this region), the output voltage is ideally a linear reproduction of the input. Figure 3.18 shows an example of the linear operation of a transistor. AC quantities are indicated by lowercase italic subscripts. Assume a sinusoidal voltage, ViII' is superimposed on VBB . Causing the base current to vary sinusoidally 100 /-LA above and below its Q-point Value of 300 /LA. This, in turn, causes the collector current to vary 10 mA above and below its Q-point value of 30 mA. As a result of the variaiation in collector current. the collector- to-emitter voltage varies 2.2 V above and below its Q-point value of 3.4 V. Point A on the load line in Figure 3.19 corresponds to the positive peak of the sinusoidal input voltage. Point B corresponds to the negative peak, and point Q corresponds to the zero value of the sine wave, as indicated. V CEQ ' I cQ , and I BQ are de Q-point values with no input sinusoidal voltage applied.
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Application of Bipolar Junction Transistors
Week 11
3.17 Waveform Distortion As previously mentioned, under certain input signal conditions the location of the Q-point on the load line can cause one peak of the V ce waveform to be limited or clipped, as shown in parts (a) and (b) of Figure 3.20. In each case the input signal is too large for the Q-point location and is driving the transistor into cutoff or saturation dur- ing a portion of the input cycle. When both peaks are limited as in Figure 3.20( c), the transistor is being driven into both saturation and cutoff by an excessively large input signal. When only the positive peak is limited, the transistor is being driven into cutoff but not saturation. When only the negative peak is limited, the transistor is being driven into saturation but not cutoff. If the base current is much smaller than the current through Rz, the bias circuit can be viewed as a voltage-divider consisting of RI and Rz, as indicated in Figure 3.21(a). If IB is not small enough to neglect compared to 1 2 , then the de input resistance, RIN(base), that appears from the base of the transistor to ground must be considered. RJN(base) is in parallel with Rz, as shown in Figure 3.21(b).
3.21 Input Resistance at the Transistor Base. To develop a formula for the dc input resistance at the base of a transistor, we will use the diagram in Figure 3.22. L VJN is applied between base and ground, and IJN is the current into the base as shown. By Ohm's law, RIN (base) = VIN/IIN Kirchhoff's voltage law applied around the base-emitter circuit yields VIN = VBE + IERE With the assumption that V BE « IERE , the equation reduces to IE == Ie = (βDcIBRE)
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Application of Bipolar Junction Transistors
Week 11
Another way to analyze a voltage-divider biased transistor circuit is to apply Thevenin's theorem. We will use this method to evaluate the stability of the circuit. First, let's get an equivalent baseemitter circuit for Figure 13.23 using Thevenin's theorem. Looking out from the base terminal, the bias circuit can be redrawn as shown in Figure 3.24(a). Apply Thevenin's theorem to the circuit left of point A, with V ee replaced by a short to ground and the transistor disconnected from the circuit. The voltage at point A with respect to ground is VTH = ( R2/R1 +R2)Vcc and the resistance is RTH = ( R2/R1 +R2)Vcc
3.18 Voltage-Divider Biased PNP Transistor As you know, a pnp transistor requires bias polarities opposite to the npn. This can be acomplished with a negative collector supply voltage. as in Figure 3.25(a), or with a positive emitter supply voltage, as in Figure 3.25(b). In a schematic, the pnp is often drawn upside down so that the supply voltage line can be drawn across the top of the schematic and ground at the bottom, as in Figure 3.26. The analysis procedure is basically the same as for an npn transistor circuit, as demonstrated in the following steps with reference to Figure 3.26. The base voltage is determined by using the voltage-divider formula.
3.19 Base Bias This method of biasing is common in relay driver circuits. Figure 3.27 shows a base-biased transistor, The analysis of this circuit for the linear region is as follows. Starting with Kirchhoff's voltage law around the base circuit, Q-Point Stability of Base Bias Notice that Equation 5-9 shows that Ic is dependent on {3De' The disadvantage of this is that a variation in {βDC causes Ic and. as a result, VCE to change, thus changing the Q-point of the transistor. This makes the base bias circuit extremely beta-dependent and very unstable.
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Application of Bipolar Junction Transistors
Week 11
3.20 Emitter Bias Emitter bias uses both a positive and a negative supply voltage. In the circuit shown in Figure 3.28 the VEE supply voltage forward-biases the base-emitter junction. Kirchhoff's voltage law applied around the base-emitter circuit in part (a), which has been redrawn in part (b) for analysis. gives the following equation:
3.21 Collector-Feedback Bias In Figure 3.29, the base resistor R B is connected to the collector rather than to V cc , as it was in the base bias arrangement discussed earlier. The collector voltage provides the bias for the baseemitter junction. The negative feedback creates an "offsetting" effect that tends to I ) " keep the Qpoint stable. If Ic tries to increase, it drops more voltage across Rc, thereby causing V to decrease. When Vc decreases, there is a decrease in voltage across R B , which decreases lB' The decrease in I B produces less Ic which, in turn, drops less voltage across Rc and thus offsets the decrease in Vc.
3.22 Q-Point Stability Over Temperature Equation 3.30 shows that the collector current is dependent to some extent on {βoc and VE . his dependency, of course, can be minimized by making Re» RB /{βoc and Vee » VBE . An mportant feature of collector-feedback bias is that is it essentially eliminates the {βoc and VBE ependency even if the stated conditions are met.
As you have learned, {βoe varies directly with temperature, and VBE varies inversely with temperature. As the temperature goes up in a collector-feedback circuit, {β goes up and VBE goes down. The increase in {βDc acts to increase Ie. The decrease in V BE acts to increase IB which, in turn also acts to increase Ie. As Ie tries to increase, the voltage drop across Rc also tries to increase. This tends to reduce the collector voltage and therefore the voltage across RB , thus reducing IB and offsetting the attempted increase in Ie and the attempted decrease in VC ' The result is that the collector-feedback circuit maintains a relatively stable Q-point. The reverse action occurs when the temperature decreases.
44
Basic Structure And Application of the Thyristor
Week 12
4.1Basic Structure of the Thyristor. The 4-layer diode (also known as Shockley diode and SUS) is a type of thyristor, which Is a class of devices constructed of four semiconductor layers. The basic construction of a 4-layer diode and its schematic symbol are shown in Figure 4.1 The pnpn structure can be represented by an equivalent circuit consisting of a pnp transistor and an npn transistor, as shown in Figure 4.2(a). The upper pnp layers form Q, and the lower npn layers form Q, with the two middle layers shared by both equivalent transistors. Notice that the base-emitter junction of Q corresponds to pn junction I in Figure 12.1, the base-emitter junction of Qz corresponds to pn junction 3, and the base-collector Junctions of both QI and Qz correspond to pn junction 2, When a positive bias voltage is applied to the anode with respect to the cathode. as shown in Figure 4.2(b), the ba<;e-emitter junctions of QI and Q2 (pn junctions land 3 in Figure ll-l(a» are forward-biased, and the common base-collector junction (pn junction 2 in Figure 4.3 is reverse-biased. Therefore, both equivalent transistors are in the linear region. The currents in a 4-layer diode are shown in the equivalent circuit in Figure 4.4. At Low-bias levels, there is very little anode current, and thus it is in the off state or forwardblocking region.
4.2 Forward-Breakover Voltage The operation of the 4-layer diode may seem unusual because when it is forward-biased, it can act essentially as an open switch. There is a region of forward bias, called the forward-blocking region, in which the device has a very high forward resistance (ideally an open) and is in the offstate. The forward-blocking region exists from VAK = 0 V up to a value of VAK called the forward-breakover voltage, VBR(F)' As VAK is increased from 0, the anode current, fA' gradually increases, as shown on the graph. As I A increases, a point is reached where 1A = Is, the switching current. At this point, VAK = VBR(F)' and the internal transistor structures become saturated. When this happens, the forward voltage drop, VAK, suddenly decreases to a low value, and the 4-layer diode enters the forward-conduction region as indicated in Figure 4.5. Now, the device is in the on state and acts as a closed switch. When the anode current drops back below the holding value, IH' the device turns off. Holding Current Once the 4-layer diode is conducting (in the on state), it will continue to conduct until the anode current is reduced below a specified level, called the holding I:O:current, IH' This parameter is also indicated on the 45
Basic Structure And Application of the Thyristor
Week 12
characteristic curve in Figure4.6. When fA falls below TH' the device rapidly switches back to the off state and enters the forward-blocking region.
4.3 Switching Current The value of the anode current at the point where the device switches from the forward-blocking region (off) to the forward-conduction region (on) is called the switching current, Is. This value of current is always less than the holding current, IH'
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Basic Structure And Application of the Thyristor
Week 13
4.4 Working Principles of the Thyristor. The 4-layer diode (also known as Shockley diode and SUS) is a type of thyristor, which is a class of devices constructed of four semiconductor layers. The basic construction of a 4-layer diode and its schematic symbol are shown in Figure 4.7. The pnpn structure can be represented by an equivalent circuit consisting of a pnp tran-sistor and an npn transistor, as shown in Figure 4.8.The upper pnp layers form Q, and the lower npn layers form Q, with the two middle layers shared by both equivalent transistors. Notice that the base-emitter junction of Q] corresponds to pn junction I in Figure 4.9, the baseemitter junction of Qz corresponds to pn junction 3, and the base-collector junctions of both QI and Qz correspond to pn junction 2. When a positive bias voltage is applied to the anode with respect to the cathode. as shown in Figure 4.10(b), the base-emitter junctions of QI and Q2 (pn junctions land 3 in Figure 4.11(a) are forwardbiased, and the common base-collector junction (pn junction2 in Figure 4.11 (a) is reverse-biased. Therefore, both equivalent transistors are in the linear region.
The currents in a 4-layer diode are shown in the equivalent circuit in Figure 4.12. At Low-bias levels, there is very little anode current, and thus it is in the off state or forwardblocking region.
As VAK is increased from 0, the anode current, fA' gradually increases, as shown on the graph. As I A increases, a point is reached where 1 A = Is, the switching current. At this point, V AK = VBR(F)' and the internal transistor structures become saturated. When this happens, the forward voltage drop, VAK, suddenly decreases to a low value, and the 4-layer diode enters the fODvard-conduction region as indicated in Figure 4.13.
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Basic Structure And Application of the Thyristor
Week 13
Now, the device is in the on state and acts as a closed switch. When the anode current drops back below the holding value, IH' the device turns off.
4.5 Holding Current Once the 4-layer diode is conducting (in the on state), it will continue to conduct until the anode current is reduced below a specified level, called the holding I: current, IH' This parameter is also indicated on the characteristic curve in Figure 4.15. When fA falls below TH' the device rapidly switches back to the off state and enters the forward-blocking region.
4.6 Switching Current. The value of the anode current at the point where the device switches from the forward-blocking region (off) to the forward-conduction region (on) is called the switching current, Is. This value of current is always less than the holding current, IH'
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Basic Structure And Application of the Thyristor
Week 14
4.7 THE SILICON-CONTROLLED RECTIFIER (SCR) Like the 4-layer diode, the SCR has two possible states of operation. In the off state, it acts ideally as an open circuit between the anode and the cathode; actually, rather than an open, there is a very high resistance. In the on state, the SCR acts ideally as a short from the anode to the cathode; actually, there is a small on (forward) resistance. The SCR is used in many applications, including motor controls, time-delay circuits, heater controls, phase controls, and relay controls, to name a few.
4.8 SCR Equivalent Circuit Like the 4-layer diode operation, the SCR operation can best be understood by thinking of Its internal pnpn structure as a two-transistor arrangement, as shown in Figure 4.16. This Structure is like that of the 4-layer diode except for the gate connection. The upper pnp layers act as a transistor, Q1, and the lower npn layers act as a transistor, Q2' Again, notice that the two middle layers are "shared."
4.9 Turning the SCR On . When the gate current, IG' is zero, as shown in Figure 4.17(a), the device acts as a 4-layer diode in the offstate. In this state, the very high resistance between the anode and cathode can be approximated by an open switch. As indicated. When a positive pulse of current (trigger) is applied to the gate, both transistors turn on (the anode must be more positive than the cathode). This action is shown in Figure 4.17 (b). I B2 turns on Q2' providing a path for IB into the Q2 collector, thus turning on Q\. The collector current of QI provides additional base current.
4.10 On-Off Control of Current Figure 4.18 shows an SCR circuit that permits current to be switched to a load by the momentary closure of switch SWI and removed from the load by the momentary closure of switch SW2.
Assuming the SCR is initially off, momentary closure of SWI provides a pulse of current into the gate, thus triggering the SCR on so that it conducts current through R L . The SCR remains in conduction even after the momentary contact of SWI is removed if the anode current is equal to or greater than the holding current. IH . When SW2 is momentarily closed. current is shunted around the SCR, thus reducing its anode current below the holding value, IH . This turns the SCR off and reduces the load current to zero.
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Basic Structure And Application of the Thyristor
Week 14
4.11 Half-Wave Power Control A common application of SCRs is in the control of ac power for lamp dimmers, electric heaters, and electric motors. A half-wave, variable-resistance, phase-control circuit is shown in Figure 4.19, 120 V ac are applied across terminals A and B: R L represents the resistance of the load (for example, a heating element or lamp filament). Resistor RI limits the current and potentiometer R2 sets the trigger level for the SCR. By adjusting R2, the SCR can be made to trigger at any point on the positive half-cycle of the ac waveform between 0° and 90°, as shown in Figure 4.20. When the SCR triggers near the beginning of the cycle (approximately 0°), as in Figure 4.21( a), it conducts for approximately 180° and maximum power is delivered to the load. When it triggers near the peak of the positive half-cycle (90°), as in Figure 4.21(b), the SCR conducts for approximately 90° and less power is delivered to the load. By adjusting R2' triggering can be made to occur anywhere between these two extremes.
4.12 Lighting System for Power Interruptions As another example of SCR applications. let's examine a circuit that will maintain lighting by using a backup battery when there is an ac power failure. Figure 4.22 shows a centertapped full-wave rectifier used for providing ac power to a low-voltage lamp, as long as the ac power is available, the battery charges through diode D3 and R. The SCR's cathode voltage is established when the capacitor charges to the peak value of the full-wave rectified ac (6.3 V rms less the drops across R2 and D). The anode is at the 6 V battery voltage, making it less positive than the cathode, thus preventing conduction. The SCR's gate is at a voltage established by the voltage divider made up of R2 and R3' Under these conditions the lamp is illuminated by the ac input power and the SCR is off..
When there is an interruption of ac power. The capacitor discharges through the closed 50
Basic Structure And Application of the Thyristor
Week 14
path D3 , R1, and R3 , making the cathode less positive than the anode or the gate. This action establishes a triggering condition, and the SCR begins to conduct. Current from the battery is through the SCR and the lamp, thus maintaining illumination.. When ac power is restored, the capacitor recharges and the SCR turns off. The battery begins recharging.
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Basic Structure And Application of the Thyristor
Week 15
4.13 An Over-Voltage Protection Circuit Figure 4.23 shows a simple over-voltage protection circuit, sometimes called a "crowbar" circuit, in a dc power supply. The dc output voltage from the regulator is monitored by the zener diode (D1) and the resistive voltage divider (R1 and R2 ). The upper limit of the output voltage is set by the zener voltage. If this voltage is exceeded, the zener conducts and the voltage divider produces an SCR trigger voltage. The trigger voltage turns on the SCR, which is connected across the line voltage. The SCR current causes the fuse to blow, thus disconnecting the line voltage from the power supply.
4.14 The Diac A diac is a two-terminal four-layer semiconductor device (thyristor) that can conduct current in either direction when properly activated. The basic construction and schematic symbol for a diac are shown in Figure 4.24. Notice that there are two terminals, labeled AI and A2 . Conduction occurs in a diac when the breakover voltage is reached with either polarity across the two terminals. The curve in Figure 4.25. illustrates this characteristic. Once breakover occurs, current is in a direction depending on the polarity of the voltage across the terminals. The device turns off when the current drops below the holding value.
4.15. Advances of the Thyristor . The basic thyristor is a 4-layer device with two terminals, the anode and the cathode. It is constructed of four semiconductor layers that fonn a pnpn structure. The device acts as a switch and remains off until the forward voltage reaches a certain value; then it turns on and conducts.
Conduction continues until the Current is reduced below a specified value. This basic thyristor is also known as a silicon unilateral switch {SUS), Shockley diode, or 4-layer diode. From the anode to the cathode; actually, there is a small on (forward) resistance. The SCR is used in many applications, including motor controls, time-delay circuits, heater controls, phase controls, and relay controls, to name a few.
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Basic Structure And Application of the Thyristor
Week 15
The SCR has many uses in the areas of power control and switching applications. A few basic applications are described in this section. . Another advantage of the SCR is that it can be used for Lighting System for Power Interruptions, also for a circuit that will maintain lighting by using a backup battery when there is an ac power failure.
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