A PIN diode is a diode with a wide, undoped intrinsic semiconductor region between a p-type semiconductor and an n-type semiconductor region. The p-type and n-type regions are typically heavily doped because they are used for ohmic contacts. The wide intrinsic region is in contrast to an ordinary PN diode. The wide intrinsic region makes the PIN diode an inferior rectifier (one typical function of a diode), but it makes the PIN diode suitable for attenuators, fast switches, photodetectors, and high voltage power electronics applications. Operation A PIN diode operates under what is known as high-level injection. In other words, the intrinsic "i" region is flooded with charge carriers from the "p" and "n" regions. Its function can be likened to filling up a water bucket with a hole on the side. Once the water reaches the hole's level it will begin to pour out. Similarly, the diode will conduct current once the flooded electrons and holes reach an equilibrium point, where the number of electrons is equal to the number of holes in the intrinsic region. When the diode is forward biased, the injected carrier concentration is typically several orders of magnitude higher than the intrinsic level carrier concentration. Due to this high level injection, which in turn is due to the depletion process, the electric field extends deeply (almost the entire length) into the region. This electric field helps in speeding up of the transport of charge carriers from P to N region, which results in faster operation of the diode, making it a suitable device for high frequency operations.

Characteristics A PIN diode obeys the standard diode equation for low frequency signals. At higher frequencies, the diode looks like an almost perfect (very linear, even for large signals) resistor. There is a lot of stored charge in the intrinsic region. At low frequencies, the charge can be removed and the diode turns off. At higher frequencies, there is not enough time to remove the charge, so the diode never turns off. The PIN diode has a poor reverse recovery time. The high-frequency resistance is inversely proportional to the DC bias current through the diode. A PIN diode, suitably biased, therefore acts as a variable resistor. This high-frequency resistance may vary over a wide range (from 0.1 ohm to 10 kΩ in some cases; the useful range is smaller, though). The wide intrinsic region also means the diode will have a low capacitance when reverse biased. In a PIN diode, the depletion region exists almost completely within the intrinsic region. This depletion region is much larger than in a PN diode, and almost constant-size, independent of the reverse bias applied to the diode. This increases the volume where electron-hole pairs can be generated by an incident photon. Some photo detector devices, such as PIN photodiodes and phototransistors (in which the base-collector junction is a PIN diode), use a PIN junction in their construction. The diode design has some design tradeoffs. Increasing the dimensions of the intrinsic region (and its stored charge) allows the diode to look like a resistor at lower frequencies. It adversely

affects the time needed to turn off the diode and its shunt capacitance. PIN diodes will be tailored for a particular use.

Photodetector and photovoltaic cell The PIN photodiode was invented by Jun-ichi Nishizawa and his colleagues in 1950. PIN photodiodes are used in fibre optic network cards and switches. As a photodetector, the PIN diode is reverse biased. Under reverse bias, the diode ordinarily does not conduct (save a small dark current or I leakage). When a photon of sufficient energy enters the depletion region s of the diode, it creates an electron, holepair. The reverse bias field sweeps the carriers out of the region creating a current. Some detectors can use avalanche multiplication. The same mechanism applies to the PIN structure, or p-i-n junction, of a solar cell. In this case, the advantage of using a PIN structure over conventional semiconductor p–n junction is the better long wavelength response of the former. In case of long wavelength irradiation, photons penetrate deep into the cell. But only those electron-hole pairs generated in and near the depletion region contribute to current generation. The depletion region of a PIN structure extends across the intrinsic region, deep into the device. This wider depletion width enables electron-hole pair generation deep within the device. This increases the quantum efficiency of the cell. Commercially available PIN photodiodes have quantum efficiencies above 80-90% in the telecom wavelength range (~1500 nm), and are typically made of germanium or InGaAs. They feature fast response times (higher than their p-n counterparts), running into several tens of gigahertz making them ideal for high speed optical telecommunication applications. Similary, silicon p-i-n photodiode have even higher quantum efficiencies, but can only detect wavelengths below the bandgap of silicon, i.e. ~1100 nm. Typically, amorphous silicon thin-film cells use PIN structures. On the other hand, CdTe cells use NIP structure, a variation of the PIN structure. In a NIP structure, an intrinsic CdTe layer is sandwiched by n-doped CdS and p-doped ZnTe. The photons are incident on the n-doped layer unlike a PIN diode. A PIN photodiode can also detect X-ray and gamma ray photons

What Is A PIN Diode: 

Conventional PN junction diodes such as the 1N4005 diode often used in power supplies consist of a region of positively doped (P) and negatively doped (N) silicon bonded together as shown below:

Structure of a PN Diode A PIN diode is a junction diode with a region of undoped (intrinsic=I) silicon layered between the conventional positively doped (P) and negatively doped (N) regions, as can be seen in the diagram below:

Structure of a PIN Diode The addition of the intrinsic region greatly alters the behavior of the diode at radio frequencies by lowering the capacitance between the P and N regions and raising the time it takes for the diode to switch between the conducting and non-conducting modes. How is the behavior of a PIN diode different from that of a conventional PN diode?  At lower frequencies, such as power line frequencies and audio frequencies, a PIN

diode behaves the same as regular diode. It passes current in only one direction. However, at radio frequencies the PIN diode acts very differently from a conventional diode. At radio frequencies, a PIN diode acts as either a small valued capacitor or as a variable resistor, depending on the DC bias applied to the diode. If the diode is reverse biased at DC, at RF it behaves as a very small value capacitor with a capacitance of approximately 1 pf. Though RF can get through a 1 pf capacitor, the amount that gets through is quite small. If the leakage through 1 pf is a problem, several diodes can be strung in series, reducing the capacitance and thus the residual leakage. Thus, for RF, the PIN diode acts as an open switch if it is reverse biased with DC.Even though the alternating RF current superimposed on the DC may occasionally forward bias the diode, the slow response time caused by the extra "I" layer prevents it from turning on. If the PIN diode is forward biased at DC, for RF it behaves like a resistor. The RF resistance is inversely proportional to the forward DC bias current. The resistance might be as high at 10,000 ohms for very small forward currents, and as low as 0.1 ohm if the forward DC bias current is 100 mA. Each type of diode has a different resistance vs. forward bias characteristic. If the forward current through the diode is near the maximum the diode is designed for, it typically has an RF resistance of only 0.1 ohm, and thus acts as a closed switch. Thus, for RF, the PIN diode acts as a closed switch if it is forward biased with the near maximum DC bias current. Even though the alternating RF current superimposed on the DC may occasionally reverse bias the diode, the slow response time caused by the extra "I" layer prevents it from turning off. If we can apply both DC and RF to a PIN diode at the same time, we can use the PIN diode as an RF switch. Reverse biasing the diode at DC turns the switch off, and forward biasing the diode at DC turns on the switch. The resulting "solid state switch" has a switching time much faster than any mechanical switch or relay. How do you apply both DC and RF to the diode at the same time?  This isn't at hard as it might seem. Since capacitors block DC while passing RF, and RF chokes block RF while passing DC, a combination of the two ought to do the trick. Take a look at the diagram below, which is a schematic of a simple single pole/single throw RF switch using a PIN diode:

Applying both RF and DC Bias at the Same Time As can be seen in the diagram, the RF is fed into and out of the diode through 0.01uf blocking capacitors. These allow the RF to pass through to the diode while preventing the DC bias from getting into the RF circuits. Meanwhile, the DC bias is applied to the diode through a pair of 2.5mH RF chokes. These allow the DC to reach the diode while preventing any RF from getting into the bias circuit. In the diagram above, if about 200V DC is connected to the DC bias terminals (negative connection going to the top) the diode will be reversed biased and the switch will be shut off. Very little current is needed from the 200V supply, since the diode is reverse biased. If about 50mA from a much lower voltage supply is allowed to flow through the diode in the forward direction (positive connection going to the top), then the diode will be forward biased and the switch will be turned on. By the way, don't let the diode arrow mislead you. As far as the RF is concerned, it isn't a diode, but a switch/resistor, and the RF is perfectly happy to go through the diode either way! The switch above can only handle low power and is designed to be used as a switch in a receiving or QRP circuit. What if I need better isolation when the switch is off? 

If the leakage through one diode when the switch is turned off is too much, then the first thing you must do is to be sure that you are applying the largest reverse bias possible. Raising the reverse bias lowers the capacitance of the diode, reducing the leakage. If raising the bias to the maximum doesn't provide enough isolation, then you need to use more than one diode in series, as shown below:

Putting Diodes in Series to Provide Better Isolation This is the same circuit as before, but two diodes are now in series, with a 1Mohm equalizing resistor connected across each to make sure that the reverse bias is split equally between the diodes. Note that you must now double the reverse bias voltage so that each diode receives the maximum it can withstand. The effective capacitance of the two diodes in series is half the capacitance of a single diode. You can use as many diodes as needed. The Ameritron QSK-5 uses a total of eight diodes in series (in a slightly different configuration) in the receive switch to get enough isolation between the input and output of the amplifier. What if I need more power handling capability?  If the switch needs to handle more power, one option is to use a larger PIN diode. However, there is practical limit to diode size, and if the power cannot be handled by a single diode, then it is possible to place diodes in parallel to handle higher power. Note that the DC bias to the diodes should be applied in series to assure that the forward bias is the same in each. This also reduces the current demand on the bias

power supply. However, the diodes must be connected in parallel for RF. See the diagram below:

Putting Diodes in Parallel for RF but in Series for DC to Increase Power Handling In the circuit above, which is used in the Ameritron QSK-5, the forward DC bias current flows from top to bottom in series through the diodes. This guarantees that the same bias current is flowing through each diode so that the RF resistance is the same for each. The diodes are, however, connected in parallel for RF, and the values of the coupling capacitors are chosen so that the RF current is split equally among the four diodes. Recall that RF doesn't care which direction it flows through the diodes, since at RF the diodes act like switches/resistors. In this case the RF resistance of each diode is roughly 0.1ohm to 0.2ohm, so that the effective parallel RF resistance is 0.025ohm to 0.05ohm, which is essentially a short circuit. When the diodes are reverse biased, the 1Mohm equalizing resisters assure that the reverse bias is split equally between all of the diodes. In the circuit above the reverse bias should be four times that needed by a single diode to insure that the diodes are fully cut off. The RF chokes used in this circuit are 100uH instead of 2.5mH, as used in the other switches shown above. This is because the 2.5mH chokes can't handle the larger

forward bias current of the larger diodes. To make sure that no residual RF gets through the chokes into the bias supply, the end of each choke is bypassed to ground with a 0.01uf capacitor.

PIN DIODES Principle of operation A photodiode is a p–n junction or PIN structure. When a photon of sufficient energy strikes the diode, it creates an electron-hole pair. This mechanism is also known as the inner photoelectric effect. If the absorption occurs in the junction's depletion region, or one diffusion length away from it, these carriers are swept from the junction by the built-in electric field of the depletion region. Thus holes move toward the anode, and electrons toward the cathode, and a photocurrent is produced. The total current through the photodiode is the sum of the dark current (current that is generated in the absence of light) and the photocurrent, so the dark current must be minimized to maximize the sensitivity of the device

When the N-type semiconductor and P-type semiconductor materials are first joined together a very large density gradient exists between both sides of the PN junction. The result is that some of the free electrons from the donor impurity atoms begin to migrate across this newly formed junction to fill up the holes in the P-type material producing negative ions. However, because the electrons have moved across the PN junction from the N-type silicon to the P-type silicon, they leave behind positively charged donor ions   ( N  D ) on the negative side and now the holes from the acceptor impurity migrate across the junction in the opposite direction into the region where there are large numbers of free electrons. As a result, the charge density of the P-type along the junction is filled with negatively charged acceptor ions   ( N  A ), and the charge density of the N-type along the junction becomes positive. This charge transfer of electrons and holes across the PN junction is known as diffusion. The width of these P and N layers depends on how heavily each side is doped with acceptor density NA, and donor density ND, respectively. This process continues back and forth until the number of electrons which have crossed the junction have a large enough electrical charge to repel or prevent any more charge carriers from crossing over the junction. Eventually a state of equilibrium (electrically neutral situation) will occur producing a “potential barrier” zone around the area of the junction as the donor atoms repel the holes and the acceptor atoms repel the electrons. Since no free charge carriers can rest in a position where there is a potential barrier, the regions on either sides of the junction now become completely depleted of any more free carriers in comparison to the N and P type materials further away from the junction. This area around the PN Junction is now called the Depletion Layer.

The PN junction

  The total charge on each side of a PN Junction must be equal and opposite to maintain a neutral charge condition around the junction. If the depletion layer region has a distance D, it therefore must therefore penetrate into the silicon by a distance of Dp for the positive side, and a distance ofDn for the negative side giving a relationship between the two of   Dp.N   Dn.N  in order to maintain charge neutrality also A = D  called equilibrium.

PN Junction Distance

  As the N-type material has lost electrons and the P-type has lost holes, the N-type material has become positive with respect to the P-type. Then the presence of impurity ions on both sides of the junction cause

an electric field to be established across this region with the N-side at a positive voltage relative to the Pside. The problem now is that a free charge requires some extra energy to overcome the barrier that now exists for it to be able to cross the depletion region junction. This electric field created by the diffusion process has created a “built-in potential difference” across the junction with an open-circuit (zero bias) potential of:

  Where: E  the zero bias junction voltage, V  thermal voltage of 26mV o is T the temperature,N  and   N   are the impurity concentrations and   n   is the intrinsic concentration. D A i

at

room

A suitable positive voltage (forward bias) applied between the two ends of the PN junction can supply the free electrons and holes with the extra energy. The external voltage required to overcome this potential barrier that now exists is very much dependent upon the type of semiconductor material used and its actual temperature. Typically at room temperature the voltage across the depletion layer for silicon is about 0.6 – 0.7 volts and for germanium is about 0.3 – 0.35 volts. This potential barrier will always exist even if the device is not connected to any external power source, as seen in diodes. The significance of this built-in potential across the junction, is that it opposes both the flow of holes and electrons across the junction and is why it is called the potential barrier. In practice, a PN junction is formed within a single crystal of material rather than just simply joining or fusing together two separate pieces. The result of this process is that the PN junction has rectifying current–voltage (IV or I–V) characteristics. Electrical contacts are fused onto either side of the semiconductor to enable an electrical connection to be made to an external circuit. The resulting electronic device that has been made is commonly called a PN junction Diode or simply Signal Diode. Then we have seen here that a PN junction can be made by joining or diffusing together differently doped semiconductor materials to produce an electronic device called a diode which can be used as the basic semiconductor structure of rectifiers, all types of transistors, LED’s, solar cells, and many more such solid state devices. In the next tutorial about the PN junction, we will look at one of the most interesting applications of the PN junction is its use in circuits as a diode. By adding connections to each end of the P-type and the Ntype materials we can produce a two terminal device called a PN Junction Diode which can be biased by an external voltage to either block or allow the flow of current through it.

PN THEORY

If a block of P-type semiconductor is placed in contact with a block of N-type semiconductor in Figure below(a), the result is of no value. We have two conductive blocks in contact with each other, showing no unique properties. The problem is two separate and distinct crystal bodies. The number of electrons is balanced by the number of protons in both blocks. Thus, neither block has any net charge. However, a single semiconductor crystal manufactured with P-type material at one end and N-type material at the other in Figure below (b) has some unique properties. The P-type material has positive majority charge carriers, holes, which are free to move about the crystal lattice. The N-type material has mobile negative majority carriers, electrons. Near the junction, the N-type material electrons diffuse across the junction, combining with holes in P-type material. The region of the P-type material near the junction takes on a net negative charge because of the electrons attracted. Since electrons departed the N-type region, it takes on a localized positive charge. The thin layer of the crystal lattice between these charges has been depleted of majority carriers, thus, is known as the depletion region. It becomes nonconductive intrinsic semiconductor material. In effect, we have nearly an insulator separating the conductive P and N doped regions.

(a) Blocks of P and N semiconductor in contact have no exploitable properties. (b) Single crystal doped with P and N type impurities develops a potential barrier. This separation of charges at the PN junction constitutes a potential barrier. This potential barrier must be overcome by an external voltage source to make the junction conduct. The formation of the junction and potential barrier happens during the

manufacturing process. The magnitude of the potential barrier is a function of the materials used in manufacturing. Silicon PN junctions have a higher potential barrier than germanium junctions. In Figure below(a) the battery is arranged so that the negative terminal supplies electrons to the N-type material. These electrons diffuse toward the junction. The positive terminal removes electrons from the P-type semiconductor, creating holes that diffuse toward the junction. If the battery voltage is great enough to overcome the junction potential (0.6V in Si), the N-type electrons and P-holes combine annihilating each other. This frees up space within the lattice for more carriers to flow toward the junction. Thus, currents of N-type and P-type majority carriers flow toward the junction. The recombination at the junction allows a battery current to flow through the PN junction diode. Such a junction is said to be forward biased.

(a) Forward battery bias repels carriers toward junction, where recombination results in battery current. (b) Reverse battery bias attracts carriers toward battery terminals, away from junction. Depletion region thickness increases. No sustained battery current flows. If the battery polarity is reversed as in Figure above(b) majority carriers are attracted away from the junction toward the battery terminals. The positive battery terminal attracts N-type majority carriers, electrons, away from the junction. The negative terminal attracts P-type majority carriers, holes, away from the junction. This increases the thickness of the nonconducting depletion region. There is no recombination of majority carriers; thus, no conduction. This arrangement of battery polarity is called reverse bias.

The diode schematic symbol is illustrated in Figure below(b) corresponding to the doped semiconductor bar at (a). The diode is a unidirectional device. Electron current only flows in one direction, against the arrow, corresponding to forward bias. The cathode, bar, of the diode symbol corresponds to N-type semiconductor. The anode, arrow, corresponds to the P-type semiconductor. To remember this relationship, Notpointing (bar) on the symbol corresponds to N-type semiconductor. Pointing (arrow) corresponds to P-type.

(a) Forward biased PN junction, (b) Corresponding diode schematic symbol (c) Silicon Diode I vs V characteristic curve. If a diode is forward biased as in Figure above(a), current will increase slightly as voltage is increased from 0 V. In the case of a silicon diode a measurable current flows when the voltage approaches 0.6 V in Figureabove(c). As the voltage increases past 0.6 V, current increases considerably after the knee. Increasing the voltage well beyond 0.7 V may result in high enough current to destroy the diode. The forward voltage, VF, is a characteristic of the semiconductor: 0.6 to 0.7 V for silicon, 0.2 V for germanium, a few volts for Light Emitting Diodes (LED). The forward current ranges from a few mA for point contact diodes to 100 mA for small signal diodes to tens or thousands of amperes for power diodes. If the diode is reverse biased, only the leakage current of the intrinsic semiconductor flows. This is plotted to the left of the origin in Figure above(c). This current will only

be as high as 1 µA for the most extreme conditions for silicon small signal diodes. This current does not increase appreciably with increasing reverse bias until the diode breaks down. At breakdown, the current increases so greatly that the diode will be destroyed unless a high series resistance limits current. We normally select a diode with a higher reverse voltage rating than any applied voltage to prevent this. Silicon diodes are typically available with reverse break down ratings of 50, 100, 200, 400, 800 V and higher. It is possible to fabricate diodes with a lower rating of a few volts for use as voltage standards. We previously mentioned that the reverse leakage current of under a µA for silicon diodes was due to conduction of the intrinsic semiconductor. This is the leakage that can be explained by theory. Thermal energy produces few electron hole pairs, which conduct leakage current until recombination. In actual practice this predictable current is only part of the leakage current. Much of the leakage current is due to surface conduction, related to the lack of cleanliness of the semiconductor surface. Both leakage currents increase with increasing temperature, approaching a µA for small silicon diodes. For germanium, the leakage current is orders of magnitude higher. Since germanium semiconductors are rarely used today, this is not a problem in practice. 

REVIEW:

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PN junctions are fabricated from a monocrystalline piece of semiconductor with both a P-type and N-type region in proximity at a junction.

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The transfer of electrons from the N side of the junction to holes annihilated on the P side of the junction produces a barrier voltage. This is 0.6 to 0.7 V in silicon, and varies with other semiconductors.

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A forward biased PN junction conducts a current once the barrier voltage is overcome. The external applied potential forces majority carriers toward the junction where recombination takes place, allowing current flow.

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A reverse biased PN junction conducts almost no current. The applied reverse bias attracts majority carriers away from the junction. This increases the thickness of the nonconducting depletion region.

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Reverse biased PN junctions show a temperature dependent reverse leakage current. This is less than a µA in small silicon diodes.