AN OVERVIEW OF WIDE BANDGAP SEMICODUCTORS Rupesh Gupta

Abstract- Among all the semiconductor materials, the extremities of property values are almost always be defined by wide bandgap materials. An overview of wide bandgap semiconductor properties is presented here and contrasted with those of traditional Si and GaAs based semiconductors. This is followed by a brief discussion on the high temperature and radio frequency high power operations. GaN has been predicted as one of the best semiconductor materials for consideration in advanced power electronic devices. An analysis of diamond as a potential wide bandgap material for future use is presented. Finally we examine some state of the art devices and a few drawbacks limiting the widespread use of wide bandgap materials.

NOMENCLATURE ni N c, N v Eg Ecrit Nd

Intrinsic carrier concentration (cm-3) Effective density of states (cm-3) Bandgap of material (eV) Crirtical electric field (N/C) Impurity concentration (cm-3)

Vikram Gupta

1. INTRODUCTION The last few decades have witnessed an increasing proliferation of performance enhancing electronics. Extensive research in Si semiconductor technology has lead to a stage where theoretical limits of this technology have been reached. However Si has not been able to satisfy the requirements of certain utility applications involving high temperature or high power operations. Presently, number of units of Si or GaAs devices are combined together to form a high power module. This technique makes the module bulky and unreliable. In this situation the wide bandgap materials with their improved properties have proved to be a promising replacement to the conventional semiconductors. The wide bandgap semiconductor devices are currently limited in performance due to several physical effects associated with material design-related issues. Solutions to these problems are emerging and with research and development in the fabrication and design techniques it would be possible to fully exploit their capabilities in the near future. This literature review aims at the realization of the enormous potential of the wide bandgap materials and to comprehend their scope for the need of the hour.

TABLE1 Physical characteristics of Si and major wide bandgap semiconductors [1]

2. PROPERTIES OF SEMICONDUCTORS

WIDE

BANDGAP

Wide band gap materials exhibit finer electrical characteristics as compared to Si. Some important properties are presented in Table 1. These are bandgap, electric breakdown field, electron and hole mobility, thermal conductivity and saturated drift velocity. A quick glance reflects the superiority of these materials. These characteristics are briefly discussed here. The band gaps of wide bandgap semiconductors are about three times those of conventional semiconductors. The problem with silicon based semiconductors is that at temperatures above 150ºC, the thermally generated carrier concentration becomes very large leading to uncontrolled conduction. This makes them unsuitable for high temperature applications. The intrinsic temperature for wide bandgap materials is quite high like 900ºC for SiC. 𝐸

𝑛𝑖2 = 𝑁𝑐 𝑁𝑣 exp⁡ (− 𝑘𝑇𝑔 )

(1)

Wider band gap means a larger amount of energy is required to generate large number of electron hole pairs, thus breakdown electric field and breakdown voltage increase. The breakdown voltage of a PN junction is given by

𝑉𝑏 = 𝜀

2 𝐸𝑐𝑟𝑖𝑡 2 𝑞 𝑁𝑑

drift velocities characteristics.

gives

excellent

reverse

recovery

The last property which we discuss here is the thermal conductivity of wide bandgap materials. High thermal conductivity implies that heat generated in these devices can more easily be conducted to the ambient leading to a very slow increase in the device temperature. This is a critical property for high temperature power applications and discussed in more detail in the next section with reference to coefficient of thermal expansion values. 3. HIGH TEMPERATURE OPERATION OF WIDE BANDGAP SEMICONDUCTORS The motivation that underlies the use of wide bandgap semiconductors for high temperature applications is that they do not require a well equipped cooling mechanism to be integrated with the devices. This saves a lot of overhead cost and provides us with a gain in weight, size and count of devices that can be deployed. Also, the intrinsic carrier concentration of these devices does not shoot up for high temperatures as was discussed in section 2 and again presented here in Fig2 for reference. Another vital aspect for high temperature operations is discussed next.

(2)

It is clear from the above equation that with a high electric breakdown voltage, doping can be increased; thus, device can be made thinner and more compact at the same breakdown voltage levels. 600 Diamond Breakdown Voltage (Volts)

500 400 300 200 100

6H-SiC 4H-SiC Si

GaN

0

Fig1. Maximum breakdown voltage normalized to Si Another important characteristic visible from Table 1 is the high drift velocities of wide bandgap materials. Higher drift velocity allows charge in the depletion region of a diode to be removed faster leading to better high frequency switching capabilities. Thus these devices can operate at frequencies higher than 20 kHz which is not possible with Si-based devices. A combination of thinner devices along with large

Fig2. Semiconductor intrinsic carrier concentration versus temperature for Si, 6H-SiC and GaN [2] The wide bandgap materials have coefficient of thermal expansion (CTE) values very close to some of the ceramic materials used for packaging. It is desirable to maximize thermal conductivity of the semiconductor material and the associated package. The closer CTE match to electrically insulating ceramics makes them easily adaptable for wide temperature range applications as there is no thermomechanical mismatch. This luxury cannot be availed by Si based devices. The material parameters CTE and thermal conductivity for the available packaging materials are tabulated along with semiconductor materials in Table2.

TABLE 2 Material parameters at 300K Package Materials

CTE (ppm/K)

AlN substrate AlSiC substrate

4.5 7

Thermal Conductivity (W/m.K) 200 200

Metal Matrix Composites 6.5

170

Beryllia substrate Semiconductor Materials Si SiC GaN C (diamond)

6.1

280

2.6 5.1 6.2 0.8

130 700 110 600-2000

As can be seen from the table SiC has a larger thermal conductivity than GaN. However, this advantage is offset by the favourable properties of GaN like larger bandgap, maximum junction temperature of operation and critical electric field [3]. CTE of GaN is very close to that of AlSiC. As many GaN devices are grown on SiC substrates, the use of AlSiC/SiC in direct contact provides a good starting platform for device growth. Also, the chemical inertness of nitrides ensures high reliability. It is therefore determined that GaN is at present, one of the best semiconductor materials for consideration in advanced power electronic devices. GaN based semiconductors have enthused research interests in materials growth and optoelectronic and electronic devices using this semiconductor system. A main hindrance to the use of GaN is its unavailability as bulk crystals. 4. RADIO FREQUENCY APPLICATIONS

HIGH

POWER

Wide band gap semiconductors are proving to be a strong candidate for radio frequency high power microwave radars and communication transmitters. Low breakdown voltage of conventional semiconductors has limited the fabrication of high power transmitters. Using wide bang gap semiconductors output power density of the order of 10-12 W/mm in microwave frequency range has been achieved [4]. MESFETs fabricated from the 4H-SiC polytype and heterojunction field-effect transistors (HFETs) fabricated using the AlGaN/GaN heterojunction are seen to be very effective in high power applications in the microwave frequency range. However, the challenges with use of wide bandgap semiconductors for such applications are associated with material purity and design. As shown in fig.3, near the interfaces, in particular, the gate source region, the channel region under the drain edge of the gate electrode and the surface region near the drain side of the gate develop deep traps that cause problems in current flow. Secondly, electrostatic disturbances caused due to thermally assisted tunnelling of electrons from the gate metal to the surface of the semiconductor results in anomalies in channel current. Thirdly the high current injection conditions through the source contact results in an undesirable nonlinear source region resistance. In order to improve the high gate voltage operation of these devices field plates have proven to be a good solution.

Fig3. AlGaN/GaN HFET structure and sources of operational non linearities [5] Field plate consists of an additional metal layer located over the gate and drain region. The field plate attracts the electric field towards itself and thus reduces the magnitude of electric field near the gate, which allows high gate voltage to be applied without causing too much gate leakage current from the gate to the substrate region. With the increasing and effective use of field plates as well as advanced fabrication techniques, wide bandgap semiconductors will prove to be an integral part of high power microwave radars and communication transmitters. 5. SiC vs GaN SiC technology is the most extensively researched upon technology among the wide bandgap materials. This is primarily due to the fact that SiC can be used with the same oxide as Si i.e. SiO2 which is required for MOS devices. On the other hand there does not exist any native oxide of GaN and thus MOS fabrication has not been possible. Another problem which restricts the use of GaN is that pure GaN wafers are not available [6]. GaN wafers are grown on SiC which are thin and very expensive. However GaN diodes have shown better performance compared to SiC diodes in terms of reverse recovery current and switching losses due to slightly higher drift velocity and electron hole mobilities. 6. DIAMOND Theoretically diamond has the best properties among the available wide bandgap materials. It has the largest thermal conductivity and bandgap of any of the materials from Table1. Diamond also has the largest electron mobility. However, very high temperatures are required for its processing owing to its hardness. It is available only as thin films or polycrystalline layers. However, recent developments in chemical vapour deposition techniques have lead to the production of layers with low defect densities and low angle grain boundaries [7]. Fabrication of elementary single diamond devices has also been achieved. It is expected that diamond electronics will be employed primarily in computer aided manufacturing and process control environments. 7. WIDE BANDGAP SEMICONDUCTOR DEVICES The wide bandgap semiconductor based devices find extensive use in military applications, RF power transmitters, high power MMICs, LEDs and laser applications The feasibility SiC based 50 KW amplifier working in the UHF spectrum has been established [8].Wide bandgap based MESFET's having enhanced properties than their

conventional counterparts are being used in microwave power amplifier and oscillator applications. The leakage currents in SiC P-N junctions are much less than that in conventional semiconductors making them useful for improved non volatile random access memory (RAM) and charge-coupled devices. Wide bandgap semiconductors have the ability to emit radiation in the blue and UV portion of the optical spectrum which makes them useful for making lasers and LEDs. For instance, GaN has a bandgap of 3.44 eV at room temperature which corresponds to a wavelength in the near ultra violet (UV) region of the optical spectrum. Thus, it can be used to emit rays in the UV region. These emerging wide bandgap material based state-of-the-art devices will prove to be the best choice for futuristic low profile, high power and high temperature applications. 8. CONCLUSIONS Wide band gap materials have an immense scope for the futuristic devices as theoretically, the maximum potential of Si and GaAs have been exploited. They offer a wide range of advantages over Si. Some of these discussed in this overview are high breakdown voltage, high thermal conductivity, high temperature ranges of operation and lower switching losses. However certain limitations restrict their widespread use. Some of the unresolved issues are those of high cost, low processing yield due to availability of these materials only as thin films and need for high temperature packaging techniques that have not yet been developed. As the ratio of the electron to hole mobility values is much higher in these materials as compared to Si, the use of wide bandgap semiconductors for bipolar devices would not be desirable. Only diamond which has a near unity mobility ratio makes it ideal for bipolar device designs, particularly in high temperature operating environments. REFERENCES [1] Leon M. Tolbert, Burak Ozpineci, S. Kamrul Islam, Madhu Chinthavali, "Wide Bandgap Semiconductors for Utility Applications," IASTED International Conference on Power and Energy Systems (PES 2003), February 2426, 2003, pp. 317-321. [2] Neudeck, P.G.; Okojie, R.S.; Liang-Yu Chen, “Hightemperature electronics - a role for wide bandgap semiconductors?” Proceedings of the IEEE, Volume 90, Issue 6, June 2002 Page(s):1065 – 1076 [3] Hudgins, J.L.; Simin, G.S.; Santi, E.; Khan, M.A., “An assessment of wide bandgap semiconductors for power devices”, IEEE Transactions on Power Electronics, Volume 18, Issue 3, May 2003, Page(s):907 – 914 [4] R.J. Trew; “SiC and GaN transistors––is there one winner for microwave power applications”, Proc. IEEE 906 (2002), pp. 1032–1047 [5] Trew, R.J.; “Wide bandgap transistor amplifiers for improved performance microwave power and radar applications”, 15th International Conference on Microwaves, Radar and Wireless Communications, 2004. MIKON-2004. Volume 1, 17-19 May 2004 Page(s):18 23 Vol.1 [6] Burak Ozpineci, Leon M. Tolbert, S. Kamrul Islam, Madhu Chinthavali, “Comparison of Wide Bandgap Semiconductors for Power Applications," 10th European Conference on Power Electronics and Applications, September 2-4, 2003, Toulouse, France.

[7] Chalker, P.R., “Wide band-gap semiconductor: how good is diamond? “, IEE Colloquium on Diamond in Electronics and Optics, 4 Nov 1993 Page(s):1/1 - 1/3 [8] Yoder M.N., “Wide bandgap semiconductor materials and devices”, IEEE Transactions on Electron Devices, Volume 43, Issue 10, Oct. 1996 Page(s):1633 – 1636