Wide and narrow bandgap semiconductors for power electronics: A new valuation

Journal of Electronic Materials, Jun 2003 by Hudgins, Jerry L

OPTIMAL SEMICONDUCTOR MATERIALS FOR LOW-VOLTAGE DEVICES

Many power-semiconductor devices are used in low-voltage applications, from 42-V automotive systems to power supplies operating digital-processor integrated circuits (ICs) at and below 1.3 V. These applications can push the physical limits of devices fabricated using Si. All applications requiring devices other than n-channel metal-oxide semiconductor field-effect transistors (includes synchronous rectifiers) will have at least one pn-junction drop. For low-voltage IC power, this becomes a significant limiting factor in using silicon. Figure 5 shows the built-in junction potential for several semiconductors. The materials with voltage drops less than Si are GaSb, Ge, InN, InAs, and InSb. The use of Ge or GaSb provides a good alternative, as each of them has electron and hole mobilities larger than Si. Low-voltage bipolar devices, such as insulated-gate bipolar transistors and diodes, can be contemplated in these materials.

A graph of the electron-to-hole mobility ratios for the semiconductors of Table I is given in Fig. 6. Very few semiconductors have a ratio below three and also have an electron mobility above 1,000 cm^sup 2^/V [middot] s. As can be seen from the plot in Fig. 6, only Ge and diamond meet these conditions. The near-unity ratio of diamond makes it ideal for CMOS circuit designs, particularly in operating environments of elevated temperatures. Where low-voltage applications have junction voltage drops as a critical limiting factor, Ge is again the material of choice. If NMOS circuit design is used, then an ultrahigh mobility material, such as InAs, could be useful, though the small bandgap energy would require operation in well-controlled temperature environments.

CONCLUSIONS

The GaN, SiC (4H), and C (diamond) are the best semiconductor material systems in which to create future power-electronic devices. Specifically, it has been shown that the traditionally used relationship between energy bandgap and critical-electric field is incorrect and underestimates the gains to be had by using wide bandgap semiconductors for high-voltage devices. A comparison of the revised equations for ronsp was given. In light of the maturity of the fabrication technology and thermomechanical-material properties, GaN appears to be the best choice, overall, for the next decade of development. The GaN/AlN system (AlGaN ternary) has a good CTE match to package materials, the second highest bandgap (compared to diamond), and can be used to create heterojunction devices that give effective electron-mobility values above 2,000 cm^sup 2^/V [middot] s.

For low-voltage applications, where junction voltage drops are critical or CMOS circuits are to be used, it has been argued that Ge is overall the best material. The GaSb is also worthy of consideration, though this material system suffers from the lack of material growth and processing infrastructure that is in place for Ge. Finally, it has been shown that high-voltage unipolar and bipolar power-electronic devices, low-voltage CMOS, and possibly high-voltage CMOS ICs can gain greatly in performance by being fabricated using C (diamond). The near match and high values of carrier mobilities as well as the large bandgap and high thermal conductivity make diamond the ideal material for electronic devices of all power levels and types.