Unipolar Device in SiC: Diodes and MOSFETs

Summary

Unipolar power devices in 4H‐SiC, which include JBS diodes and power MOSFETs, have been reviewed in this chapter. In 4H‐SiC JBS diodes, the focus has been on improving the surge current capability and reduction in conduction losses. Improvement in surge current capability was achieved by utilizing minority carrier injection from p⁺ grid and conductivity modulation of the drift layer. Reduction in conduction losses was achieved by using more aggressively doped drift layers, enabled by enhanced shielding of the Schottky barriers using trench structures. Further reduction of forward voltage drop and conduction losses was achieved by using Schottky barrier metal with lower barrier height, which reduced the threshold voltage or knee voltage of the diode. 4H‐SiC DMOSFETs with voltage ratings ranging from 900 V to 15 kV have been developed. Various surface passivation processes resulting in significantly higher MOS channel mobility values over conventional NO or N₂O anneals were reported. However, further improvements in reliability and stability were desired. High‐performance 4H‐SiC trench MOSFETs were also developed. Devices using trench bottom p‐implants, double‐trench protection, and asymmetric implant protection produced very good results. Advanced power MOSFET structures, including superjunction MOSFETs and MOSFETs with integrated JBS diodes, were successfully demonstrated. Superjunction MOSFETs with lower blocking voltage (around 1.2 kV) used multi‐epi growth approach, and higher voltage devices used trench etch and epitaxial fill approach. It was observed that the 4H‐SiC superjunction MOSFETs have much lower rate of increase in on‐resistance at elevated temperatures than conventional structure. 4H‐SiC power MOSFETs with integrated JBS diodes provided a very effective way of bypassing the built‐in pn junction diode, suppressing the growth of stacking faults and minimizing reverse recovery of the body diode.