Surge-energy robustness is essential for power devices in many applications such as automotive powertrains and electricity grids. While Si and SiC MOSFETs can dissipate surge energy via avalanche, the GaN high-electron-mobility transistor (HEMT) has no avalanche capability and withstands surge energy by its overvoltage capability. However, a comprehensive study into the surge-energy robustness of the cascode GaN HEMT, a composite device made of a GaN HEMT and a Si metal-oxide-semiconductor field-effect-transistor (MOSFET), is still lacking. This work fills this gap by investigating the failure and degradation of 650-V-rated cascode GaN HEMTs in single-event and repetitive unclamped inductive switching (UIS) tests. The cascode was found to withstand surge energy by the overvoltage capability of the GaN HEMT, accompanied by an avalanche in the Si MOSFET. In single-event UIS tests, the cascode failed in the GaN HEMT at a peak overvoltage of 1.4~1.7 kV, which is statistically lower than the device's static breakdown voltage (1.8~2.2 kV). In repetitive UIS tests, the device failure boundary was found to be frequency-dependent. At 100 kHz, the failure boundary (~1.3 kV) was even lower than the single-event UIS boundary. After 1 million cycles of 1.25-kV UIS stresses, devices showed significant but recoverable parametric shifts.

Physics-based device simulation and modeling were then performed to understand the circuit test results. The electron trapping in the buffer layer of the GaN HEMT can explain all the above failure and degradation behaviors in the GaN HEMT and the resulted change in its dynamic breakdown voltage. Moreover, the GaN buffer trapping is believed to be assisted by the Si MOSFET avalanche. An analytical model was also developed to extract the charges and losses produced in the Si avalanche in a UIS cycle. These results provide new insights into the surge-energy and overvoltage robustness of cascode GaN HEMTs.