Physical Understanding on Short-Circuit Failure for Cascode GaN HEMTs

Short-circuit (SC) robustness is an important concern for power devices in switching mode power supplies. For individual transistors such as Si and SiC MOSFETs, the SC capability has been widely reported. However, for the cascode gallium nitride high electron mobility transistor (GaN HEMT), which consists of a Si MOSFET and a depletion-mode GaN HEMT (DHEMT), the inside mechanisms of SC have not been fully investigated yet. In this article, by setting two individual devices, both experiments and numerical simulations are performed to reveal the electrothermal failure mechanisms of cascode GaN HEMTs during SC operation. The electrical characteristics for the Si MOSFET and the DHEMT are separately extracted. It is confirmed that the DHEMT withstands a much higher electrothermal stress than the Si MOSFET during SC operation. Thermal failures tend to occur in the DHEMT. Furthermore, thermal and mechanical simulations are deployed to analyze the failure mechanism within DHEMT structure. The thermal-induced mechanical stress due to different thermal expansion rates of heterojunction layers is the source of failure. Finally, an analysis is conducted on the unique gate control mechanism that limits the SC capability of the cascode configuration. A design guidance is proposed for trade-off relationship between SC robustness and device on-resistance in cascode GaN HEMT.