As the devices (i.e. DUT1~4) were not failed, more stringent stress was further used to investigate the device failure mechanism. The DUT5 was stressed at V _GS=6 V,V _DD=70 V with a larger SC pulse duration ofT _SC=20 μs. DUT5 failed within the16^th SC cycle with the SC waveform as shown in Fig. 7. It can be seen that the SC current I _Dexhibits an abrupt increase at 20.5 μs. The failed device behaves as the typical electrical shorting behavior between the drain and gate terminal. The failed device is then decapsulated and drain electrode burnout was observed in Fig. 8 (a). Since the SC current before the device failure is 17.4 A that is even lower than that as shown in Fig. 2 (b), the device failure is prone to be induced by the substantially high SC energy and its accumulation effect during the repetitive SC events (e.g. the1^st ~15^thSC cycles) prior to eventual device failure.

We compare the SC energy (E _SC) of a single SC pulse of DUT-1~5 as illustrated in Fig. 8 (b). It is worth noticing that E _SC increases with the enhanced SC stress. Besides, the E _SC of DUT1-4 are much lower compared with DUT-5 attributes to the longerT _SC. More importantly, the much higher SC energy can cause a higher local temperature close to the drain electrode [3], [4]. The local temperature fluctuates between on-state and off-state in the repetitive SC cycles, generating cyclic mechanical stress of thermal expansion and contraction phases. Together with the repetitive SC events, the mechanical stress leads to accumulative degradation at drain electrodes during the 1^stto 15^th SC cycles. After that, in the16^th SC cycle, the accumulative degradation exceeds to a critical level and leads to fatigue failure and then the subsequent gate-to-drain short circuit with abrupt increase in SCI _D as observed in Fig. 7(c). Consequently, the substantially high current results in the catastrophic thermal burned up of the two drain electrodes as shown in Fig. 8 (a).

Conclusion: The SC capability and device failure mechanism of the 100 V p-GaN gate HEMTs were investigated by the repetitive SC stress. In the repetitive SC stress with 150 SC cycles atV _GS=6 V,V _DD=40~70 V andT _SC=10 μs, the device exhibits strong repetitive SC capability without device failure. However, significant positive shift in V _th stems from electron trapping in the p-GaN/AlGaN gate stack is observed prior to the device failure. Accordingly, the electron-filled traps in turn to result in a suppressed gate leakage and off-state drain leakage current in the device after SC stress. By further enhancing the SC stress condition with a larger SC duration of 20 μs, the device can withstand ~15 cycles at V _GS=6 V and V _DD=70 V. The resultant significantly higher SC energy leads to thermal fatigue crack formation at the drain electrode due to the local temperature fluctuations, which results in the consequent thermal burnout.