Study on Threshold Voltage Drift for SiC MOSFET Under Avalanche Stress

In this article, a dedicated testing system is developed to realize low-delay threshold voltage (VTH) characteristic testing of silicon carbide (SiC) MOSFET devices after an avalanche stress. The developed low-delay testing system enables VTH detection within milliseconds after the avalanche, facilitating the study of VTH drift behavior under different gate-source turn-off voltages (VGS-OFF) and repeated avalanche events. Experimental results of 1200 V commercial devices indicate that after a single avalanche stress, VTH drifts positively by about 0.11 V when VGS-OFF is 0 V. However, if the avalanche stress is monitored at a negative bias of VGS-OFF, VTH exhibits a negative drift. The drift increases as a more negative gate bias is applied. When VGS-OFF reaches 6 V, the VTH drift saturates at approximately −0.226 V. After repeated avalanche cycles, the threshold drift does not saturate until VGS-OFF is −10 V. Furthermore, verification shows that the VTH drift is minimized when VGS-OFF is −3 V. The absolute value of VTH drift shows a non-monotonic variation with avalanche cycles: it starts to increase with the number of avalanche cycles, reaching a peak at approximately 1000 cycles, and further increasing the avalanche cycles. The magnitude of the drift gradually decreases after reaching a peak. TCAD simulations reveal that this phenomenon could be attributed to the ionization of donor/acceptor traps at the SiC/SiO2 interface and the consequent modulation of channel hole concentration. After excitation by electric fields of varying intensities, the ionization of acceptor and donor traps undergoes differential changes, consequently leading to a non-monotonic drift in threshold voltage.