Due to the excellent the higher critical electric field strength and rapid heat extraction of SiC,  SiC gate turn-OFF (GTO) thyristors are considered as an advanced solution for increasing the power density and efficiency of medium voltage industrial drives and megawatt industrial loads. In such applications, the SiC GTOs are exposed to the repetitive discharging stress (including the high current, large power loss and the related thermal fatigue), and therefore may lead to the long-term degradation. However, investigations on the long-term degradation and mechanisms under repetitive discharging stress are rarely reported, which is the motivation of the present work.

    In this work, the degradations of electrical parameters for SiC GTOs under cyclic discharge stress are investigated comprehensively. The devices were repetitively stressed with a 5.0 kA sine wave pulse of ~40 μs duration, and the static characteristics were measured to record the degradations after the discharge tests. Interestingly, it demonstrates that the decrease of threshold gate current (I_Gth) to turn-on the devices and the increase of gate leakage current are the dominant degradation modes until the devices’ failure. Meanwhile, the on-state resistance presents no essential change after 6000 stress cycles, indicating the insignificant bipolar degradations. The capacitance-voltage characteristic of anode-gate (A-G) junction is positively shifted after discharge stress, which indicates the new generation of negative charge in the SiO₂ layer between anode and gate originating from the hot-carrier capture by the interface or oxide defects. The simulation based on the poisson's equation proves that the negative charge in the SiO₂ layer repels the electrons and forms the additional depletion region in the gate, which leads to the increase of on-state resistance of A-G junction and thus the decrease of I_Gth. Meanwhile, scanning electron microscope reveals that a conductive path through the A-G junction is formed with the accumulated deterioration during repetitive stress, and thus leads to the short-circuits of anode-gate and anode-cathode after failure.
   
    More details can be seen in the attachment.