Degradation Mechanism Analysis for SiC Gate Turn-Off Thyristor Under Repetitive Pulse Power Stress

SiC gate turn-OFF (GTO) thyristors are considered as an advanced solution to increase the power density and efficiency for pulsed power applications, where the SiC GTOs are exposed to the repetitive pulsed power stress, including the high current/bus voltage, large power loss and the resulted thermal stress. In this work, the long-term degradation and mechanisms under cyclic pulse stress are comprehensively investigated. The devices were repetitively stressed with a 5.0 kA sine wave pulse of ~40 μs duration, and the dynamic and static characteristics were recorded to monitor the degradations after the pulse stress. Interestingly, it demonstrates that the decrease of threshold gate current (<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Gth</sub>) to turn-on the devices and the increase of gate leakage current are the dominant degradation modes, while the pulse current and voltage signals keep constant until the failure. The <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">AG</sub>/<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">AG</sub> characteristics and deep-level transient spectroscopy (DLTS) measurements reveal that the enhanced electron trapping by the split interstitial carbon atoms at SiC/SiO2 interface localized between anode and gate is the main reason for the threshold gate current instability, which promotes the field-effect surface passivation. Meanwhile, Scanning Electron Microscope images show that the cyclic pulse stress finally results in the thermal runway and the formations of voids and cracks localized at the anode-gate boundary. Based on the IGth decrease and the negative shift of the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">AG</sub> waveform when the devices approaches the final failure, it is speculated that this burning-out behaviors might be resulted from the uneven turn-on behaviors of the different cells, where the prematurely triggered cells need to withstand much higher pulse currents.