Switching Transients Optimization and Investigations into Threshold Voltage Shift for SiC MOSFETs using a Current-Controlled Gate Driver

Abstract

Silicon Carbide Metal-Oxide Semiconductor Field Effect Transistors (SiC MOSFET) show superior properties over conventional silicon (Si) based MOSFETs: featuring higher voltage and temperature operation, low on-resistance and faster switching speed. These characteristics facilitate higher efficiency, power density and arguably better reliability. However, the mainstream commercial SiC MOSFETs are often fitted to the packages that are previously designed for Si-based devices, introducing significant oscillation during the turn-on and turnoff periods. Moreover, oxidation in the gate oxide introduces defects, resulting in a more fragile gate oxide compared to Si MOSFETs . Oxidation introduces the shift of the threshold voltage (Vth), which remains a concern for the consumers of SiC MOSFETs. This thesis investigates the causes of oscillations and causes of the oxidation and proposes methods to minimize their impact on the devices. A detailed analysis on switching transients is presented using a small signal model for every stage in voltage and current changes during the turn-on and turn-off periods. This analysis provides guidance for the design of optimized gate driving signals. The benefits of these gate driving signals are compared with conventional turnon and turn-off signals and results show that the proposed new signals provide lower voltage and current overshoots and lower losses. For the investigation into Vth shift, the phenomenon of Vth shift in short gate stress time and long gate stress time is discussed. The gate driver developed to limit oscillations is altered by embedding a new way in measuring Vth shift. This allows to receive feedback from the gate oxide degradation. In addition, the same measuring circuit can be used to measure the SiC chip temperature. Thus, the proposed technique can be also be used as a temperature sensitive electrical parameter (TSEP).