Novel characterization techniques for the study of the dynamic behavior of silicon carbide power MOSFETs

Abstract

This dissertation provides insight into the dynamic behavior of SiC power MOSFETs from their inherent static IV and CV characteristics. While conventional dynamic measurements extracted from a DPT or a similar dynamic test-bench yield accurate quantitative data, the static IV and CV characteristics of a power semiconductor device offer more qualitative information to delve into the root mechanisms responsible for its dynamic behavior. Conventional characterization techniques are limited to power levels way below those which the power device withstands in the application. As a result, the static IV and CV characteristics attained by available measurement solutions are reduced to a limited scope of bias conditions insufficient to infer information about the dynamic behavior of the power device. This work tackles this gap and proposes novel measurement techniques that enable the characterization of the static IV and CV characteristics of SiC power MOSFETs at the full range of bias conditions the power device goes through in the application. Iso-thermal IV characteristics of a commercially available SiC power MOSFET are measured up to 40 kW power (instantaneous 50 A and 800 V) at junction temperatures ranging from 25°C to 175 °C. The CV characteristics are mapped at drain-source and gate-source bias combinations of VDS = 0 - 40 V and VGS = 0 - 20 V, respectively, at junction temperatures ranging from 25°C to 150 °C. The results of these measurements reveal unique insights into the electrical characteristics of SiC power MOSFETs which impact their performance in the application and explain unclear phenomena observed in their dynamic behavior. On the one hand, the intrinsic capacitances of the SiC power MOSFET extend their non-linearity, function of both VGS and VDS, to the saturation region of the power device. Moreover, they are also affected by the junction temperature of the power device. The impact of these in the voltage commutation speed of the device under different switching conditions is thoroughly analyzed in the thesis. On the other hand, the IV characteristics of the SiC power MOSFET reveal the existence of short channel effects that drastically affect the transconductance of the power device in its high voltage saturation region. Furthermore, the measurements show a positive temperature coefficient of the drain current in the high voltage saturation region of the SiC power device, attributed to the density of trap energy states in the SiC/SiO2 interface. These effects effectively lower the plateau voltage of the device and lead to faster current commutation speeds in the application than those expected from the datasheet values. The insights revealed by the proposed characterization techniques are intended to help fine-tune semiconductor technology processes and improve the accuracy of simulation models to achieve a higher grade of optimization in the design of future SiC-based energy conversion circuits.