Measurement Issues Affecting Threshold-Voltage Instability Characterization of SiC MOSFETs

Abstract

This work focuses on measurement issues that affect the accuracy of positive bias temperature instability measurements of SiC power MOSFETs using a conventional sweep technique to characterize the threshold voltage, VT. Threshold-voltage shifts occurring during stress are readily recoverable during the measurement, resulting in an underestimation in VT degradation. Recovery of VT due to gate-stress relaxation before or during the measurement was a major source of measurement error that was mitigated by performing an immediate sweep down in gate voltage, VGS, from the stress bias toward threshold. Allowing the gate bias to drop to zero just prior to measuring by sweeping VGS positively resulted in smaller observed degradation due to VT recovery. This result is important, especially in cases where the gate stress has to be completely removed before making any electrical measurements. The VT shift caused by bias stress can quickly recover and yield test results that underestimate the effect of the applied stress. A full recovery of VT was observed following a positive gate bias stress for conditions where the gate was either subject to a negative gate voltage for a few seconds, or when VGS was maintained at zero volts for several minutes.