Unipolar and Bipolar Pulsed Gate Stresses and Threshold Voltage Shifts in GaN e-HEMTs

Abstract

In GaN e-HEMTs, Threshold Voltage (VTH) shift from gate voltage (VGS) stress depends on the VGS magnitude, stress time, recovery time (time between stress removal and VTH measurement), temperature and pulse polarity (0 to + VGS or -VGS to + VGS). In this paper, unipolar (0 to VGS) and bipolar (-VGS to + VGS) pulsed gate stresses have been performed on GaN e-HEMTs with different VGS magnitudes. The pulse frequency ranges from 25 Hz to 200 kHz. The results show negative VTH shifts for unipolar VGS pulses (between 5V and 8V) at long recovery times (above 500 seconds). The VTH shift is proportional to the VGS pulse magnitude. When the recovery time is increased from 50 milliseconds to 500 seconds, the measured VTH shift becomes more negative, indicating a faster release of trapped electrons than holes. In bipolar stresses, results show both positive and negative VTH shifts with no clear magnitude or frequency dependencies.