Positive VTH Shift in Schottky p-GaN Gate Power HEMTs: Dependence on Temperature, Bias and Gate Leakage

In this article, we present an extensive analysis of the positive threshold voltage instability in Schottky p-GaN gate enhancement-mode devices, investigated by a custom setup allowing an extended observation window, from the microsecond to hundreds of seconds. We show that a matrix of experiments can be specifically designed to investigate the voltage, temperature and leakage dependence of the threshold voltage instability induced by a positive gate bias, and to identify them. The original results indicate that the observed positive threshold voltage shift can be ascribed to the trapping of electrons at defects located in the AlGaN barrier. Remarkably, the trapping rate is strongly dependent on temperature at low bias, while it is not temperature-dependent at high bias, indicating the existence of both temperature and leakage-assisted trapping processes. This result was confirmed by investigating the correlation between dc leakage measurements and the time constant of threshold voltage transients. On the other hand, the recovery process is found to be thermally activated, with an activation energy of 0.26 eV: the trapped electrons are thermally emitted into the conduction band and are pushed toward the channel by the intrinsic electric field.