Two-stage threshold voltage shift under semi-on DC stress in power p-GaN HEMTs

Hard-switching operation is a critical reliability concern for p-GaN high-electron-mobility transistors (HEMTs), yet degradation driven by switching transients remains insufficiently clarified. In this work, a semi-on stress, defined at the bias corresponding to the maximum switching power loss, was introduced to decouple switching-transient-induced degradation from quasi-static on-/off-state stress. Under semi-on stress, devices initially exhibit a positive threshold-voltage (Vth) shift, reduced on-state current (Ion), and suppressed gate current (Ig). Combined with a reversal of transfer characteristics hysteresis polarity after stress, these signatures indicate electron trapping in the AlGaN barrier that increases the effective injection barrier and alters the dominant gate-leakage path. In parallel, reduced off-state leakage suggests additional electron trapping in the GaN layer. Importantly, these short-term signatures vanish under source-floating semi-on stress and weaken at elevated temperature, supporting hot-carrier injection from the channel as the primary driver of early degradation. At longer stress times, however, a gradual negative shift in Vth emerges, indicating a different degradation pathway. Under high-temperature semi-on stress, a post-stress Ig increase and a negative Vth shift are observed even with a floating source, evidencing thermal-assisted hole generation and trapping in the AlGaN barrier. Comparing source-grounded and source-floating semi-on stress at high temperature, the larger negative Vth shift under source-grounded conditions further points to hole generation enhanced by self-heating during high-current operation. These results show that long-term semi-on stress is governed not only by hot-electron injection but also by thermal-assisted hole generation processes, providing a physics-based framework for assessing p-GaN HEMT reliability under realistic hard-switching operation.