Trap Dynamics and Long-Term Reliability in Depletion-Mode GaN HEMTs Under Sequential Electrical Stress

The long-term electrical stability of GaN high-electron-mobility transistors (HEMTs) is strongly influenced by charge trapping under electrical stress, which can affect threshold voltage (VTH), on-resistance (RDS), and gate leakage current (IGS). In this work, the temporal evolution of these parameters is systematically investigated in depletion-mode GaN-on-SiC HEMTs subjected to prolonged sequential electrical stress. Three nominally identical devices from the same wafer were exposed to the same stress sequence, including negative gate biases from −5 to −9 V, positive gate-bias recovery, repeated negative gate stress, and drain-current stress up to 0.5 A under controlled temperature conditions. IGS was monitored continuously during stress, whereas VTH and RDS were periodically extracted using brief measurement interruptions. The study focuses on cumulative long-term parameter evolution rather than switching-cycle or fast dynamic RDS(on) behavior. Within the sequential negative-bias protocol, the −5 to −7 V stress segments exhibited stronger time-dependent evolution of VTH and RDS, whereas the subsequent −8 and −9 V segments showed reduced incremental drift. Because these conditions were applied sequentially, this behavior reflects the combined influence of gate-bias magnitude and accumulated stress history rather than an isolated voltage-acceleration relationship. Positive gate bias produced partial recovery of VTH and RDS, indicating a reversible component of the electrically induced parameter shift. Reapplication of −5 V stress after the recovery phase resulted in altered degradation kinetics, demonstrating a pronounced dependence on prior stress history. Drain-current stress at zero gate bias produced comparatively limited long-term parameter drift within the investigated range, whereas combined positive gate bias and drain current increased electrical instability. These results demonstrate that the electrical state established by previous stress conditions strongly influences subsequent GaN HEMT behavior. Although the present measurements do not uniquely identify trap energies or spatial locations, they provide a reproducible experimental basis for evaluating history-dependent trapping and long-term parameter stability in GaN power devices.