High-temperature reverse-bias (HTRB) stress in a dry or a humid ambient is applied to power n-channel U-shaped trench-gated MOSFET (UMOSFETs). The HTRB is shown to induce negative-bias temperature instabilities (NBTI) in a parasitic p-channel MOSFET occurring in the n-channel UMOSFET during the stress. The manifestations of the NBTI were gate-controlled shifts in the threshold voltage, V th, and in the drain-to-source leakage current, I DSS, following only humid HTRB stress. SEM inspection of UMOSFETs deprocessed to passivation showed the presence of passivation cracks on the edge termination only in devices that degraded with the HTRB stressing. Our results lead us to conclude that only humid HTRB/NBTI stress causes changes in V th and I DDS. During the stress water or hydrogen species diffuse into the gate oxide through passivation cracks at the edge of the UMOSFET, and react with the gate oxide in the presence of holes and release protons. The protons are not confined to the gate oxide at the trench-bottom and are able to migrate up the oxide on the trench sidewalls.