Robust Vth stability in normally off GaN HEMTs with a stacked p <b>+</b> /p <b>−</b> -NiO gate: Suppression of interface trapping and electric-field crowding

We report a normally off AlGaN/GaN high-electron-mobility transistor (HEMT) featuring a stacked p+/p−-NiO gate with an integrated p−-NiO reduced-surface-field (RESURF) terminal, enabling simultaneous suppression of interface traps and reduction of gate-edge electric-field crowding. The lightly doped p−-NiO transition layer improves the NiO/AlGaN interface quality by suppressing nickel vacancy-related defect states, resulting in a low interface trap density (Dit ≈ 1 × 1012 cm−2 eV−1)—over an order of magnitude lower than that of single-layer p+-NiO-gated devices. Furthermore, this layer lowers the vertical electric field across the AlGaN barrier, thereby reducing band bending and consequently suppressing Poole–Frenkel emission of accumulated holes into the GaN channel as well as electron injection from the two-dimensional electron gas into the NiO gate. The combined reduction in trap density and carrier trapping effectively stabilizes the threshold voltage and suppresses gate leakage. Moreover, the p−-NiO RESURF region flattens the lateral electric-field distribution and reduces the peak electric field at the gate edge by over 51%, enabling a high off-state breakdown voltage (BV) of 1586 V. The p+/p−-NiO gate HEMT operates in the enhancement mode with a threshold voltage of 1.9 V and exhibits only a 0.03 V shift under 0–5 V gate-bias stress. These results demonstrate that the integration of a RESURF region into the stacked p+/p−-NiO gate enables high BV and robust threshold-voltage stability in normally off GaN HEMTs.