Metal–insulator–semiconductor p-type GaN high-electron-mobility transistor with an Al2O3/AlN deposited by atomic layer deposition was investigated. The selected insulator, AlN has been proven to have a good interface with GaN. A traditional p-GaN device without an Al2O3/AlN layer was processed for comparison. Due to the Al2O3/AlN layer, the gate leakage was lower, and the threshold voltage was higher, at 4.7 V. Additionally, excellent turn-on voltage was obtained. Furthermore, low current degradation and smaller VTH shift at high temperatures was also observed. Hence, growing a good-quality Al2O3/AlN layer can achieve an enhancement-mode operation with superior stability and high gate swing region.