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The physical mechanism of passivation of AlGaN/GaN HEMTs by AlN thin film prepared with plasma-enhanced atomic layer deposition (PEALD) is investigated by characterizing Ni- Al2O3/AlN-GaN/AlGaN/GaN metal-insulator-semiconductor (MIS) diodes. The dielectric stack Al2O3/AlN (13/2 nm) exhibits similar capability in suppressing the current collapse in AlGaN/GaN HEMTs as the 4-nm PEALD-AlN thin film used in our previous work but delivers much lower vertical leakage to facilitate the capacitance-voltage characterizations. Exceptionally large negative bias (<; -8 V) is required to deplete the 2-D electron gas in the MIS diode's C-V measurement. By virtue of quasi-static C-V characterization, it is revealed that positive fixed charges of ~ 3.2 × 1013 e/cm2 are introduced by the PEALD-AlN. The positive fixed charges are suggested to be polarization charges in the monocrystal-like PEALD-AlN. They can effectively compensate the high-density slow-response acceptor-like interface traps, resulting in effective suppression of current collapse.