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 $\hbox{Ni}{-} \hbox{Al}_{2}\hbox{O}_{3}/\hbox{AlN}{-}\hbox{GaN}/\hbox{AlGaN}/\hbox{GaN}$ metal–insulator–semiconductor (MIS) diodes. The dielectric stack $ \hbox{Al}_{2}\hbox{O}_{3}/\hbox{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 ( $ < -\hbox{8}\ \hbox{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 $\sim\hbox{3.2} \times \hbox{10}^{13}\ \hbox{e}/\hbox{cm}^{2}$ 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.

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