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An effective passivation technique that yields low off-state leakage and low current collapse simultaneously in high-voltage (600-V) AlGaN/GaN high-electron-mobility transistors (HEMTs) is reported in this letter. The passivation structure consists of an AlN/SiNx stack with 4-nm AlN deposited by plasma-enhanced atomic layer deposition and 50-nm SiNx deposited by PECVD. The AlN/ SiNx-passivated HEMTs with a gate-drain distance of 15 μm exhibit a high maximum drain current of 900 mA/mm, a low off-state current of 0.7 μA/mm at VDS = 600 V, and a steep subthreshold slope of 63 mV/dec. Compared with the static on-resistance of 1.3 mΩ·cm2, the dynamic on-resistance after high off-state drain bias stress at 650 V only increases to 2.1 mΩ·cm2. A high breakdown voltage of 632 V is achieved at a drain leakage current of 1 μA/mm .