In this paper, a novel gate dielectric and passivation technique for GaN-on-Si AlN/GaN metal–insulator–semiconductor high-electron-mobility transistor (MIS-HEMT) is proposed. This technique features the AlN thin film grown by thermal atomic layer deposition (ALD) at 360 °C without plasma enhancement. A 10 nm AlN thin film serving as gate dielectric and passivation layer in the access region was grown. Compared with the Schottky gate AlN/GaN HEMT (SG-HEMT), the fabricated thermal ALD-grown AlN MIS-HEMT exhibits enhanced Ion/Ioff ratio, reduction of gate leakage by 5 orders of magnitude at a bias of 5 V, and suppressed current collapse degradation.