Inversion n -channel GaN metal-oxide-semiconductor field-effect-transistors (MOSFETs) using atomic-layer-deposited Al ₂ O ₃ as a gate dielectric have been fabricated, showing well-behaved drain I-V characteristics. The drain current was scaled with gate length (varying from 1 to 16 μ m ), showing a maximum drain current of ∼10 mA / mm in a device of 1 μ m gate length, at a gate voltage of 8 V and a drain voltage of 10 V . At a drain voltage of 0.1 V , a high I _on /I _off ratio of 2.5×10⁵ was achieved with a very low off-state leakage of 4×10^-13 A /μ m . Both MOSFET and MOS capacitor showed very low leakage current densities of 10^-8 A / cm ² at biasing fields of 4 MV / cm . The interfacial density of states was calculated to be (4–9)×10¹¹ cm ^-2 eV ^-1 near the midgap.