We demonstrate GaAs-based, metal-oxide-semiconductor field-effect transistors (MOSFETs) with excellent performance using an Al 2 O 3 gate dielectric, deposited by atomic layer deposition (ALD). This achievement is very significant because Al 2 O 3 possesses highly desirable physical and electrical properties as a gate dielectric. These MOSFET devices exhibit extremely low gate-leakage current, high transconductance, and high dielectric breakdown strength. A short-circuit, current-gain, cutoff frequency (f T ) of 14 GHz and a maximum oscillation frequency (f max ) of 25.2 GHz have been achieved from a 0.65-µm gate-length device. The interface trap density (D it ) of Al 2 O 3 /GaAs is evaluated by the hysteresis of drain-source current, I ds , versus gate-source bias, V gs , and the frequency dispersion of transconductance, g m .