AlGaN/GaN metal oxide semiconductor high electron mobility transistors (MOSHEMTs) with thick (>35 nm), high-κ (TiO₂/NiO), submicrometer-footprint (0.4 μm) gate dielectric are found to exhibit two orders of magnitude in lower gate leakage current (~1 nA/mm up to +3-V applied gate bias), higher IMAX (709 mA/mm), and higher drain breakdown voltage, compared to Schottky barrier (SB) HEMTs of the same geometry. The maximum extrinsic transconductance of both the MOSHEMTs and the SBHEMTs with 2 × 80-μm gate fingers is measured to be 149 mS/mm. The addition of the submicrometerfootprint gate oxide layer only results in a small reduction of the current gain cutoff frequency (21 versus 25 GHz, derived from S-parameter test data) because of the high permittivity (κ ≈ 100) of the gate dielectric. This high-performance submicrometer-footprint MOSHEMT is highly promising for microwave power amplifier applications in communication and radar systems.