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Enhancement-mode pseudomorphic In0.22Ga0.78As-channel MOSFETs with an ultrathin native InAlP oxide gate dielectric are demonstrated. The gate dielectric, a 3.5-nm-thick oxide grown by wet thermal oxidation from epitaxial InAlP, reduces the gate leakage below that of HFETs based on the same epitaxial structure by more than 105 times. The devices operate in enhancement mode, with 0.25-μm gate length devices exhibiting a measured threshold voltage of 0.25 V, a peak intrinsic transconductance of 245 mS/mm, and a saturation drain current density of 165 mA/mm. A record-high current gain cutoff frequency fT of 60 GHz has also been achieved. These results indicate the applicability of InAlP oxide-based InGaAs-channel MOSFETs for use in single power supply RF applications.