An experimental investigation on the impact of postgate CF4 plasma treatment on InP metal-oxide-semiconductor field-effect transistors is presented. Fluorine (F) has been incorporated into the atomic-layer-deposited Al2O3 gate dielectric by postgate CF4 plasma treatment. The effects of radio-frequency (rf) power and plasma treatment time were studied systematically. An appropriate amount of F incorporated in Al2O3 gate dielectric brings great improvement in electrical characterization. The treatment of rf power of 20 W for 5 min is the optimum condition for the Al2O3/InP stack. It is believed to be due to the great reduction of the oxide fixed charge in the Al2O3 bulk.