An atomic layer passivation (ALP) structure for GaAs is studied by fabricating metal–insulator–semiconductor (MIS) diodes. An atomically thin GaP layer is grown on a (100) surface of GaAs to form the ALP structure. MIS diodes are fabricated on the GaP surface by depositing SiO2as the insulator. Between 1 MHz and 20 Hz, the maximum capacitances are very close to the insulator capacitance without frequency dispersion. The accumulation and inversion conditions are observed in the capacitance–voltage characteristics of the diodes at room and low temperature. The capacitance–voltage characteristics of diodes with and without ALP are compared. The results show that ALP unpins the surface Fermi level which can be displaced nearly throughout the GaAs band gap by the applied gate voltage. Interface trap density is estimated to be about 5×1011cm−2 eV−1near the midgap. The influence of the SiO2plasma deposition process on the interface characteristics is also described, and the mechanism of unpinning is discussed.