Electrical properties of surface passivation films for GaAs ICs were studied by measuring the surface I-V characteristics on semi-insulating wafers and the surface state parameters. Silicon dioxide and silicon nitride films were formed by plasma CVD processes. Characteristic activation energies of 0.3-0.5 eV were observed in both linear and superlinear regions of I-V curves and in electron emission from surface states, which suggests surface state conduction. It is shown that surface passivation becomes increasingly important in GaAs VLSIs.