Al-SiO2-GaAs structures are prepared and characterized by electrical measurements. The GaAs (100) surfaces are cleaned and passivated by chemical solutions. A passivation treatment based on ammonium polysulfide (NH4)(2)S-x is applied, then an insulating layer is deposited by electron-beam evaporation of an amorphous SiO2. By adjusting the passivation and the insulator deposition parameters, the accumulation and inversion conditions are observed by a capacitance-voltage (C-V) technique, showing that the Fermi level is unpinned. The interface-states density D-it as calculated using the method of Terman applied to high-frequency C-V characteristics presents a minimum of 10(11) cm(-2) eV(-1).