Abstract The conductance and capacitance of tunnel metal/oxide/semiconductor (MOS) structures with plasma SiO 2 (thickness 30–80 Å) as the oxide were measured at various frequencies. The experimental current-voltage curves and breakdown oxide field were also investigated. The surface state spectrum in the silicon gap for samples with different thicknesses and at various oxygen pressures is given. The surface state capture cross section for an n-Si(111) MOS with plasma SiO 2 is about 10 −18 cm 2. For all thicknesses investigated negative charge was observed in the surface states.