Molecular beam epitaxial GaAs layers of electron concentration 1.69×1017 cm−3, and various thicknesses d=0.25, 0.50, 1.00, and 2.00 μm, have been grown on semi‐insulating GaAs substrates and characterized by the Hall effect and capacitance‐voltage (C‐V) techniques. A plot of sheet Hall concentration ns vs d gives accurate values of (ND−NA) and (ws+wi), the sum of the surface and interface free‐carrier depletion widths, respectively. The C‐V measurements verify the value of ND−NA, and also give a good estimate of wi. By comparing the value of wi with depletion theory, it is shown unambiguously that the interface depletion is mainly due to interface states, of concentration 1.2×1012 cm−2 (below midgap). This result has important technological implications.