Molecular beam epitaxial GaAs layers of electron concentration 1.69×1017cm−3, and various thicknessesd=0.25, 0.50, 1.00, and 2.00 &mgr;m, 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 concentrationnsvsdgives accurate values of (ND−NA) and (ws+wi), the sum of the surface and interface free‐carrier depletion widths, respectively. TheC‐Vmeasurements verify the value ofND−NA, and also give a good estimate ofwi. By comparing the value ofwiwith depletion theory, it is shown unambiguously that the interface depletion is mainly due to interface states, of concentration 1.2×1012cm−2(below midgap). This result has important technological implications.