The strength of the surface field on GaAs epitaxial layers of various thicknesses and doping concentrations grown by metalorganic chemical vapor deposition on a semi‐insulating GaAs substrate was studied as a function of applied bias using modulation spectroscopy. While for small applied potentials the square of the field strength increases linearly with respect to reverse bias, for larger values of bias it saturates. It is shown that this saturation appears when the width of the depletion layer in the epilayer approaches the width of the epilayer itself.