The built-in electric fields, surface Fermi level, and surface state density of a series of InP surface-intrinsic-n+ structures are calculated from the Franz–Keldysh oscillations of the modulation spectra of photoreflectance (PR) and electroreflectance (ER). Those results indicate that for samples with similar doping concentrations in the buffer layer and the substrate, the only built-in electric field across the top layer increases as their top layer thickness decreases. The surface Fermi level, however, remains roughly constant. For samples with a semi-insulated substrate, two built-in electric field exist separately in the top layer and at the interface between the buffer layer and the substrate. In addition, ER spectra are taken with the modulation electric field across the top layer. Although the built-in electric field across the top layer derived from the ER spectra increases as the top layer thickness decreases, the surface Fermi level, again, remains roughly constant. An approach of room temperature photoreflectance, based on the thermionic-emission theory and the current transport theory, is employed to estimate the surface state density. The surface state density of the InP SIN+ structure is found to diminish with an increase in the thickness of the top layer. © 2000 American Vacuum Society.