The techniques of the photoreflectance and electroreflectance (ER) were used to study the built-in electric fields and the surface Fermi levels of InP surface-intrinsic-n+(SIN+)structures. The substrates ofSIN+structures are either Fe-doped semi-insulated InP or Sn-dopedN+InP with the same doping concentrations as its buffer layer. The built-in electric field and the Fermi level were calculated from the Franz–Keldysh oscillations of the photoreflectance spectra. Our studies found that for the samples with the same doping concentration in the buffer layer and substrate, the built-in electric field increases as their top layer thickness decreases. The surface Fermi level, on the other hand, remains approximately constant. For samples with a semi-insulated substrate, the photoreflectance spectra indicate the simultaneous existence of two built-in electric fields, one in the top layer and the other at the interface region between the buffer layer and substrate. ER spectra were measured with the application of a modulation electric field across the top layer. The built-in electric field across the top layer obtained from the ER spectra increases as the top layer thickness decreases while the surface Fermi level, again, remains approximately constant. ©1997 American Institute of Physics.