Photoreflectance spectroscopy of surface-intrinsic n+-doped (s-i-n+) GaAs has been measured at various power densities (Ppu) of a pump beam. Many Franz–Keldysh oscillations (FKOs) were observed above the band-gap energy, which will enable the electric-field strength (F) to be determined from the periods of the FKOs. Field F thus obtained is subject to photovoltaic effects. In order to reduce the photovoltaic effects from the pump beam, Ppu was kept below 10 μW/cm2 in the previous experiments. Here, we demonstrate that the built-in field can be determined at a larger Ppu by using fast Fourier transform techniques. © 1999 American Institute of Physics.