The electroreflectance ͑ER͒ spectra of a surface-intrinsic n ϩ-type doped ͑100͒ GaAs have been measured at various polarization angles of the probe beam. Several Franz-Keldysh oscillations were observed above the band-gap energy, thus enabling heavy-and light-hole transitions to be separated by the application of the fast Fourier transform to the ER spectra. From this, the ratios of the amplitudes of the light-to heavy-hole transitions versus angle of polarization were obtained. At a large incident angle ͑80°͒, the strength of the field of the probe beam in the normal direction of the sample (F z) was varied from zero to a larger component. It was found that the ratios increased with increasing F z which is consistent with the theory that the light-hole transition becomes more enhanced with z-polarized light.