Franz-Keldysh oscillations in Si-delta-doped GaAs have been studied by the application of fast Fourier transformations to the piezoreflectance spectra. In such studies, we iind that the Franz-Keldysh oscillations of the piezoreflectance and its associated Fourier transformation can be described by a model calculation which considers the energy gap modulation in a uniform electric field. Owing to the character of nonelectromodulation, the Franz-Keldysh oscillation lineshape in the piezoreflectance has no modulation electric field distortion such as that which occurs in the photoreflectance and electroreflectance experiments. We demonstrate that the piezoreflectance accuracy is better than the photoreflectance or electroreflectance for the measurement of a uniform built-in electric field. The fast Fourier transformation of piezoreflectance spectra exhibits two separate heavy- and light-hole frequencies at temperatures from 20 to 425 K, Using these separated peaks, the ratio between the heavy- and light-hole effective reduced mass, the surface Fermi level, and the broadening parameters of the Franz-Keldysh oscillations are measured and discussed. (C) 1998 American Institute of Physics. [S0021-8979(98)04114-0].