In this article we present the analysis and simulation of nonlinearities in the GaInAs/InP p-i-n photodetector at high illuminations. It is shown that the nonlinearities are generated principally in the low-field region of the intrinsic layer. The frequency response, especially for the nonlinear characteristics, is illustrated to differ significantly between top-illumination and substrate-illumination operations at common biasing level. (C) 1996 John Wiley & Sons, Inc.