A simple analytical model for calculating the high-frequency barrier capacitance of GaAs thin-film epitaxial structures both in the dark and under extrinsic illumination is presented. The model accounts for the space charge region at the film-substrate interface. The model shows that the photocapacitance of these structures has the form of a narrow peak located at the bias voltage, at which the barrier capacitance drops abruptly with reverse bias. The underlying physical mechanism is discussed. A method for predicting the MESFET threshold voltage based on the measured photocapacitance is proposed and tested. (C) 2004 Elsevier Ltd. All rights reserved.