The effects of optical illumination on the microwave characteristics of a conventional MESFET have been studied theoretically. The effect of the changes in the intrinsic parameters of the device under illumination has been utilized to calculate the Y parameters of the device at microwave frequencies. The small-signal equivalent circuit of the device in the illuminated condition has been obtained with the various components determined from the Y parameters. The attractive feature of the present model is that it is fully compatible with commonly used circuit simulation packages like SPICE. © 1995 John Wiley & Sons. Inc.