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Commercial metal-semiconductor-field-effect transistors (MESFET's) have opaque gate. We present here the frequency-dependent characteristics of an ion-implanted GaAs MESFET with opaque gate under illumination. The incident light enters the device through the gate-source and gate-drain spacings. Two photovoltages are developed: one across the Schottky junction due to generation in the side walls of the depletion layer below the gate and the other across the channel-substrate junction due to generation in the channel-substrate depletion region. The frequency dependence of the two photovoltages along with channel charge, drain-source current, transconductance and channel conductance of the device have been studied analytically and compared with the published theoretical results. For the first time, a commercially available GaAs optically illuminated field-effect transistor (OPFET) has been analyzed for frequency dependent characteristics instead of the transparent/semitransparent gate OPFET.