Bias dependence of the photocurrent in planar metal-semiconductor-metal systems under de and ac optical illumination conditions has been examined experimentally. Making use of molybdenum-silicon-molybdenum structures with a long undepleted neutral region, we measured their dc. and low frequency (100 Hz to 100 kHz) photoresponse properties. It was confirmed that, in addition to opto-electronic conversion function, these structures showed an appreciable voltage controllability of the photocurrent in the device.