The effect of the optical spot position relative to the electrodes in metal-semiconductor-metal (MSM) structures has been studied. Theoretically, we have found that the I-V characteristic depends strongly on the position and the intensity of the optical spot. Besides being very sensitive to the optical spot position, at zero applied bias, the MSM can be used as a current source where the magnitude and the direction of the output current depend on the position and the intensity of the optical spot. Being a planar structure with high signal-to-noise ratio makes the MSM a unique structure for these types of applications.