The PHOTOFET method is applied to submicrometre MESFETS processed on ion-implanted Cr-doped substrates. It is demonstrated that the magnitude of the photoconductivity both under extrinsic and intrinsic illumination is determined by the substrate leakage current, which is a function of the device drain bias. A semiempirical modeling of the dependence of the photoconductivity on the device bias gives a power-law relation between the leakage current and the drain bias.