The extrinsic photoconductivity of an n-type GaAs thin-film structure under backgating has been studied. It is shown that this photoconductivity is a nonmonotonic function of negative substrate voltage, namely, the magnitude of the photoconductivity shows a maximum. This is due to a sharp increase in the magnitude of the photoconductivity caused by an illumination-induced change in the backgating threshold voltage and to its subsequent decrease caused by the accumulation of excess carriers near the film–substrate interface. The calculated results are in a qualitative agreement with the experimental data.