Results obtained with lead sulfide photoconductive insulated‐gate field‐effect transistors (photo‐IGFET’s) are presented. Detectivity modulation is provided by the gate electrode. Then‐type PbS film was depleted of carriers by applying a negative gate potential, and the detectivity was increased by a factor of 200 while the noise remained constant. Detectivities of 2×1010cm Hz1/2/W were obtained at 173 °K and 500 Hz for a 180° field of view and a 300 °K background. The devices exhibited stable operating characteristics and showed no significant performance change after 18 months under ambient laboratory conditions. They were fabricated entirely by vacuum deposition in a single pumpdown, and the various conducting, insulating, and semiconducting films were deposited successively through accurately registered metal shadow masks.