An infrared imager, wherein a transparent gate is separated from a very narrow bandgap semiconductor (such as HgCdTe) by a thin dielectric. The gate is biased to create a depletion well in a semiconductor, and photo-generated carriers are collected in the well. The gate voltage is sensed to measure the accumulated charge. Preferably the accumulated charge is not sensed directly from the gate, but the gate output is repeatedly averaged with another capacitor, so that the output of the imager is sensed as an average over a number of read cycles, which provides a greatly improved signal-to-noise ratio. Preferably an array of the MIS detection devices is formed in a thin layer of HgCdTe, which is bonded to a silicon substrate containing a corresponding array of the averaging capacitors with addressing and output connections, and via holes through the HgCdTe are used to connect each detection device to its corresponding averaging capacitor.