MOSFETS operate at 4.2 K and act as impedance transformers for high impedance (1010Ω) ir detectors. Several types, both n‐channel and p‐channel, have operated successfully at this temperature and withstood the thermal shock of repeated cycling from room temperature. Low device power dissipation and relatively low noise can be achieved by optimizing drain current (ID ) and drain‐to‐source voltage (VDS ) for the particular device. Power dissipations as low as 0.4 mW have been recognized.