The theory of operation, construction, and performance data of an infrared detector based upon the PEM effect in InSb are presented. Theoretical derivations of the spectral noise equivalent input, noise equivalent power, and responsivity are made which show the dependence of these figures of merit upon the electrical properties of the semiconductor, sample thickness, surface area, front and back surface recombination velocities, and magnetic induction. Performance of the detector operating at room temperature shows a spectral response extending to 7.5 microns and a response time of less than 1 microsecond. The noise equivalent input of a detector having a 0.71‐mm2sensitive area has a minimum value at 6.6 microns of 6.7×10−10w.