Improvement of the previously reported photocapacitive MIS infrared detectors has led to the development of exceptional room‐temperature devices. Unoptimized peak detectivities on the order of 1013W−1cm Hz1/2, a value which exceeds the best obtainable from existing solid‐state detectors, have now been consistently obtained in Si and GaAs devices using high‐capacitance LaF3or composite LaF3/native‐oxide insulating layers. The measured spectral response of representative samples is presented and discussed in detail together with a simple theory which accounts for the observed behavior. The response of an ideal MIS photocapacitor is also contrasted with that of both a conventional photoconductor and ap‐i‐nphotodiode, and reasons for the superior performance of our detectors are given. Finally, fundamental studies on the electrical, optical, and noise characteristics of our MIS structures are analyzed and discussed in the context of infrared‐detector applications.