We demonstrate for the first time that a quantum well infrared photodetector based on bound‐to‐continuum state intersubband transitions can have a photoconductive gain much greater than unity similar to extrinsic photoconductors. An optical gaing=8.1 was determined by comparing the optical absorption, responsivity, and noise characteristics of two multiquantum well detectors which were identical in every respect except for having a different number of periods (2 and 20). The results suggest that since the photocarrier lifetime &tgr;Lis not transit time limited, detector optimization to increase &tgr;Lor optical coupling can be expected to lead to an improved detectivityD*.