QWIPs belong to the category of so called photon detectors; the absorption of an infrared photon results directly in some specific quantum event, such as the photoelectric emission of electrons from a surface, or electronic inter band transitions in semiconductor materials. Therefore, the output of photon detectors is governed by the rate of absorption of photons and not directly by the photon energy. Photon detectors typically require cooling down to cryogenic temperatures in order to get rid of excessive dark current, but in return their general performance is high. QWIP’s are most often used as photo-conductive detectors. In this type of detectors photo-generated charge carriers increase the conductivity of the device material.
In QWIP’s the quantum wells (QW) are formed by layers of different materials with different band gap. This gives rise to potential-wells for charge carriers in the conduction band as well as in the valence band.