The most important mechanism of dark current inn‐doped quantum‐well infrared photodetectors (QWIPs) is due to interaction of electrons with longitudinal optical phonons. Theoretical expressions are derived for the carrier lifetime, and for generation currents originating from both photoexcitation as well as from thermal excitation in a single quantum well. Detector gain is discussed briefly. Calculated values of thermal generation currents and the ratio of photocurrent to thermal current are found to accord well with experimental data. Finally the conditions of background radiation limitation of QWIPs are investigated and the theory gives a temperature of background radiation limitationTBLIP=81 K for a 9 &mgr;m cutoff detector with a two‐dimensional grating and optical cavity, for 300 K background temperature, opticsfnumber=1 with 100% optical transmission, provided that a photocurrent to dark current ratio of 1 criterion is used. ©1995 American Institute of Physics.