We propose a simple distributed model for intersubband quantum well infrared photodetectors(QWIPs), which explicitly takes into account the injecting properties of the contacts. We show that the QWIP operation with multiple QWs involves the formation of a high‐field domain near the emitter, caused by the modulation of the bound electron density in the QWs by applied voltage and infrared radiation. The external characteristics of the QWIP (total current, differential resistance, and quasistatic capacitance) are strong functions of the voltage and radiation intensity.