Numerical work has shown that, at low operating temperatures or large incident photon fluxes, carriers deplete from the quantum wells near the emitter contact in a quantum-well infrared photodetector (QWIP). A physical model is developed in this work to describe, with closed-form analytical expressions, the accumulation and depletion of carriers in QWIPs. In QWIPs having the same growth sequence (layer widths and compositions) in each period, carrier depletion is found to occur only in one or two QWs near the emitter contact at the small applied biases for which the electron drift velocity is linear in the electric field. At intermediate applied biases for which the electron drift velocity is saturated, carrier depiction is found to be partial, uniform (throughout the depletion region), and abrupt, with the total charge in the depletion region fixed and with the depletion width increasing linearly with applied bias. At a large applied bias, carriers are found to be uniformly accumulated in the device. Carrier depletion or accumulation in QWIPs arises from the different dependences on the local electric field of the different physical mechanisms which are responsible for the carrier injection from the contacts (via thermionic emission or thermionic field assisted tunneling) and for the photoconduction process (via drift).
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