Physical model of depletion and accumulation in quantum-well infrared photodetectors

Numerical work has shown that, at low operating temperatures or large incid ent photon fluxes, carriers deplete from the quantum welts 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 expressi ons, the accumulation and depletion of carriers in QWIP's. In QWIP's having the same growth sequence (layer widths and compositions) in each period, c arrier depletion is found to occur only in one of two QW's 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 t he electron drift velocity is saturated, carrier depletion is found to be p artial, uniform (throughout the depletion region), and abrupt, with the tot al charge in the depletion region fixed and with the depletion width increa sing linearly with applied bias. At a large applied bias, carriers are foun d to be uniformly accumulated in the device, Carrier depletion or accumulat ion in QWIP's arises from the different dependences on the local electric f ield of the different physical mechanisms which are responsible for the car rier injection from the contacts (via thermionic emission or thermionic hel d assisted tunneling) and for the photoconduction process (via drift).