We report a strongly nonexponential behavior of the transient photocurrent in quantum-well infrared photodetectors (QWIPs) in response to a step-like infrared illumination. The transient photocurrent displays an overshoot on the time scale 0.1–1 ms at low temperatures (T<70 K), exceeding the steady-state photocurrent by as much as ≈50%. The overshoot behavior is attributed to a nonlinearity of responsivity caused by the modulation of the electric field in QWIP under relatively high illumination power, when the photocurrent exceeds the dark current. This explanation is confirmed by the experimental data and numerical simulation. These effects can play an important role when QWIPs operate in nonlinear regimes, such as in a heterodyne mode or in low-temperature and low-background applications.

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