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 stead
y-state photocurrent by as much as approximate to 50%. The overshoot behavi
or is attributed to a nonlinearity of responsivity caused by the modulation
of the electric field in QWIP under relatively high illumination power, wh
en 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 het
erodyne mode or in low-temperature and low-background applications. (C) 200
1 American Institute of Physics.