We show that the transient intersubband photocurrent in a GaAs/AlGaAs quantum‐well infrared photodetector (QWIP) consists of two dynamical components, which are associated, respectively, with the drift motion of photoexcited carriers and with the extra injection current induced by the generated nonequilibrium space charges. The decay time τ of the latter component depends critically on the temperature of the sample and the applied bias voltage. For a temperature of T=22 K, decay times in excess of 15 μs are found. A simple model of the refilling process of the space charges is presented, showing, that the slow component corresponds approximately to the dielectric relaxation time of the structure along the growth direction. While the slow component in the present 8‐period structure corresponds to about 50% of the total photocurrent, it is negligible in standard QWIP structures containing 40 or more periods, where ⩾99% of the intersubband photocurrent proceeds on a picosecond time scale. © 1996 American Institute of Physics.