We present an investigation of Al0.3Ga0.7As/GaAs quantum well infrared photodetectors (QWIPs) through detailed ensemble Monte Carlo simulations. Both two-dimensional and three-dimensional electrons are simulated with realistically evaluated scattering rates. Transport of the excited electrons is accurately modelled including the reflections from well-barrier interfaces. The details incorporated into the simulator clarified some important phenomena, as well as verifying the previous predictions. Under large bias, well accumulation occurs non-uniformly, being highest near the emitter. Contrary to previous assumptions, the L valley is found to be the origin of a significant portion of the captured electrons even under typical bias voltages. Gamma-L transfer, while decreasing carrier mobility, also increases capture probability and decreases the electron lifetime, having a twofold effect on device gain. The above findings explain the large difference between the gains of AlxGa1-xAs/GaAs (with x similar to 0.3) and InP/In0.53Ga0.47As (or GaAs/InxGa1-xAs) QWIPs, as well as the bias dependence of gain. The average barrier electron velocity is close to the saturated electron velocity in bulk Al0.3Ga0.7As under moderate and large bias; however, low-field mobility is significantly lower than that in bulk material. While complementing previous work, our results offer a deeper understanding of some important QWIP characteristics by resolving the details of transport and electron dynamics in the device.