We present a study of high energy electron transport in GaAs using an analytical model of the band structure. This model is based on piecewise polynomial approximation of the dispersion relation in different regions of the Brillouin zone. It accounts for the first two conduction bands and reproduces all important features of the full band structure. We have used this model to set up a Monte Carlo simulation of electron transport accounting for impact ionization. It has been shown that this “extended valley” model yields essentially the same results as the most rigorous full band Monte Carlo calculations. We have found a large influence of high energy anisotropy on electron transport. Another important result is that most impact ionization events are due to electrons in the second conduction band. ©1997 American Institute of Physics.