Steady-state velocity-field characteristics in AlxGa1-xAs are calculated for a wide range of composition, doping, and temperature variations, using Monte Carlo techniques. The results are related to the scattering mechanisms which are responsible for the observed transport parameters. A single empirical expression is formulated to fit all the Monte Carlo data. This expression incorporates all the nonlinear transport effects and would be useful for numerical simulation of device models. A general methodology is also presented for formulating such expressions, and the results are applicable to modelling field-dependent mobility in AlxGa1-xAs devices.