AlxGa1-xAs (x=0.0–0.9) homojunction avalanche photodiodes are characterized in the temperature range of 11–300 K. For all compositions, the breakdown electric field strength decreases with temperature, and the rate of change is smaller at temperatures below 50 K than at higher temperatures. This results from the fact that spontaneous phonon emission dominates carrier scattering in the 0 K limit. The rate of change of the breakdown field strength is smallest for Al0.6Ga0.4As compared with other compositions. This is likely due to higher alloy scattering at x=0.6. A Monte Carlo model that provides good fits to experimental data is presented. © 2002 American Institute of Physics.