We have used Monte Carlo simulation methods employing both realistic band structure and a simpler analytical approximation to investigate impact ionization in bulk Al_xGa_1-xAs and also submicron p⁺in⁺ diodes for x40%. The calculated impact ionization rates in bulk Al_xGa_1-xAs compared well with previous experiments and the electron- and hole-initiated current multiplication characteristics of the p⁺in⁺ diodes were found to agree very well with our experimental results for both the analytical and the numerical models.