Digital alloy and random alloy Al<sub>0.85</sub>Ga<sub>0.15</sub>As<sub>0.56</sub> Sb<sub>0.44</sub> avalanche photodiodes (APDs) exhibit low excess noise, comparable to Si APDs. Consequently, this material is a promising multiplication layer candidate for separate absorption, charge, and multiplication structure APDs with high gain-bandwidth product. Characterization of the impact ionization coefficients of electrons (<italic>α</italic>) and holes (<italic>β</italic>) plays an important role in the simulation of avalanche photodiodes. The multiplication gain curves of eight p<sup>+</sup>-i-n<sup>+</sup> and n<sup>+</sup>-i-p<sup>+</sup> APDs covering a wide range of avalanche widths have been used to determine the electric field dependence of the impact ionization coefficients of Al<sub>0.85</sub>Ga<sub>0.15</sub>As<sub>0.56</sub>Sb<sub>0.44</sub>. A large impact ionization coefficient ratio between that of electrons to holes was seen across a wide electric field range. Simulations of the avalanche multiplication in these structures using a random path length (RPL) model gave good agreement with experimental results over almost three orders of magnitude, and a mixed injection method was employed to verify the extracted impact ionization coefficients. Interestingly, no difference in the impact ionization coefficients was seen between digital alloy and random alloy Al<sub>0.85</sub>Ga<sub>0.15</sub>As<sub>0.56</sub>Sb<sub>0.44</sub>. This knowledge of impact ionization coefficients is beneficial for the future utilization of the Al<sub>x</sub>Ga<sub>1-x</sub>As<sub>y</sub>Sb<sub>1-y</sub> material system.