The quality and reliability of predictions from numerical simulations of GaAs/AlGaAs devices, such as heterojunction bipolar transistors, depend on model parameters. These parameters include the variations with doping and carrier concentrations of the valence‐ and conduction‐band edges and of the effective intrinsic carrier concentrations for heavily dopedp‐ andn‐type gallium arsenide. The Klauder self‐energy method is used to calculate the effects of interactions among carriers and dopant ions in heavily doped GaAs at 300 K. The carrier‐carrier interactions of exchange and correlation are estimated by interpreting optical‐absorption measurements and by calculations based on degenerate theory. When densities exceed 1019cm−3inp‐type and 1017cm−3inn‐type gallium arsenide, carrier‐dopant ion interactions and carrier‐carrier interactions become of the same order of magnitude and should be included in calculations of band‐structure changes and of properties which depend on the density of states, such as carrier transport and effective intrinsic carrier concentrations.