Using the best available physical models is essential for predictive numerical simulations of advanced, high performance GaAs transistors. Among the many input parameters for numerical simulations, the effective intrinsic carrier concentrations, n ie, minority carrier mobilities, μ, and recombination lifetimes, τ, are very critical parameters. The results from recent theoretical calculations for n ie were implemented into a two-dimensional, drift-diffusion simulator for GaAs transistors. In order to compare predicted and measured d.c. common emitter gains, NPN GaAs homojunction bipolar transistors with different but heavily doped bases and similarly doped emitters that have widths between 0.1 and 0.45 μm were fabricated by molecular beam epitaxy. The predicted gains of 8, 25, 46, 72 for these transistors agreed very well with their measured gains of 9, 22, 42 and 70 at high current, respectively. Without using the new theoretical data for n ie but setting n ie equal everywhere to the intrinsic carrier concentration, n i, the predicted gains became 4, 14, 27 and 35, respectively. Sensitivity analyses on mobilities, lifetimes, and n ie showed that physically correct n ie values are quantitatively very important for predictive simulations of GaAs bipolar transistors.