Gallium to arsenic ratio in the gas phase, and doping of GaAs in the presence and absence of stable dopant halide are analyzed thermodynamically for the case of epitaxial growth of GaAs. (Ga/As)g in the gas phase does not change with change in AsCl 3 inlet partial pressure and a theoretical value for this ratio is found to be 2.9. Sulfur from H 2S and silicon from various sources are the dopants selected for the thermodynamic analyses representing the cases of the absence and presence of stable dopant halides respectively. Incorporation of sulfur in the epitaxial GaAs is a function of temperature and the partial pressure of H 2S. Incorporation of silicon is a function of temperature, HCl partial pressure and the partial pressure of any injected water vapor.