The mechanism of background impurity incorporation in the metalorganic vapor phase epitaxy of GaAs using triethylgallium and arsine was investigated over a wide range of growth parameters. The growth temperature, total reactor pressure, AsH3/Ga(C2H5)3 ratio, and the linear gas velocity were altered in order to ascertain the primary determinants of the background impurity incorporation in the ethyl based chemistry. The GaAs electrical and optical properties are, in general, independent of AsH3/Ga(C2H5)3 ratio but strongly dependent on growth temperature and reactor pressure. Reductions in the reactor pressure, at a constant mass flow rate, result in a linear decrease in the unintentional impurity incorporation, while not changing the growth rate. Substantial improvements in the layer purity with greatly reduced AsH 3 consumption can be achieved. A general model of impurity incorporation which relates the effect of reactor pressure and impurity incorporation is presented.