A model is developed for the incorporation of donors and acceptors in epitaxial GaAs which indicates that the thinp‐type region often observed at the layer‐substrate interface inn−p+ structures is caused by the electric field associated with surface states and substrate doping. The model also predicts impurity gradients at the outer surface, about which little is known experimentally. Since results from the model are in agreement with many experimental observations, surface states and substrate doping are believed to be the major cause of impurity gradients in epitaxial GaAs.