The forces on dopant impurities and other defects are crucial in determining defect motion and diffusion in semiconductor heterostructures. Impurity ions and other charged defects feel electrostatic forces, just as electrons do. However, at heterojunction interfaces, electrons feel additional forces associated with band-edge distontinuities. Here, the analogous forces for ions and charged defects are derived, and given a simple physical interpretation. The donor or acceptor level is found to play much the same role for the ion that the band edge plays for the electron or hole in defining the effective potential. These forces can in general be spatially discontinuous, because of their dependence on charge state. © 1990 The American Physical Society.