Poisson's equation is solved in the depletion approximation to give an expression for the sheet free-carrier charge transferred from a conductive semiconductor layer to acceptor (or donor) states at interfaces or in the bulk material. The principal goal of the paper is to show that a relatively thin, undoped buffer layer between the substrate and active layer can dramatically lower the free-carrier loss to substrate interface states. Data on molecular-beam epitaxial, n-type GaAs agree well with the theory, but show that there still is some loss at the interface between the active layer and buffer layer.