We present a procedure for extracting the parameters of a simple model for localized states at the exposed surfaces of GaAs/AlGaAs heterostructures. Such a procedure is based on a comparison between experimental data on test structures and results from the solution of the Poisson-Schrodinger equation in the growth direction of the heterostructure. We show that a model of surface states based on just two parameters is sufficient for obtaining good agreement with experiments and significant improvement with respect to the assumption of Fermi level pinning. In addition, the procedure proposed allows the extraction of the donor concentration in the doped layer and the unintentional doping in the substrate.
