We have found evidence that the surface depletion charge density in molecular beam epitaxial n-GaAs doped heavily with Si approaches the Si concentration. In situ metallization of the as-grown surface of GaAs uniformly doped with Si at 1×1020 cm-3 yields a specific contact resistivity of 1.3 μΩ cm2, indicating a space-charge density about equal to the silicon density despite a measured bulk electron density of 4×1018 cm -3. This contact resistivity is among the lowest for nonalloyed ohmic contacts to n-GaAs. We attribute the large discrepancy between surface space-charge density and bulk electron density to the amphoteric behavior of silicon in GaAs. Surface Fermi-level pinning and arsenic stabilization create a surface depletion region where donor site selection predominates, whereas the extrinsic electron density in the bulk causes self-compensation.