Generation of native defects during growth of heavily Si-doped GaAs and their effects on carrier concentrations in heavily doped and adjoining lightly Si-doped GaAs layers are investigated. The mechanism of their intrusion from the heavily doped layer into the lightly doped layers is discussed. The behavior of the native defects during annealing after growth is also studied. As Si doping concentration increases, the concentration of triply ionized gallium vacancy (VGa3-), generated by a Frenkel-pair defect formation process, increases. The limit of free-carrier concentration in the heavily doped layers is caused by this VGa and not by electron occupation of a highly localized state of the donor-related DX center. VGa also causes carrier compensation in the adjoining underlayers. However, the carrier concentration in the adjoining overlayer grown on the heavily doped layer is not affected. We infer that drift is the predominant process for VGa flow into the lightly doped layers. This drift is caused by the electric field induced by a surface–Fermi-level pinning, mainly in an early growth stage of the heavily doped layer. On the other hand, the diffusion process of VGa from the heavily doped layer during growth is negligible. Therefore, the carrier concentration in the layers grown on the heavily doped layer is not affected. During annealing after growth the VGa, which is supersaturated in the lightly doped underlayer, disappears as the result of a first-order reaction, so the carrier concentration is recovered. These results not only suggest a carrier compensation mechanism, but are also useful in improving the characteristics of devices consisting of structures with heavily Si-doped GaAs layers. © 1997 American Institute of Physics.