  The wave function and binding energy for shallow donors in GaAs are
calculated within the tight binding (TB) approach, for supercells containing up
to two million atoms. The resulting solutions, coupled with a scaling law,
allow extrapolation to the bulk limit. A sharp shallow-deep transition is
obtained as the impurity perturbation increases. The model allows investigating
the quantitative consistency between the effective mass theory and the TB
formalism. Although the calculated binding energies are in excellent agreement,
anisotropies and the overall decay obtained in the TB envelope function can not
be afforded by the hydrogenlike effective mass prediction.
