Si incorporation processes on a GaAs(1 1 1)A surface were investigated by using the first-principles pseudopotential method. We found that the most stable adsorption site for a single Si atom is a Ga lattice site of the GaAs lattice, but that the Si atom is pushed out from the site when Ga-As-Si microstructures are formed on the surface. We also found that a Si atom remains at the Ga lattice sire when it couples with three As adatoms. These findings qualitatively explain the experimentally observed properties of Si-doped GaAs layers, i.e., that the layers are p-type under conventional growth conditions and they become n-type under very high As pressure.