We have theoretically investigated the electronic structure of Si delta -doped GaAs inserted into a quantum well under an applied electric field. For uniform distribution we have studied the influence of the electric field on the donor concentration. The electronic properties such as the effective potential. the density profile, the subband energies, the subband occupations and the Fermi energy level have been calculated by solving the Schrodinger and Poisson equations self-consistently. From our calculations, we have seen that the change of the electronic properties as dependent on the applied electric field is more pronounced at low doping concentration. The high electric fields can induce a spatial separation between confined electrons and ionized dopants in the delta -doped GaAs structure, resulting in enhanced free-carrier mobility in devices.