A shallow donor refers to a donor that contributes an electron that exhibits energy states equivalent to atomic hydrogen with an altered expected mass i.e. the long range coulomb potential of the ion-cores determines the energy levels. Essentially the electron orbits the donor ion within the semiconductor material at approximately the bohr radius. This is in contrast to deep level donors where the short range potential determines the energy levels, not the effective mass states. This contributes additional energy states that can be used for conduction.