The shallow donor or acceptor in a compound semiconductor is more complex than in an elemental semiconductor for two reasons:
(1) the interaction of the electron or hole with the alternatingly charged ions of the lattice, and
(2) the differentiation between incorporating the defect on an anion or cation site.
Although the degree of ionicity of good compound semiconductors is small, the above-stated effects are not negligible. For example, isochoric acceptors in GaP should be well described by point-charge quasi-hydrogenic models. However, the two isochoric acceptors, GaP:ZnGa and GaP:SiP, have substantially different ground-state energies—64 and 204 meV, respectively. This difference cannot be explained by site-dependent screening, even though neighboring anions are expected to screen more effectively since they are surrounded by more electrons. It needs a more sophisticated analysis, beyond that of the effective mass approximation (Bern – holc and Pantelides 1977).
The degree of ionicity in compound semiconductors also determines the coupling of electrons with the lattice, i. e., with phonons; it is described by Frohlich’s coupling constant ac. Such interaction can be included by considering, instead of a Bloch electron, a polaron to interact with the defect center, with an effective
(see Sak 1971) where EqH is the quasi-hydrogen energy. However, this approximation is not sufficient to explain the observed variations of the ground-state energies indicated above.
The modified hydrogenic effective mass approximation describes reasonably well the level spectrum of excited states of shallow donors and acceptors in compound semiconductors with a sufficiently large e/mp ratio, i. e., for many III-V and II-VI compounds—see Grimmeiss (1986).