Example: replace Ga by Si in GaAs

Si has one valence electron more → introduces extra electron: donor

Si⁴⁺ weakly binds the electron:
hydrogenic (shallow) donor state

Example: replace Ga by Zn in GaAs

Zn has one valence electron less
→ introduces extra hole: acceptor

Zn²⁺ weakly binds the hole:
hydrogenic (shallow) acceptor state