Suitable substitutional donors include Si on a Ga site and O on an N site. The former replaces a group-III atom with a group-IV atom; the latter replaces a group-V atom with a group-VI atom. Each provides an extra valence electron relative to its host site. Si is a plausible substitutional cation-site dopant, and the compact oxygen atom is compatible with an anion site. Site compatibility matters: merely choosing an element with an extra electron without considering which sublattice it occupies is insufficient.
A standard substitutional acceptor choice is Mg on a Ga site: a group-II atom replaces group III, leaving one fewer valence electron. Its size is sufficiently compatible with the cation sublattice to make substitution plausible. This identifies the intended acceptor chemistry; actual hole density also depends on activation, incomplete ionization and compensation. Size and valence rules alone do not guarantee a shallow, fully ionized acceptor.
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