 Solution

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.