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Asked in GUJCET 2007 · Doping and carrier concentration
Idea: n-type means the majority carriers are electrons, so the dopant must supply a spare electron.
Germanium is tetravalent: each atom uses all four of its valence electrons in covalent bonds.
A pentavalent (group-15) dopant completes those four bonds and still has a fifth electron left over, loosely bound and free to conduct.
Of the four choices only phosphorus is pentavalent. Boron and aluminium are trivalent, so they give p-type germanium, and gold is neither a donor nor an acceptor.
So doping intrinsic germanium with phosphorus gives an n-type semiconductor.
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