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Asked in GUJCET 2026 · Doping and carrier concentration
Idea: a trivalent dopant belongs to group 13 and brings three valence electrons. Put into silicon or germanium it leaves one covalent bond an electron short, creating a hole, so the crystal becomes p-type.
Aluminium, indium and boron are all group-13 elements, so all three are trivalent acceptors.
Antimony sits in group 15 and brings five valence electrons. Four of them complete the bonds and the fifth is left loosely bound and free, so antimony is a pentavalent donor and gives n-type material.
So antimony is the impurity that is not trivalent.
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