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Asked in NEET 2023 (mpr) · Doping and carrier concentration
Idea: count the valence electrons of the dopant against germanium's four.
Germanium forms four covalent bonds with its four valence electrons.
A trivalent atom put in its place can complete only three of those bonds, leaving one bond short of an electron. That vacancy is a hole, and it accepts an electron from a neighbouring bond.
Holes then outnumber electrons and the crystal is p-type.
Of the four dopants offered only boron is trivalent, so boron is the answer.
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