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Asked in GUJCET 2024 · Doping and carrier concentration
Idea: the type of an extrinsic semiconductor is fixed by how many valence electrons the dopant has compared with the four of silicon or germanium.
A pentavalent atom (P, As, Sb, Bi) has one electron too many. It donates that electron to the conduction band, so electrons are the majority carriers and the crystal is n-type.
A trivalent atom (B, Al, Ga, In) has one electron too few. One covalent bond is left incomplete, which is a hole, so holes are the majority carriers and the crystal is p-type.
Indium is trivalent; antimony, arsenic and phosphorus are all pentavalent.
So indium is the impurity that makes the crystal p-type.
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