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Asked in GUJCET 2009 · Doping and carrier concentration
Idea: an atom put into the lattice arrives electrically neutral, its nucleus and its own electrons together.
Doping silicon with a trivalent atom therefore adds no net charge. It only changes where the electrons sit, leaving one covalent bond short of an electron.
Each hole created this way is matched by the extra positive charge of the acceptor core left behind in the lattice, so positive and negative charge still balance exactly.
An isolated p-type crystal is therefore electrically neutral. Holes carry a current when a field is applied, but that does not charge the crystal.
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