Extrinsic Semiconductors

NCERT states that a p-type semiconductor is formed by doping tetravalent silicon with a trivalent acceptor impurity. Such an atom has only three valence electrons, so when it bonds with four neighbouring silicon atoms one bond is left incomplete, creating a hole that can accept an electron from a nearby bond. This makes holes the majority carriers, and the material conducts primarily through hole movement. Boron, with three valence electrons, is a classic trivalent acceptor and is therefore the correct choice. Phosphorus is wrong because with five valence electrons it is a pentavalent donor that produces n-type material. Antimony is likewise wrong because it too is pentavalent and donates electrons rather than creating holes. Germanium is wrong because it is tetravalent like silicon and cannot upset the carrier balance to favour holes. As stated in NCERT Class 12, Chapter 14, trivalent dopants create acceptor levels just above the valence band, and only a tiny amount of dopant, roughly one atom in a million, dramatically raises the hole concentration far above the intrinsic value. A consistency check confirms electrical neutrality, since each acceptor that captures an electron becomes a fixed negative ion balancing the mobile positive hole.