Extrinsic Semiconductors

NCERT explains that doping a tetravalent silicon crystal with a pentavalent impurity such as arsenic introduces atoms with five valence electrons. Four of these electrons form covalent bonds with neighbouring silicon atoms, while the fifth is loosely bound and easily released into the conduction band even at room temperature. Such pentavalent impurities are called donors because they donate free electrons, producing an n-type semiconductor in which electrons are the majority carriers and thermally generated holes are the minority carriers. The option naming a p-type is wrong because p-type material requires a trivalent acceptor impurity, not a pentavalent one. The option calling it intrinsic is wrong because deliberate doping destroys the equal carrier balance of a pure crystal. The option pairing n-type with holes as majority carriers is wrong because in n-type material electrons, supplied by donors, dominate. As stated in NCERT Class 12, Chapter 14, pentavalent doping yields donor levels just below the conduction band. A consistency check confirms that the crystal stays electrically neutral, since each donor atom that gives up an electron becomes a fixed positive ion balancing the mobile electron.