Intrinsic Semiconductors

According to NCERT, an intrinsic semiconductor is a pure crystal in which conduction arises solely from thermally generated carriers. When thermal energy at room temperature breaks a covalent bond in germanium, it frees one electron into the conduction band and simultaneously leaves behind one vacancy in the valence band called a hole. Because each broken bond creates exactly one electron and one hole, their number densities are equal, written $n_e=n_h=n_i$, the intrinsic carrier concentration. The option that only electrons appear is wrong because every liberated electron necessarily leaves a hole behind. The option that only holes appear is wrong for the same reason, since the freed electron itself contributes to conduction. The option about positive ions drifting is wrong because the lattice ions are fixed in place; it is the mobile electrons and holes, not ions, that carry current. As stated in NCERT Class 12, Chapter 14, in an intrinsic semiconductor the electron and hole concentrations are always equal. A consistency check confirms that pair generation conserves charge, since creating an electron and a hole together keeps the crystal electrically neutral overall.