Energy Bands In Solids

NCERT describes solids using the band theory, where the valence band holds the bonded electrons and the conduction band, separated by a forbidden energy gap of about 1.1 eV in silicon, holds electrons that are free to conduct. At absolute zero no thermal energy is available to lift electrons across this gap, so the valence band stays completely filled and the conduction band remains completely empty. A completely full band cannot carry current because there are no vacant states for electrons to move into, and an empty band has no carriers, so the crystal behaves as an insulator. The option about band overlap is wrong because overlapping bands describe a metal, not a semiconductor at 0 K. The option about electrons freezing onto nuclei is wrong because electrons remain in their bonds, not attached to individual nuclei in a classical sense. The option about an infinite gap is wrong because the gap of silicon is a fixed small value near 1.1 eV and does not grow at low temperature. As stated in NCERT Class 12, Chapter 14 (Semiconductor Electronics), conduction requires partially filled bands. A consistency check confirms that as temperature rises, some electrons cross the small gap and conductivity increases, which is the defining semiconductor behaviour.