Semiconductors & p–n Junctions — A/L Physics

Energy Bands in Solids In a solid, discrete atomic energy levels broaden into bands: the valence band (filled with electrons) and the conduction band (empty or partially filled). The band gap (forbidden gap) is the energy difference between them. In conductors, bands overlap; in insulators, the gap is large (>3 eV); in semiconductors, the gap is small (~0.7–1.1 eV for Si, Ge), allowing electrons to jump to the conduction band with thermal or optical energy.

Intrinsic vs. Extrinsic Semiconductors An intrinsic semiconductor is pure (e.g., Si, Ge) with equal numbers of electrons and holes. Extrinsic semiconductors are doped with impurities to increase conductivity: n-type (doped with pentavalent atoms like P, As — extra electrons as majority carriers) and p-type (doped with trivalent atoms like B, Al — extra holes as majority carriers). Doping dramatically increases carrier concentration; for example, adding 1 ppm of phosphorus to silicon increases conductivity by ~10⁶ times.