What is Doping?

Replace one silicon atom in a million by phosphorus (five valence electrons): four bonds are satisfied and the fifth electron, bound by a mere $45\,\mathrm{meV}$ (Exercise 24.10), is free at room temperature. Such donors make an n-type semiconductor, conduction by electrons; boron (three electrons) is an acceptor, grabbing a bond electron and releasing a mobile hole — a missing electron that moves, responds to fields, and carries positive charge as genuinely as a bubble carries buoyancy: p-type. Doping swings the carrier density — and hence conductivity — across six orders of magnitude at will: the knob that turns sand into circuitry.