EASA / CAR Part-66 · Module 4 · 4.1 Semiconductors · Cat B1/B2
Pure silicon is a poor conductor because all four valence electrons are locked into covalent bonds. Add a pentavalent donor and you get a spare electron; add a trivalent acceptor and you get a hole. Both stay electrically neutral overall.
Silicon has four valence electrons. In pure (intrinsic) material every one of them is shared in a covalent bond with a neighbour, so at absolute zero there are no free carriers at all. Heat breaks a few bonds, and each broken bond releases one free electron and leaves one hole behind, always in pairs. That is why intrinsic material has n = p = ni.
Doping replaces an occasional silicon atom with an impurity. A pentavalent atom (phosphorus, arsenic, antimony) has five valence electrons: four bond, the fifth is loosely held and becomes a free electron, so the material is N-type and the donor atom is the fixed positive ion left behind. A trivalent atom (boron, aluminium, gallium, indium) has only three, leaving a vacancy, a hole, so the material is P-type. Watch the readout: doping raises the majority carriers and, by mass action (n·p = ni²), pushes the minority carriers down just as far.
A common exam trap: doped material is still electrically neutral. Every free electron is balanced by a fixed positive donor ion, and every hole by a fixed negative acceptor ion. "N-type" names the majority carrier, not a net charge.
Raise the temperature and watch ni climb. In intrinsic or lightly doped material the extra carriers dominate and resistance falls, the negative temperature coefficient that distinguishes a semiconductor from a metal. In heavily doped material the carrier count is already set by the dopant, so the lattice vibration that impedes them wins and resistivity rises slightly: that stability is exactly why devices are doped well above the intrinsic level. Germanium's smaller band gap (0.7 eV against silicon's 1.1 eV, giving the familiar 0.3 V and 0.7 V junction drops) makes far more pairs at a given temperature, which is why germanium leaks more and why silicon took over in aircraft equipment.