n-type and p-type semiconductors

At room temperature a pure semiconductor conducts too poorly to build useful devices. Adding a tiny amount of a suitable impurity, a few parts per million, raises its conductivity many times over.

Deliberately adding impurity is doping, the impurity atoms are dopants, and the product is an extrinsic (impurity, doped) semiconductor. The dopant must not distort the lattice, so its atoms should be about the same size as Si or Ge.

Pentavalent dopants (valency 5): arsenic (As), antimony (Sb), phosphorus (P). Trivalent dopants (valency 3): indium (In), boron (B), aluminium (Al).

n-type: a pentavalent atom bonds four of its electrons with its Si neighbours. The fifth is barely held, needing only about 0.01 eV (Ge) or 0.05 eV (Si) to free it, against a gap of about 0.72 eV (Ge) or 1.1 eV (Si), so it is free at room temperature.

Because it gives away an electron, a pentavalent dopant is a donor. The electrons it supplies depend on the doping level, not on temperature, while the intrinsic pairs from Si atoms grow only weakly with temperature.

Extra electrons raise the recombination rate, so the holes fall even below the intrinsic number. Electrons are the majority carriers and holes the minority: nₑ ≫ nₕ (Eq. 14.3).

p-type: a trivalent atom bonds with three neighbours, leaving the fourth bond one electron short, a hole. An electron from a nearby bond can fill it, moving the hole along. Each acceptor atom gives one hole.

Once it takes that fourth electron, the acceptor becomes a fixed negative core. Holes are the majority carriers and electrons the minority: nₕ ≫ nₑ (Eq. 14.4).

The crystal stays electrically neutral overall: the charge of the extra mobile carriers exactly balances the opposite charge on the fixed ionised dopant cores.

Plentiful majority carriers mop up thermally made minority carriers, so doping indirectly lowers the minority concentration below nᵢ.