Physics · Semiconductor Electronics : Materials, Devices And Simple Circuits · NEET
An intrinsic semiconductor is pure (only Si or Ge). Its charge carriers come only from heat breaking bonds, so ne = nh, and its conductivity is very low at room temperature. An extrinsic semiconductor is a pure semiconductor with a small impurity added by doping. This raises the number of carriers many times, so its conductivity is much higher and can be controlled. Extrinsic = intrinsic + added dopant.
Doping is the deliberate adding of a small, carefully chosen impurity to a pure semiconductor to increase its conductivity. The added atoms are called dopants. Only a few parts per million (ppm) are added. The dopant must be nearly the same size as Si or Ge atoms so it fits into the crystal lattice without distorting it, replacing only a very few original atoms.
A pure (intrinsic) semiconductor has very low conductivity at room temperature, so no useful electronic device can be built from it. Adding a suitable impurity raises the number of free charge carriers manifold, so the conductivity goes up a lot. This controlled conductivity is what makes diodes, transistors and ICs possible.
Two types are used for tetravalent Si or Ge. (i) Pentavalent dopants (valency 5): Arsenic (As), Antimony (Sb), Phosphorus (P). They donate one extra electron, so they are called donor impurities and give an n-type semiconductor. (ii) Trivalent dopants (valency 3): Indium (In), Boron (B), Aluminium (Al). They create one hole, so they are called acceptor impurities and give a p-type semiconductor.
A donor atom is a pentavalent dopant. Four of its electrons bond with four Si neighbours; the fifth is weakly bound and easily becomes free, so it donates an electron for conduction. An acceptor atom is a trivalent dopant. It bonds with only three Si atoms; the missing fourth bond is a hole that can accept an electron from a neighbour, leaving a hole free for conduction.
No. The extra carriers from dopants depend mainly on the doping level, not on temperature. The ionisation energy of a donor electron is very small (about 0.01 eV for Ge, 0.05 eV for Si), so at room temperature almost all dopant atoms are already ionised. The intrinsically generated carriers still rise weakly with temperature, but the dopant contribution is set by how much impurity was added.
For a p-type semiconductor, which of the following statements is true?
A p-type extrinsic semiconductor is obtained when Germanium is doped with:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is impure by design. A pure semiconductor has a tiny, controlled impurity added by doping. The impurity is intentional and useful, unlike random contamination.
Only a few parts per million (ppm). This tiny amount can raise the conductivity of the semiconductor many times over.
So it can occupy a lattice site without distorting the original crystal structure. If sizes differ too much, the lattice bends and the dopant will not fit cleanly. That is why we pick dopants from the neighbouring third and fifth groups.
n-type (made with pentavalent donor dopants like As, Sb, P, where electrons are the majority carriers) and p-type (made with trivalent acceptor dopants like B, Al, In, where holes are the majority carriers).
No. The crystal stays electrically neutral. A donor gives a free electron but keeps a fixed positive ion core; an acceptor gives a free hole but keeps a fixed negative ion core, so the charges balance.
