P- and N-type Semiconductors

A pure or intrinsic semiconductor has charge carriers which are thermally generated. These are relatively few in number. By 'doping' a semiconductor with a tiny amount of impurity such as one part in a million, thus forming a so-called extrinsic semiconductor, a considerable increase can be made to the number of charge carriers.

Arsenic atoms, for example, have five electrons in their outermost or valence band. When an atom of arsenic is added to a germanium crystal, the atom settles in a lattice site with four of its electrons shared with neighbouring germanium atoms, Fig. 39.4 (i). The fifth electron may thus become free to wander through the crystal. Since an impurity atom may provide one free electron, an enormous increase occurs in the number of electron carriers. The impure semiconductor is called an 'n-type semiconductor' or n-semiconductor, where 'n' represnts the negative charge on an electron. Thus the majority carriers in an n-semiconductor are electrons. Positive charges or holes are also present in the n-semiconductor.

These are thermally generated, as previously explained, and since they are relatively few they are called the minority carriers. The impurity (arsenic) atoms are called donors because they donate electrons as carriers.

P-semiconductors are made by adding foreign atoms which are trivalent to pure germanium or silicon. Examples are boron or indium. In this case the reverse happens to that previously described. Each trivalent atom at a lattice site attracts an electron from a neighbouring atom, thereby completing the four valence bonds and forming a hole in the neighbouring atom, Fig. 39.4 (ii). In this way an enormous increase occurs in the number of holes. Thus in a p-semiconductor, the majority carriers are holes or positive charges. The minority carriers are electrons, negative charges, which are thermally generated. The impurity atoms are called acceptors in this case because each 'accepts' an electron when the atom is introduced into the crystal.

Summarising: In a n-semiconductor, conduction is due mainly to negative charges or electrons, with positive charges (holes) as minority carriers. In a p-semiconductor, conduction is due mainly to positive charges or holes, with negative charges (electrons) as minority carriers.

P-N Junction

By a special manufacturing process, p- and n-semiconductors can be melted so that a boundary or junction is formed between them. This junction is extremely thin and of the order 10^-3 mm. It is called a p-n junction, Fig. 39.5 (i).

When a scent bottle is opened, the high concentration of scent molecules in the bottle causes the molecules to diffuse into the air. In the same way, the high

concentration of holes (positive charges) on one side of a p-n junction, and the high concentration of electrons on the other side, causes the two carriers to diffuse respectively to the other side of the junction, as shown. The electrons which move to the p-semiconductor side recombine with holes there. These holes therefore disappear, and an excess negative charge A appears on this side Fig. 39.5 (ii).

In a similar way, an excess positive charge В builds up in the n-semiconductor when holes diffuse across the junction. Together with the negative charge A on the p-side, an e.m.f. or p.d. is produced which opposes more diffusion of charges across the junction. This is called a barrier p.d. and when the flow ceases it has a magnitude of a few tenths of a volt. The narrow region or layer at the p-n junction which contains the negative and positive charges is called the depletion layer. The width of the depletion layer is of the order 10^-3 mm.

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