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Intrinsic semiconductors

Conduction in a pure (intrinsic) semiconductor Substances such as silicon and germanium have resistivities between those of insulators and those of conductors. These substances are known as semiconductors, and they form the basis of many of the devices that we take for granted in a technological society. Pure semiconductors are usually referred to as intrinsic semiconductors, since their conductivity is not affected by any external factors. As we shall see in Section 5.3, trace impurities greatly alter the conductivity of semiconductors.

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In an intrinsic semiconductor, electric current is carried by moving electrons, as it is in metals, although the number of charge carriers in silicon is perhaps a billion times fewer than in copper. However, in addition to electrons, intrinsic semiconductors can also be considered to contain moving positive charges that carry current. This can be explained as follows.

Lattice structure of the atoms in an intrinsic semiconductor

Some of the electrons in an atom of an intrinsic semiconductor are held less tightly than others. This means that in a piece of intrinsic semiconductor material at room temperature there will always be a few free electrons that have been 'shaken free' of their atoms by thermal excitation (when the material has absorbed energy from the surroundings). When an electron leaves an atom in this way, the atom becomes positively charged (Figure 5.13a). The effect of an electron leaving an atom is therefore to create a positive charge in the semiconductor lattice. This positive charge is called a hole. When an electric field is applied to the semiconductor (that is, when it is connected to a source of e.m.f.) the electrons and holes move in opposite directions, and the semiconductor exhibits intrinsic conduction.

This happens because, under the influence of this electric field, electrons still bound to atoms in the lattice are able to move

through the lattice from an atom to a nearby hole, thus causing the hole to appear to move through the lattice (Figure 5.13c). This motion happens in the opposite direction to

the motion of the electrons.