A semiconductor is a substance whose resistivity is between that of a good conduc- tor and a good insulator.
A semiconductor is a substance where the outer shell electrons are not free to move, but only require a little energy to free them for conduction. Group IV elements, such as silicon and germanium, have four valance electrons1.Taking silicon as an example, as it is the most commonly used semiconductor, when found in its pure state the silicon atoms are structured in a tight grid, with each of the valence (outer shell) electrons forming a covalent bond with a surrounding atom. Fig 19.1 shows this arrangement; in reality the structure is three-dimensional rather than two-dimensional.At low temper- ature, close to zero kelvin, all of the valence electrons are bound to this structure. Therefore there are no electrons free for conduction, making silicon an insulator at low temperature, see Fig 19.1(a).
The resistance of a semiconductor changes with temperature. This is because adding heat to the structure frees more charge carriers, thus increasing the conductivity, see Fig 19.1(b).
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Fig 19.1: Outer shell electron arrangement of silicon (a) at a temperature close to zero kelvin and (b) at room temperature
Conduction electrons
As the pure semiconductor heats up, some of the valence electrons are given enough thermal energy to break free from their covalent bond and they become conduction electrons. As electrons are negatively charged, sometimes these electrons are referred to as ‘negative electrons’.
1Valence electrons are electrons from the outermost shell of the atom; they are involved in chemical bonding. SEMICONDUCTORS 333
