Electrical conduction in solids

Electrical conduction in any solid happens through the transport of charge carriers, and how well a solid conducts depends on several factors: its temperature, the number of charge carriers available, how easily those carriers can move through the material (their mobility), the material's crystal structure, and the type and density of defects present. Based on how well they conduct, solids fall into three broad groups.

CONDUCTORS (metals) have an enormous number of free electrons available for conduction -- typically of the order of $10^{28}$ electrons per cubic metre -- which is what makes metals such good conductors. INSULATORS, such as glass, wood or rubber, have a comparatively very small number of free electrons, of the order of $10^{23}$ per cubic metre. SEMICONDUCTORS -- silicon, germanium, gallium arsenide, gallium nitride, cadmium sulphide among the commonly used ones -- sit between the two: their conductivity lies between that of a metal and that of an insulator, and, crucially, the number of charge carriers in a semiconductor can be deliberately controlled, and its internal structure designed, to suit a particular requirement. This tunability is exactly why semiconductors are indispensable across the electronics industry, from a cell phone or a solar cell to something as demanding as a satellite or the International Space Station. Table 14.1 lists real conductivity values for a range of common conductors, semiconductors and insulators, for comparison.

Temperature affects conductors and semiconductors in opposite ways. As shown in Fig. 14.1, when the temperature of a semiconductor is increased, its electrical conductivity also INCREASES; the electrical conductivity of a metal, by contrast, DECREASES as its temperature rises. This opposite temperature dependence is itself a very useful, exploitable property, and is central to many electronic devices built from semiconductors.

Semiconductors can be further classified into three broad families. ELEMENTAL semiconductors -- silicon and germanium -- are single-element materials. COMPOUND semiconductors -- such as cadmium sulphide and zinc sulphide -- are made from a compound of two or more elements. ORGANIC semiconductors -- anthracene, doped phthalocyanines, polyaniline and similar carbon-based compounds -- are a comparatively recent discovery and, so far, find far fewer applications than the elemental and compound families, which remain the two workhorses of the electronics industry. …