ABSTRACT
Materials may be classified as conductors, semiconductors or insulators. The classification depends on the value of resistivity of the material. Good conductors are usually metals and have resistivities in the order of 107 to 108m. Semiconductors have resistivities in the order of 103 to 3 ð 103m. The resistivities of insulators are in the order of 104 to 1014m. Some typical approximate values at normal room temperatures are:
Conductors:
Aluminium 2.7 ð 108m Brass (70 Cu/30 Zn) 8 ð 108m Copper (pure annealed) 1.7 ð 108m Steel (mild) 15 ð 108m
Semiconductors:
Silicon 2.3 ð 103m } at 27°C
Insulators:
Glass ½ 1010m Mica ½ 1011m PVC ½ 1013 m Rubber (pure) 1012 to 1014m
In general, over a limited range of temperatures, the resistance of a conductor increases with temperature increase. The resistance of insulators remains approximately constant with variation of temperature. The resistance of semiconductor materials decreases as the temperature increases. For a specimen of each of these materials, having the same resistance (and thus completely different dimensions), at say, 15°C, the variation for a small increase in temperature to t °C is as shown in Fig. 11.1
11.2 Silicon and germanium The most important semiconductors used in the electronics industry are silicon and germanium. As the temperature of these materials is raised above room temperature, the resistivity is reduced and ultimately become
conductors. For this reason, silicon should not operate at a working temperature in excess of 150°C to 200°C, depending on its purity, and germanium should not operate at a working temperature in excess of 75°C to 90°C, depending on its purity. As the temperature of a semiconductor is reduced below normal room temperature, the resistivity increases until, at very low temperatures the semiconductor becomes an insulator.
