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 10−7 to 10−8m, semiconductors have resistivities in the order of 10−3 to 3×103 m, and the resistivities of insulators are in the order of 104 to 1014 m. Some typical approximate values at normal room temperatures are:
Conductors:
Aluminium 2.7 ×10−8 m Brass (70 Cu/30 Zn) 8 ×10−8 m Copper (pure annealed) 1.7 ×10−8 m Steel (mild) 15 ×10−8m
Semiconductors: (at 27◦C) Silicon 2.3 ×103 m Germanium 0.45m
Insulators:
Glass ≥1010 m Mica ≥1011 m PVC ≥1013 m Rubber (pure) 1012 to 1014 m
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 and 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 Figure 11.1.
