Materials Classified According to Conductivity

Materials are often classified according to their DC conductivity as conductors, semiconductors, (electronic), and insulators. For electronic conductors (charge carriers electrons), this classification is based on energy levels and the Fermi-Dirac statistics of the free electrons. A division between metals and semiconductors is sometimes based on the temperature coefficient of the conductivity, a Semiconductors (as for ionic conductors) have a positive da/dT whereas metals have a negative da/dT. 

For ionic conductors, the classification is a little more problematic. They are in the conductivity range of pure semiconductors, but the charge carriers and therefore the conduction mechanism is very different. It is often found with liquid or solid electrolytes that the difference in conductivity is the result of different mobilities and not so much a different number of charge carriers (Table 2.7). 

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