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Conductivity arises due to motion of charge carriers in a material when placed in an electric field. The primary charge carriers in a metal are electrons. In gases and solutions, the carriers could be charged ions. The density of carriers in a metal is very high, being of the order of  per m  .
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Charge carriers in semiconductors are electrons and vacancies, the latter being known as holes . Typical carrier density in a semiconductor is about  which is substantially lower than that of metals. Insulators have negligible carrier densities |
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Electrical properties of materials are best understood in terms of their electronic structure. We know that the energy levels of isolated atoms are discrete. When atoms are brought together to form a solid, these energy levels spread out into bands of allowed energies. The effect is qualitatively understood as follows by considering what happens when a collection of  atoms, which are initially far apart are brought closer. |
