| On the basis of their electrical conductivities solids may be divided into three classes: metals, insulators, and semiconductors. Metals at room temperature have a high electronic conductivity which decreases with increasing temperature, while insulators have an extremely low ionic conductivity which increases rapidly with temperature. Semiconductors, like metals, conduct electronically, but their resistance at room temperature is much higher than that of metals and has a negative temperature coefficient.;A solid may be a member of more than one class. For example, it is believed that if most semiconductors were prepared in an extremely pure state, they would behave as insulators. On the other hand, insulators to which some excess cation or anion has been introduced exhibit both electronic and electrolytic conductivities in some cases. However, there is evidence to indicate that a few solids have a true "intrinsic" semiconduction which is independent of their impurity content. |
