"A consequence of the discovery of electricity was the observation that metals are good conductors while nonmetals are poor conductors. The latter were called insulators. Metallic onductivity is typically between 106 and 104 (Ω cm)–1, while typical insulators have conductivities of less than 10–10 (Ω cm)–1. Some solids with conductivities between 104 and 10–10 (Ω cm)–1 are classified as semiconductors. ... semiconductors have an energy gap while semimetals and metals have no such gap. However, very impure semiconductors show a more or less metallic behavior and with many substances, the art of purification is not so far advanced that a distinction can easily be made. The transition between semiconductors and insulators is even more gradual and depends on the ratio of the energy gap to the temperature of investigation. Very pure semiconductors may become insulators when the temperature approaches the absolute zero."