Elementary Properties of Semiconductors
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 conductivity is typically between 106 and 104(Ωcm)−1, while typical insulators have conductivities of less than 10−10(Ωcm)−1. Some solids with conductivities between 104 and 10−16(Ωcm)−1 are classified as semiconductors. However, substances such as alkali-halides whose conductivity is due to electrolytic decomposition shall be excluded. Also we restrict our discussion to chemically uniform, homogeneous substances and prefer those which can be obtained in monocrystalline form. Even then we have to distinguish between semiconductors and semimetals. This distinction is possible only as a result of thorough investigation of optical and electrical properties and how they are influenced by temperature, magnetic field, etc. Without giving further explanations at this stage, the statement is made that 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 by, e.g., zone refining [1.1, 2] 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 become insulators when the temperature approaches the absolute zero.
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