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 (Ω cm)−1, while typical insula
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1.1 Insulator - Semiconductor - Semimetal - Metal 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 ( n cm)-l , while typical insulators have conductivities of less than 10- 10 ( n cm)-l . Some solids with conductivities between 104 and 10- 10 ( n cm)-l 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 mono crystalline 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 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. Typical elemental semiconductors are germanium and silicon. An inspection of the periodic table of elements reveals that these materials belong to the fourth group while typical metals such as the alkalis are in the first group and typical nonmetals such as the halogens and the noble gases which crystallize at low temperatures are in the seventh and eighth group, respectively. Other semiconducting elements in the fourth group are diamond which is a modification of carbon, while gray tin (a - Sn), which is stable only at low temperatures is a semimetal. All fourth-group semiconductors crystallize in a structure known as the diamond structure in which neighboring atoms are arranged in tetrahedral symmetry. In the third group, the lightest element boron, and in the sixth group, the heavy elements selenium and tellurium, are semiconductors. A typical semimetal is the heaviest fifth group element, bismuth, and also the lighter elements of this group, arsenic and antimony, may be classified as such although they are at present less thoroughly investigated. K. Seeger, Semiconductor Physics © Springer-Verlag Berlin Heidelberg 2004
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1. Elementary Properties of Semiconductors
Typical compound semiconductors are the III-V compounds such as gallium arsenide, GaAs, and indium antimonide, InSb, and the II-VI compounds such as zinc sulfide, ZnS (zinc blende). They crystallize in the zinc blende structure which can be obtained from the diamond structure by replacing the carbon atoms alterna
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