Electroluminescence
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3.1 General The III-V family of materials contains some of the compounds with semiconducting properties most like those of the classical group IV elemental semiconductors germanium and silicon, fulfilling some early predictions made by Welker [3.1]. Our insight into and consequent ability to control these properties was initially developed through detailed study of the crystal growth, metallurgical and electronic properties of these perfectly covalently bonded elemental semiconductors. Germanium and silicon have relatively low melting points and can be readily grown in large single crystal form. Techniques such as zone melting were developed to refine these crystals to the point whereby their electrical properties could be tailored through the addition of microscopic quantities of certain key impurities which produce the well-known donor and acceptor centers. The possible existence of two dominant forms of conductivity within different regions of a given single crystal containing different major dopant species led to the design of pn junction rectifiers with essentially ideal electrical characteristics and the invention of the bipolar transistor. The active regions of these devices, the depletion layers between the p and n regions in which hole and electron conductivity, respectively, predominates, can be formed well within the bulk of the single crystal. They can lie well away from the external surfaces whose characteristics were difficult to assess and control in the early work. The basic principles of the energy band theory of crystals, from which the concepts of bipolar conductivity emerge, are especially appropriate to covalently bonded semiconductors. However, key properties of relatively low electrical carrier mass, relatively long scattering times and therefore high carrier mobilities extend also into the compound semiconductors whose electronic basis represents only a small perturbation from perfect covalent bonding. The III-V compounds come closest to this ideal. They form extended crystals in which the outer electronic shells of each III-V atom pair contribute 8 electrons to the interatomic bonding process, just as in the elemental semiconductors. The same basic crystal structure (zincblende) results, without the inversion symmetry of the elemental (diamond structure) semiconductor lattice. A relatively minor variation in the stacking sequence of atom planes along a (111)-type axis produces an alternate form, the wurtzite crystal structure. These differences in crystal structure, diamond~zincblende-,wurtzite represent reductions in the crystal symmetry, reductions in
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the space group and consequent reductions in the degeneracy of the electronic band structure, seen most clearly at key symmetry points [3.2]. One aspect of this symmetry reduction which has been clarified only relatively recently is discussed in Section 3.3. However, the essential point here is the close approach of certain of the III-V compound semiconductors to the near ideal electronic behavior of the now classical elemen
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