Section 12_Semiconductors

Section 12_Semiconductors - Physics 927 E.Y.Tsymbal Section...

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Physics 927 E.Y.Tsymbal Section 12: Semiconductors Crystal structure and bonding Semiconductors include a large number of substances of widely different chemical and physical properties. These materials are grouped into several classes of similar behavior, the classification being based on the position in the periodic table of the elements. The best-known class is the Group IV semiconductors - C (diamond), Si, Ge, - all of which lie in the fourth column of the periodic table. They have been studied intensively, particularly Si and Ge, which have found many applications in electronic devices. The elemental semiconductors all crystallize in the diamond structure. The diamond structure has an fcc lattice with a basis composed of two identical atoms, and is such that each atom is surrounded by four neighboring atoms, forming a regular tetrahedron. Group IV semiconductors are covalent crystals, i.e., the atoms are held together by covalent bonds. These bonds consist of two electrons of opposite spins distributed along the line joining the two atoms. The covalent electrons forming the bonds are hybrid sp 3 atomic orbitals. Another important group of semiconductors is the Group III-V compounds, so named because each contains two elements, one from the third and the other from the fifth column of the periodic table. The best-known members of this group are GaAs and InSb (indium antimonite), but the list also contains compounds such as GaP, InAs, GaSb, and many others. These substances crystallize in the zincblend structure which is the same as the diamond structure, except that the two atoms forming the basis of the lattice are now different. Thus, in GaAs, the basis of the fcc lattice consists of two atoms, Ga and As. Because of this structure, each atom is surrounded by four others of the opposite kind, and these latter atoms form a regular tetrahedron, just as in the diamond structure. The bonding in the III-V compounds is also primarily covalent. The eight electrons required for the four tetrahedral covalent bonds are supplied by the two types of atoms, the trivalent atom contributing its three valence electrons, and the pentavalent atom five electrons. The bonding in this group is not entirely covalent. Because the two elements in the compound are different , distribution of the electrons along the bond is not symmetric, but is displaced toward one of the atoms. As a result, one of the atoms acquires a net electric charge. Such a bond is called heteropolar, in contrast to the purely covalent bond in the elemental semiconductors, which is called homopolar. The distribution of electrons in the bond is displaced toward the atom of higher electronegativity. In GaAs, for instance, the As atom has a higher electronegativity than the Ga, and consequently the As atom acquires a net negative charge, whose value is ± 0.46 e per atom (a typical value in Group III-V compounds). The Ga atom correspondingly acquires a net positive charge of 0.46 e . Charge transfer leads to an ionic contribution to the bonding in Group III-V compounds. Their bonding is therefore
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This note was uploaded on 03/11/2012 for the course PHYSICS 927 taught by Professor Staff during the Fall '11 term at UNL.

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Section 12_Semiconductors - Physics 927 E.Y.Tsymbal Section...

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