Materials electronic

Materials play an important role ia the electronics iadustry. The effectiveness of the electrical performance of the system, its reUabiUty, and its cost aU. depend on the packagiag materials used, which are chosen for their properties and appHcations. As a result, the practicing engineer must have ready access to current information on the materials that can be used ia product development. This article gives an overview of the various material choices for the elements of an electronic product. 

Kirk-Othmer Encyclopedia of Chemical Technology (4th Edition) 

Semiconductors (qv) are materials with resistivities between those of conductors and those of insulators (between 10 and 10 H-cm). The electrical properties of a semiconductor determine the hmctional performance of the device. Important electrical properties of semiconductors are resistivity and dielectric constant. The resistivity of a semiconductor can be varied by introducing small amounts of material impurities or dopants. Through proper material doping, electron movement can be precisely controlled, producing hmctions such as rectification, switching, detection, and modulation. 

Material Dielectric constant at high frequency Density, kg/m Knoop hardness, kg/mm Thermal conductivity, W/(m-K) Melting point, °C 

Other properties, eg, strength, resistivity, heat capacity, and thermal expansion, are given in the reference (2,4) from which this table is compiled. 

Semiconductor materials are rather unique and exceptional substances (see Semiconductors). The entire semiconductor crystal is one giant covalent molecule. In benzene molecules, the electron wave functions that describe probabiUty density ate spread over the six ting-carbon atoms in a large dye molecule, an electron might be delocalized over a series of rings, but in semiconductors, the electron wave-functions are delocalized, in principle, over an entire macroscopic crystal. Because of the size of these wave functions, no single atom can have much effect on the electron energies, ie, the electronic excitations in semiconductors are delocalized. 

Good semiconductors are drawn from the central columns. Groups 13, 14, and 15 (111,IV, and V), of the Periodic Table, where the atoms tend to be nonpolar. Eor this reason, and because of the giant size of the wave functions, the electron-atom interaction is very weak. The electrons move as if in free space, colliding with the atomic lattice rather infrequendy. 

Electrons excited into the conduction band tend to stay in the conduction band, returning only slowly to the valence band. The corresponding missing electrons in the valence band are called holes. Holes tend to remain in the valence band. The conduction band electrons can estabUsh an equihbrium at a defined chemical potential, and electrons in the valence band can have an equiUbrium at a second, different chemical potential. Chemical potential can be regarded as a sort of available voltage from that subsystem. Instead of having one single chemical potential, ie, a Fermi level, for all the electrons in the material, the possibiUty exists for two separate quasi-Fermi levels in the same crystal. 

The idea of having two distinct quasi-Fermi levels or chemical potentials within the same volume of material, first emphasized by Shockley (1), has deeper implications than the somewhat similar concept of two distinct effective temperatures in the same block of material. The latter can occur, for example, when nuclear spins are weakly coupled to atomic motion (see Magnetic spin resonance). Quasi-Fermi level separations are often labeled as Im p Fermi s name spelled backwards. 

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King D A and Woodruff D P (eds) 1988 Surface properties of electronic materials The Chemical Physics of Solid Surfaces and Heterogeneous Cafa/ys/svol 5 (Amsterdam Elsevier)... 

EIGHTGENERATION - SEMICONDUCTORLASERS] (Vol 15) -in electronic materials ELECTRONIC MATERIALS] (Vol 9)... 

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