Semiconductor atomic density

The carbides with the NaCl structure may be considered to consist of alternating layers of metal atoms and layers of semiconductor atoms where the planes are octahedral ones of the cubic symmetry system. (Figure 10.1). In TiC, for example, the carbon atoms lie 3.06A apart which is about twice the covalent bond length of 1.54 A, so the carbon atoms are not covalently bonded, but they may transfer some charge to the metal layers, and they do increase the valence electron density. 

Simple calculation gives a comparable distribution of the electrode potential in the two layers, (64< >h/64( sc) = 1 at the surface state density of about 10cm" that is about one percent of the smface atoms of semiconductors. Figure 5—40 shows the distribution of the electrode potential in the two layers as a function of the surface state density. At a surface state density greater than one percent of the surface atom density, almost all the change of electrode potential occurs in the compact layer, (6A /5d )>l, in the same way as occurs with metal electrodes. Such a state of the semiconductor electrode is called the quasi-metallic state or quasi-metallization of the interface of semiconductor electrodes, which is described in Sec. 5.9 as Fermi level pinning at the surface state of semiconductor electrodes. 

Other semiconductors, such as Ge and GeAs, have atomic densities of 4.4 X 10 atoms/cm, a metal such as aluminum has about 6 X lO atoms/cm, and metals such as cobalt, nickel, and copper have about 9 X lO atoms/cm. ... 

Dopant atoms chemical impurities that are deliberately introduced into the semiconductor lattice to provide control over the conductivity and Fermi level of the solid Doping the introduction of specific chemical impurities into a semiconductor lattice to control the conductivity and the Fermi level of the semiconductor Effective density of states the number of electronic states within ikT of the edge of an energy band, where k is the Boltzmann constant and T is the temperature Energy bands a cluster of orbitals in which the individual molecular orbitals are packed closely together to form an almost continuous distribution of energy levels... 

Thus the higher the free-carrier concentration in the material, the smaller the penetration depth of the applied field into the medium. For electron concentrations of 10 cm (10 m ) or larger, the space charge is restricted to distances on the order of one atomic layer or less, because the large free-carrier density screens the solid from the penetration of the electrostatic field caused by the charge imbalance. For most metals, almost every atom contributes one free valence electron. Because the atomic density for most solids is on the order of 10 cm (10 m ), the free-carrier concentration in metals is in the range of 10 -10 cm (10 -10 m ). Thus Fv and d are small. For semiconductors or insulators, however, typical free-carrier concentrations at room temperatures are in the range of cm lO -... 

True reconstruction implies a variation in the atomic density in the topmost layer of the substrate if compared with the corresponding bulk plane. Such a situation is found not only with numerous clean semiconductor surfaces but also with the (10 0) and (11 0) surfaces of the fee 5d metals [18]. 

Most vitreous covalent semiconducting alloys contain a Group VI element as a major constituent and thus are LP semiconductors. The density of states at the top of the LP valence band tails into the gap because of local energy fluctuations of the LP electrons. These occur because of the lack of long-range order but are enhanced with the presence of (i) dangling bonds, (ii) other chalcogens, and (iii) electropositive atoms. The conduction band tail is broadened by the presence in the alloy of weaker bonds. 

One of the earliest models for charge transport in delocalized bands is the Drude model [12], which assumes the charge carriers as free to move under the influence of an applied electric field, but subject to damping forces due to collisions. This model is valid in semiconductors, where the density of carriers is much lower that the atomic density in metals, where the density of carriers is much higher, it leads to inconsistencies that were removed by taking quantum effects into account. 

For many surfaces, the displacements of the atoms from their bulk truncated positions are more pronounced than a simple relaxation in which only the interlayer spacings change. These may involve lateral displacements of atoms within the surface layers and/or a change in the surface layer atomic density. First, we discuss some examples of reconstructions at metal surfaces and then some examples at semiconductor surfaces. 

Moderately doped diamond electrodes are well suited for revealing the semiconductor and structural aspects of the diamond electrochemistry. The structural effects often boil down to a difference in the acceptor concentration in the diamond, rather than reflecting the surface atomic density or other purely surface properties. To reveal these fine effects, the electrochemical behavior... 

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. 

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