Semiconductor conductivity, degeneration

In the classical electron transport model in metals or semiconductors, for a material with a free electron concentration n and an average electron scattering time (also called relaxation time) r, the DC conductivity is Oo = ne2r/to. In this classical expression, m (m or m ) is the conductivity effective mass, which is an average mass different from the DoS effective mass (see for instance [4]. In cubic semiconductors with degenerate CB extrema, the conductivity effective mass for electrons is ... 

The film system on metal electrodes in ionic melts often belongs to the imi-metal type. This is consistent with the general model of polyvalent cmiductor, which predicts the degeneration of semiconductor conductivity into metal one at higher temperatures [1, 2]. 

The semiconductor dependence o (T) for stoichiometric compositions obtains the most common electrical property of binary chalcogenides. Deviation fi-om stoichiometry influences the electrical conductivity value and even slight deviations from stoichiometry lead to degeneration of the electron gas. It also changes the semiconductor conductivity to the metal one. Conduction changes occur in some systems as a result of polymorphic transformations under temperature variations for fixed compositions [1]. 

Another class of conducting oxides are degenerate semiconductors, obtained by heavy doping with suitable foreign atoms. Two oxides, n-Sn02 (doped with Sb, F, In) and n-ln203 (doped with Sn), are of particular interest. These are commercially available in the form of thin optically... 

Degenerate semiconductors can be intrinsic or extrinsic semiconductors, but in these materials the band gap is similar to or less than the thermal energy. In such cases the number of charge carriers in each band becomes very high, as does the electronic conductivity. The compounds are said to show quasi-metallic behavior. 

Since the electron state density near the Fermi level at the degenerated surface (Fermi level pinning) is so high as to be comparable with that of metals, the Fermi level pinning at the surface state, at the conduction band, or at the valence band, is often called the quasi-metallization of semiconductor surfaces. As is described in Chap. 8, the quasi-metallized surface occasionally plays an important role in semiconductor electrode reactions. 

The concept of a mobility edge has proved useful in the description of the nondegenerate gas of electrons in the conduction band of non-crystalline semiconductors. Here recent theoretical work (see Dersch and Thomas 1985, Dersch et al. 1987, Mott 1988, Overhof and Thomas 1989) has emphasized that, since even at zero temperature an electron can jump downwards with the emission of a phonon, the localized states always have a finite lifetime x and so are broadened with width AE fi/x. This allows non-activated hopping from one such state to another, the states are delocalized by phonons. In this book we discuss only degenerate electron gases here neither the Fermi energy at T=0 nor the mobility edge is broadened by interaction with phonons or by electron-electron interaction this will be shown in Chapter 2. 

Perhaps the most direct evidence of the high effective mass in such materials comes from the work of Shapira et al. (1972) on the conductivity of EuTe and the work of Shapira and Reed (1972) on EuS, an antiferromagnetic material, with sufficient non-stoichiometry to make it a degenerate n-type semiconductor. Figure 3.5 shows the temperature variation of the resistivity for various magnetic... 

Another requirement for efficient photovoltaic energy conversion is that the solar cells have a semiconductor junction with a large built-in potential (Vbi). The doped layers in a p-i-n cell are mainly responsible for determining VH. Ideally, one would want wide-band-gap doped layers that are degenerate or highly conductive. In such a case, the Fermi levels would lie very close to the band edges, and the built-in potential would approach the band gap. 

The bulk electronic properties of extrinsic semiconductors are largely determined by the level of doping that is used to make the materials n-type or p-type. For non-degenerate semiconductors, the electron concentration in the conduction band and the hole concentration in the valence band are related to the Fermi energy EF and to the effective densities of states in the conduction and valence bands (Nc and Ny respectively) by... 

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