Theories of electronic conduction

The theories of conductivity are concerned with the calculation of the energy-dependent conductivity o(E). There is no conductivity within the localized states at zero temperature, and Sections 7.4.1 and 

A further consideration in applying the conductivity theories to a-Si H is the possibility of long range inhomogeneities in the material. The conductivity is a spatial average over the complete conduction path of the carrier, which may be different from the local conductivity at any particular site. The role of inhomogeneities are considered in Section 7.4.5. 

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For many years, during and after the development of the modem band theory of electronic conduction in crystalline solids, it was not considered that amorphous materials could behave as semiconductors. The occurrence of bands of allowed electronic energy states, separated by forbidden ranges of energy, to become firmly identified with the interaction of an electronic waveform with a periodic lattice. Thus, it proved difficult for physicists to contemplate the existence of similar features in materials lacking such long-range order. 

Fig. 17.8. Model of a particulate redox enzyme upon which the theory of electron conduction enzymes is based. Site X in the particle acts as electrode for the redox couple X and develops an equilibrium potential determined by the extent of the reduction of X. Site Y, on the opposite side, acts in the same way for Y. The potential difference causes an electronic current between sites X and Y and within the particle (from Ref. 35 with permission).

Evidence of the electrical conductivity of DNA and of its important mechanisms has been discussed for a long time and has led to a theory of electron conduction in biopolymers [25, 82]. From this it appeared that the major carrier of conductivity is either electronic or ionic, depending on the temperature of the sample, the water content, and the fact that the conductivity of native samples is higher than that of denatured samples. Following electrochemical oxidation of dsDNA and ssDNA in electrolyte solutions over a wide range of pH, interesting electrochemical properties of a glassy carbon electrode with dsDNA or ssDNA adsorbed on the electrode surface were observed [68]. 

FIG. 11.9 Schematic representation of the band theory of electronic conduction (A) insulators (B) semi-conductors and (C) conducting materials. The hatched bands are allowed bands, the cross hatched bands are occupied. 

There are four experimental techniques that are commonly used to gain information about the electronic transport in a-Si H dc conductivity, the drift mobility, thermopower, and the Hall effect. Sections 7.1 and 7.2 describe these measurements and how the information about electronic conduction in Sections 7.3 and 7.4. 

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