Conduction in semiconductors

Let us dwell on existing key models describing chemisorption induced response of electric conductivity in semiconductor adsorbent. Let us consider both the stationary values of electric conductivity attained during equilibrium in the adsorbate-adsorbent system and the kinetics of the change of electric conductivity when the content of ambient atmosphere changes. Let us consider the cases of adsorption of acceptor and donor particles separately. In all cases we will pay a special attention to the issue of dependence of the value and character of signal on the structure type of adsorbent, namely on characteristics of the dominant type of contacts in microcrystals. 

Yu, D. Wang, C. Wehrenberg, B. L. Guyot-Sionnest, P. 2004. Variable range hopping conduction in semiconductor nanocrystal solids. Phys. Rev. Lett. 92 216802-216806. 

At the early stages the photoconductivity of solid solutions of the leucobase of malachite green in various organic media was investigated [285]. In these systems, carrier transport occurs by direct interaction between the leucobase molecules. No direct participation of the organic matrix in the charge transfer was observed. A model was proposed which links charge transfer in these systems with impurity conduction in semiconductors. 

It is clear from Equation 11.3 that resistivity should approach within 10% of the bulk value when the film thickness exceeds about four times the mean free path. The better the conductor, the smaller the mean free path. Thus, the resistivity approaches the bulk value as the film thickness reaches typical values of 100-200 nm for metallic conductors, or perhaps as much as several micrometers for semiconductors, depending on the intrinsic or doped carrier density. For sufficiently thick metallic films with K 1, the temperature coefficient of resistivity becomes positive, as bulk electron-phonon scattering becomes the primary contribution to resistivity [5]. Conduction in semiconductor films remains activation-limited, and retains a negative temperature coefficient. Figure 11.1 illustrates the dependence of resistivity on film thickness for sputtered... 

As Figure 4.3 illustrates, when thermal energy promotes a bonding electron from the valence band to the conduction band, the released electrons are free to migrate throughout the lattice. However, the vacancies (i.e., holes) left behind are also free to move - in the opposite direction as electrons. One may consider these holes as positively charged species formed from loss of an electron. Thus, electrons and holes represent the two types of carriers that correspond to electrical conductivity in semiconductors. 

The electrical conductivity of electrons in a solid depends on the ability of an electron to move to a higher energy level when accelerated by an electric field. The energy change is very small, so that only partially filled bands can conduct. In semiconductors thermal energy will promote a few valence-band electrons into the conduction band. These electrons can now move in the field. So can the electrons in the valence band whose energies are just below the levels of the promoted electrons. 

We normally define the energy level of electrons in a solid in terms of the Fermi level, eF, which is essentially equivalent to the electrochemical potential of electrons in the solid. In the case of metals, the Fermi level is equal to the highest occupied level of electrons in the partially filled frontier band. In the case of semiconductors of covalent and ionic solids, by contrast, the Fermi level is situated within the band gap where no electron levels are available except for localized ones. A semiconductor is either n-type or p-type, depending on its impurities and lattice defects. For n-type semiconductors, the Fermi level is located close to the conduction band edge, while it is located close to the valence band edge for p-type semiconductors. For examples, a zinc oxide containing indium as donor impurities is an n-type semiconductor, and a nickel oxide containing nickel ion vacancies, which accept electrons, makes a p-type semiconductor. In semiconductors, impurities and lattice defects that donate electrons introduce freely mobile electrons in the conduction band, and those that accept electrons leave mobile holes (electron vacancies) in the valence band. Both the conduction band electrons and the valence band holes contribute to electronic conduction in semiconductors. 

On this basis, solids can be divided into insulators, in which the highest occupied band (the valence band) is completely hlled, while the lowest unoccupied band (the conduction band) is completely empty and metals present a partly empty and partly hlled band (the conduction band). Semiconductors are a particular case of insulators where the energy gap between the top of the valence band and the bottom of the conduction band is small enough that, at nonzero temperature, the smoothing out of the Fermi-Dirac distribution causes an appreciable number of states at top of the valence band to be empty and an equivalent number of states at bottom of the conduction band to be hlled. Note that the conductivity in semiconductors is highly temperature dependent. 

In metals, free electrons are solely responsible for conduction. In semiconductors, the conducting species are electrons and/or electron holes. In ceramics, however, because of the presence of ions, the application of an... 

Theoretical studies of catalytic conversion in a flow reactor reveal that a compensation effect will be observed under certain restrictive conditions. It appears that the compensation effect is observed when two or more coupled transport processes are involved and consequently may be a general law. Compensation effects have been observed in electronic conductivity in semiconductors, diffusion of atoms in solids, etc however, more work is needed to establish its generality. 

Shklovskii, B.I. 1979. Hopping conduction in semiconductors subjected to a strong electric fields. Fiz Tekh Poluprovodn (Leningrad) 13 93 [Sov Phys Semicond 13 53). 

Abstract Most radiation related to nuclear properties is outside the visible part of the electromagnetic spectrum or involves submicroscopic particles, hence is invisible. Detectors -devices to sense the radiation, and perhaps measure its properties - are essential. The emphasis in research has moved from the characteri2ation of radioactivity, through simple nuclear reactions, to explorations of the extremes of nuclear matter, but the central importance of suitable radiation detectors has persisted. This chapter emphasi2es detectors associated with measurements of radioactivity, as opposed to nuclear reactions. Thus, much of the current creative work is excluded, but otherwise the scope of these volumes would at least double. Detectors are classified broadly as based on ionization of gases, conduction in semiconductors, or scintillation. The concluding section is an introduction to systems based on two or more components of one of these basic types. 

Percolation theory has been successfully applied to such diverse phenomena as hopping conductivity in semiconductors [130], gelation in polymer melts [131], permeability of porous rocks [129, 130], spreading of epidemics [132], and spreading of wildfires [133]. In PEFC research, percolation theory has been employed for establishing relations between water uptake of polymer electrolyte membranes and their proton conductivities [98]. Moreover, percolation concepts... 

The change in the dielectric properties of the polymer matrix, mainly of the dielectric constant, leads to a dramatic change in conductance due to the peculiarities of the hopping mechanism. In the course of our research [41] we treated the hopping conduction in polymer-nanocomposites in the framework of the Shklovski-Efros model of hopping conduction in semiconductors. The basic equation which determinies the composite conductivity in this case is ... 

Bhandari, C.M. and Rowe, D.M. (1988) Thermal Conduction in Semiconductors, John Wiley Sons, Inc., New York. 

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