Semiconductor, conductivity free electron mobility

Electrical conductivity is due to the motion of free charge carriers in the solid. These may be either electrons (in the empty conduction band) or holes (vacancies) in the normally full valence band. In a p type semiconductor, conductivity is mainly via holes, whereas in an n type semiconductor it involves electrons. Mobile electrons are the result of either intrinsic non-stoichiometry or the presence of a dopant in the structure. To promote electrons across the band gap into the conduction band, an energy greater than that of the band gap is needed. Where the band gap is small, thermal excitation is sufficient to achieve this. In the case of most iron oxides with semiconductor properties, electron excitation is achieved by irradiation with visible light of the appropriate wavelength (photoconductivity). 

The electrical conductance of semiconductors is derived from the mobility of charge carriers, holes h+ in the valence band and free electrons e in the... 

Technically this means rather more than bad conductor . Metals conduct electricity because some of their electrons come free of their parent atoms and are at liberty to roam through the material. Their motion corresponds to an electrical current. A semiconductor also has wandering electrons, but only a few. They are not intrinsically free, but can be shaken loose from their atoms by mild heat some are liberated at room temperature. So a semiconductor becomes a better conductor the hotter it is. Metals, in contrast, become poorer conductors when hot, because they gain no more mobile electrons from a rise in temperature and the dominant effect is simply that hot, vibrating atoms obstruct the movement of the free electrons. 

In equation 3, ran is the effective mass of the electron, h is the Planck constant divided by 2/rr, and Eg is the band gap. Unlike the free electron mass, the effective mass takes into account the interaction of electrons with the periodic potential of the crystal lattice thus, the effective mass reflects the curvature of the conduction band (5). This curvature of the conduction band with momentum is apparent in Figure 7. Values of effective masses for selected semiconductors are listed in Table I. The different values for the longitudinal and transverse effective masses for the electrons reflect the variation in the curvature of the conduction band minimum with crystal direction. Similarly, the light- and heavy-hole mobilities are due to the different curvatures of the valence band maximum (5, 7). 

The microwave conductivity is proportional to the number of free electronic charge carriers multiplied by their respective mobilities (the contribution of ions and dipoles can be neglected in a first approximation.). The microwave sensitivity constant S must be obtained by calibration. The potential-dependent photoinduced microwave conductivity (PMC) of a semiconductor resulting from illumination has been calculated analytically as a function of the interfacial charge-transfer and surface recombination rates (Schlichthorl and Tributsch, 1992). The starting point is the general set of equations of the form... 

The microwave reflectivity of a semiconductor depends on the sample conductivity, i.e. on the density and mobility of free electrons and holes. Photoexcitation of the sample brings about an instantaneous increase of electron and hole densities and a corresponding change in microwave reflectivity. After excitation by a laser pulse, the decay of minority carriers by bulk and surface recombination, as well as by trapping, can be followed by the change in microwave reflectivity. For small changes in carrier density, the relationship between microwave reflectivity and carrier density is linear. 

A series of examples has become known recently, and more are reported in this volume, of catalytic reactions on oxide surfaces, involving electron transfer from reactant molecules to the catalyst, or vice versa. The general electronic concept of catalytic activation, first established for metals and alloys, has thus been extended to semiconductors. It appears certain that mobile quasi-free electrons or positive holes can migrate to the surface and can there bind reactant molecules in a charged or polarized state. This presupposes the presence of electrons in the conduction band (or of holes in the valence band), which in normal oxide semiconductors contains appreciable concentration of electrons only at elevated temperatures. Hence, the examples mentioned refer to high-temperature catalysis (N2O decomposition, CO oxidation). At ordinary temperatures, only those substances capable of releasing electrons from surface atoms or surface bonds, i.e., solid Lewis bases, are suitable as catalysts. This has been shown (I) to be true for the decomposition of ozone by various metal oxides. 

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