Electron mobility selected semiconductors

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). 

As can be seen, a wide variety of energy gaps exists for these semiconductors. However, it is the electron-mobility which is the deciding factor In the selection of materials for LED s. That and their relationship to the energy required for visible emission. These include ZnSe, AlP, GaP, AlAs, AlSb, CdTe. GaAs, and InP. However, AsAs. GaP and AlSb are indirect band-gap semi-conductors and have not been found useful for high brightness LED s. 

Here, the potentiometric selectivity coefficient is given with respect to the hydroxyl ion. Single-crystal lanthanum fluoride is a wide bandgap semiconductor in which the electrical conductivity is due only to the hopping mobility of fluoride ions through the defects in the crystal. It does not respond to the La3+ ion because of the slow ion exchange of that ion. Hydroxyl ion is the only other ion that has appreciable mobility, and is the only known interference. For this reason, the measurements with a fluoride electrode are always done below pH 7, which circumvents this interference. As shown later, the consideration of ionic and/or electronic conductivity of the membrane plays a critical role also in the design of the internal contact in nonsymmetric potentiometric sensors. 

The bulk conductivity a depends on the concentration of charge carriers and on their mobility, either of which can be modulated by exposure to the gas. The first prerequisite of such an interaction is the penetration of the analyte to the interior of the layer. The second is the ability of the gas to form a charge-transfer complex with the selective layer. This process then constitutes secondary doping, which affects the overall conductivity. For a mixed semiconductor, the overall conductivity is determined by the combined contribution from the holes (p) and electrons (n), as given by the general conductivity equation. 

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