Electronic semiconductor

Rhenium hexafluoride is used for the deposition of rhenium metal films for electronic, semiconductor, laser parts (6—8), and in chemical vapor deposition (CVD) processes which involve the reduction of ReF by hydrogen at elevated (550—750°C) temperatures and reduced (<101.3 kPa (1 atm)) pressures (9,10). 

Electronics, semiconductors 36, 3674 16 30 Intel, Texas Instruments, Motorola... 

Electrothermal atomizers are also suitable for AFS as, when an inert gas atmosphere is used, quenching will be minimized. In the nuclear, electronic, semiconductor and biomedical industries where detection limits have to be pushed as low as 1 part in lO (or 0.1 pg g- in the original sample), electrothermal atomization with a laser as excitation source (LIF-ETA) may be used. Figure 6.5 shows schematically a common way of observing the fluorescence in LIF-ETA. The fluorescence signal can be efficiently collected by the combination of a plane mirror, with a hole at its centre to allow excitation by the laser, positioned at 45° with respect to the longitudinal axis of the tube and a lens chosen to focus the central part of the tube into the entrance slit of the fluorescence monochromator. 

Pulsed curing systems are widely used in the manufacture of medical devices, electronics, semiconductors, and optical fibers. Pulsed xenon lamps can be made in a variety of shapes to fit specific requirements, such as 360° illumination. Examples of different designs are in Figures 3.5 and 3.6. 

Thermal treatment of polyacrylonitrile at 200-300 °C leads to the appearance of conjugated bonds and electronic semiconductor properties. The main photoconductivity maximum is situated at 420 nm. An increase of the temperature treatment shifts the photosensitivity to the long wavelengths. The estimated mobilities were from 10" 7 to 104m2 V"1 s"1. The main results obtained proved the sufficiency of the conjugated bonds for the appearance of the semiconductive properties. 

In an exactly analogous but opposite manner, doping germanium with arsenic (five valence electrons) results m an excess of electrons and a donor (the arsenic donates the fifth electron or n-type (n = negative electrons) semiconductor. The conduction can be viewed in terms of an enogy diagram in which the electrons can be removed from the impurity arsenic atoms to the conduction band of the semiconductor (Fig. 7.27). 

The properties are very sensitive to composition and the charge carriers are apparently positive. Other studies have shown poly(acenaph-thalene) to be only slightly photo-conductive while the nitrated polymer exhibits a photocurrent dependent upon the degree of nitration (100). Since the number of mobile n electrons is the same as in poly (vinyl naphthalene), the authors conclude that some form of stereoregularity is required for enhanced conductivity. Complexes of poly(vinyl anthracene) with halogen molecules show enhanced conductivity and reduced activation energy which is thought to be typical of an electronic semiconductor (101). 

Phthalocyanines were chosen for these experiments because they are electronic semiconductors and because they are quite stable materials — an important consideration in fabricating any practical gas-detecting device. A considerable body of literature exists describing the physical and chemical properties of the phthalocyanines. A review of the work prior to 1965 is contained in the chapter by A. B. P. Lever in Volume 7 of Advances in Inorganic Chemistry and Radiochemistry (2). Electrical properties of phthalocyanines have been receiving increased attention in recent years. The photoconductivity of metal-free phthalocyanine has been studied in detail (3,4). Electrical properties of lead phthalocyanine have been studied extensively, especially by Japanese workers (5, ,7,8i). They have also studied the alteration of the conductivity of this material upon exposure to oxygen ( ,10.). The effects of a series of adsorbed gases (0, , CO, and NO) on the conductivity of iron phthalo-... 

Consumer Electronics Semiconductors Cellular Phones Computers Accessories Digital Cameras Fuel-Cell Technology LCD Displays Memory Products... 

Other than the mutually photosensitive components, coupling between one photosensitive and another nonsensitive (e.g., very wide bandgap) semiconductor may also have positive effects on the photocatalytic performance of the sensitive one. For example, Kisch and Weiss (Weiss et al. 2001 Kisch and Weiss 2002) studied the Si02-supported CdS photoelectrode in an organic addition reaction, and found that the enhanced photocatalytic activity was related to the changes in bandgap and flat-band potential of CdS, which originates from an electronic semiconductor-support interaction mediated by [SiJ-O-Cd-S bonds. 

Kisch, H. and H. Weiss (2002). Tuning photoelectrochemical and photocatalytic properties through electronic semiconductor-support interaction. Advanced Functional Materials, 12(8), 483 488. 

Weiss, H., A. Fernandez and H. Kisch (2001). Electronic semiconductor-support interaction -a novel effect in semiconductor photocatalysis. Angewandte Chemie-Intemational Edition, 40(20), 3825-3827. 

As well known from semiconductor physics, in non-metals electrons are, at finite temperatures, excited from the highest occupied band to the lowest unoccupied band to form excess electrons in the conduction band and electron holes in the valence band. Owing to long-range order each crystal possesses a certain amount of free electronic carriers. The mixed conductor which exhibits both ionic and electronic conductivity, will play an important role in this text, since it represents the general case, and pure ionic and electronic (semiconductors) conductors follow as special cases. 

Most classical electronic semiconductors, such as Ge, Si, GaAs, InP, GaP, CdSe or CdS, are characterized by an electronic structure, in which the upper state of the valence band is occupied by electrons involved in the chemical bonding between the atoms of the crystal. Holes occurring at the surface are equivalent to missing bonds, which leads to a weakening of the structural stability and finally, in contact with a liquid, to an anodic decomposition. Taking a binary semiconductor as an example, the decomposition reaction is given for instance by... 

Although, of course, Ge atoms are immobile, electrons can move in from a neighbouring Ge atom to the effect that the Ge atom moves the other way. Phenomenologically speaking, holes are mobile. Their mobilities should not be underestimated in Ge crystals at room temperature, they are about half as high as those for electrons. The upshot is that in electronic semiconductors, electrons and holes assume the functions that anions and cations have in electrolytes and, consequently, in establishing the space charge density in the double layer. 

The range of possible applications of multiscale simulations methods to electrochemical systems is extensive. Electrochemical phenomena control the existence and movement of charged species in the bulk, and across interfaces between ionic, electronic, semiconductor, photonic and dielectric materials. The existing technology base of the electrochemical field is massive and of long-standing [15, 16]. The pervasive occurrence of these phenomena in technological devices and processes, and in natural systems, includes ... 

In all of these old and new industries, the key scientific cornerstone is the understanding of electrochemical phenomena, which control the existence, movement, and reaction of species in the bulk and at the interfaces between phases. The range of such materials is truly staggering and includes ionic, electronic, semiconductor, photonic, and dielectric materials. 

The development of semiconductors is clearly among the most significant technological achievements to evolve from the study of solid-state chemistry and physics. Aside from their well-known applications in computers and electronics, semiconductors are also used in a wide variety of optical devices such as lasers, light-emitting diodes, and solar panels. The diversity of applications can be readily understood with only a basic understanding of the theory behind these materials. 

In the preceding subsection, the number of electronic defects was fixed by the doping level, especially at lower temperatures, and the concepts of donor and acceptor localized levels were discussed. The band picture for nonstoichiometric electronic semiconductors is very similar to that of extrinsic semiconductors, except that the electronic defects form not as a result of doping, but rather by varying the stoichiometry of the crystal. 

Chapman et al. (182-184) extended the work of Bevan et al. (19) by establishing a temperature difference of about 20° between the two platinum electrodes, and using the Seebeck effect (22,185) they determined the thermoelectric potential (g). From the sign of it was ascertained that chromia is a p-t3rpe (oxygen excess or positive hole) semiconductor in an oxygen atmosphere, and an -type (metal excess or electron) semiconductor in a hydrogen atmosphere. 

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