Applications, semiconductors conducting composites

Novel materials are thus needed to improve the mechanical and chemical stability of the sensor for practical applications in various conditions and, on the other hand, to improve the immobilization scheme in order to ensure sensor stability and the spatial control of biomolectdes. The most important materials for chemical and biochemical sensors include organic polymers, sol-gel systems, semiconductors and other various conducting composites. This chapter reviews the state-of-the-art biosensing materials and addresses the limitations of existing ones. 

By dispersing carbon black into POD-DPE, conductive composites can be obtained. The presence of carbon black enhances the thermal stability. The resistivity decreases continuously with an increase of pressure. Further, the composite shows a t5 ical semiconductor behavior, characterized by an increase of conductivity with temperature. The thermal and electrical properties and pressure sensitivity make this compound a good candidate for the application in manufacture of pressure sensors for high ambient temperatures. 

The interest of physicists in the conducting polymers, their properties and applications, has been focused on dry materials 93-94 Most of the discussions center on the conductivity of the polymers and the nature of the carriers. The current knowledge is not clear because the conducting polymers exhibit a number of metallic properties, i.e., temperature-independent behavior of a linear relation between thermopower and temperature, and a free carrier absorption typical of a metal. Nevertheless, the conductivity of these specimens is quite low (about 1 S cm"1), and increases when the temperature rises, as in semiconductors. However, polymers are not semiconductors because in inorganic semiconductors, the dopant substitutes for the host atomic sites. In conducting polymers, the dopants are not substitutional, they are part of a nonstoichiometric compound, the composition of which changes from zero up to 40-50% in... 

Chemical vapor deposition in semiconductor processing applications is usually conducted in resistance- or induction-heated quartz tube furnaces. In this type of furnace, it is difficult to achieve uniform deposits consistently, especially if several substrate wafers are processed simultaneously. Part of the difficulty is due to changes in the composition of the reactant gas mixture as it passes through the tube. The creation of uniform turbulence in the gas mixture is also difficult. 

Semiconductor lasers, also known as diode lasers, obtain population inversion between the conduction band and the valence band of a p -junction diode. Various compositions of the semiconductor material can be used to give differ ent output wavelengths. Diode lasers can be tuned over small wavelength inter vals. Such lasers produce outputs in the IR region of the spectrum. They have become extremely useful in CD players, CD-ROM drives, laser printers, and spectroscopic applications, such as Raman spectroscopy. 

The use of zeolite-hosted semiconductor oxides as chemicai sensors towards oxidizing or reducing gases might be attractive. Since the alteration of the conductivity depends on changes of the oxide stoichiometry [93,94], shorter diffusion distances in smaller clusters should result in shorter response times of the sensors. Fast response is a prerequisite for the application of sensors based on changes of the bulk composition, e.g. in air/fuel ratio control devices. 

Chemical bonding and electrical conductivity provide five major categories of engineered materials metals, polymers, ceramics/glasses, composites, and semiconductors. The properties of these materials are dependent on atomic- and microscopic-scale stmcture, as well as on the way in which a given material is processed. Materials science enables the selection of the optimal material for a given application, see also Ceramics Glass Physical Chemistry Polymers, Synthetic Semiconductors. 

Dielectric materials will not conduct electricity and as such are of critical importance as capacitive elements in electronic applications and as insulators. It could be argued, with some justification, that without the discovery of new compositions with very high charge-storing capabilities, i.e., relative dielectric constants k > 1000, the impressive miniaturization of semiconductor-based devices and circuits would not have been as readily implemented. In addition, the traditional use of ceramics as insulators in high-power applications is still a substantial economic activity. 

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