Semiconducting conductivity

PROPERTIES OF SPECIAL INTEREST Insulating, semiconducting, conducting, and nonlinear optical properties. 

The incorporation of a variety of semiconducting, conducting, dielectric, and magnetic filler materials (e.g., another CP/ICP, QCNs, nanoparticu-... 

The idea of a possible semiconductive mechanism (electrons and holes) in biomaterials is old, as illustrated by the book by Pethig (1979). Later, Takashima (1989) did not mention semiconductivity as a possible mechanism. Indications of an electronic, semiconductive conduction for the DNA molecule have appeared again (Fink and Schdnenberger, 1999). Such experiments are performed under nonphysiological conditions in vacuum, which implies that every water molecule free to do so has disappeared. 

This exponential dependence is typical for semiconduction Conductivity increase with temperature can be described by the relationship... 

Semiconductors may also be made from a maferial which is normally an insulator by infroducing an impurify, a process known as doping. Figure 9.9 shows fwo ways in which an impurify may promote semiconducting properties. In Figure 9.9(a) fhe dopanf has one more valence election per atom fhan fhe hosf and confribufes a band of filled impurify levels 1 close to fhe conduction band of fhe hosf. This characterizes an n-fype semiconductor. An example is silicon (KL3s 3p ) doped wifh phosphoms (KL3s 3p ), which reduces fhe band gap to abouf 0.05 eY Since kT af room femperafure is abouf 0.025 eY the phosphoms... 

The highly conductive class of soHds based on TTF—TCNQ have less than complete charge transfer (- 0.6 electrons/unit for TTF—TCNQ) and display metallic behavior above a certain temperature. However, these soHds undergo a metal-to-insulator transition and behave as organic semiconductors at lower temperatures. The change from a metallic to semiconducting state in these chain-like one-dimensional (ID) systems is a result of a Peieds instabihty. Although for tme one-dimensional systems this transition should take place at 0 Kelvin, interchain interactions lead to effective non-ID behavior and inhibit the onset of the transition (6). 

Polymers. The Tt-conjugated polymers used in semiconducting appHcations are usually insulating, with semiconducting or metallic properties induced by doping (see Flectrically conductive polymers). Most of the polymers of this type can be prepared by standard methods. The increasing use of polymers in devices in the last decade has led to a great deal of study to improve the processabiUty of thin films of commonly used polymers. 

Polyaniline (PANI) can be formed by electrochemical oxidation of aniline in aqueous acid, or by polymerization of aniline using an aqueous solution of ammonium thiosulfate and hydrochloric acid. This polymer is finding increasing use as a "transparent electrode" in semiconducting devices. To improve processibiHty, a large number of substituted polyanilines have been prepared. The sulfonated form of PANI is water soluble, and can be prepared by treatment of PANI with fuming sulfuric acid (31). A variety of other soluble substituted AJ-alkylsulfonic acid self-doped derivatives have been synthesized that possess moderate conductivity and allow facile preparation of spincoated thin films (32). 

Several kinds of conduction mechanisms are operative in ceramic thermistors, resistors, varistors, and chemical sensors. Negative temperature coefficient (NTC) thermistors make use of the semiconducting properties of heavily doped transition metal oxides such as n-ty e Ti O andp-ty e... 

Both the polymers are dark in color and exhibit semiconductivity and paramagnetism. The electric conductivity measurements are performed on peUets and on thin films in sandwich and surface ceUs. 

The polysdanes are normally electrical insulators, but on doping with AsF or SbF they exhibit electrical conductivity up to the levels of good semiconductors (qv) (98,124). Conductivities up to 0.5 (H-cm) have been measured. However, the doped polymers are sensitive to air and moisture thereby making them unattractive for practical use. In addition to semiconducting behavior, polysilanes exhibit photoconductivity and appear suitable for electrophotography (qv) (125—127). Polysdanes have also been found to exhibit nonlinear optical properties (94,128). 

The carbon black in semiconductive shields is composed of complex aggregates (clusters) that are grape-like stmctures of very small primary particles in the 10 to 70 nanometer size range (see Carbon, carbon black). The optimum concentration of carbon black is a compromise between conductivity and processibiUty and can vary from about 30 to 60 parts per hundred of polymer (phr) depending on the black. If the black concentration is higher than 60 phr for most blacks, the compound is no longer easily extmded into a thin continuous layer on the cable and its physical properties are sacrificed. Ionic contaminants in carbon black may produce tree channels in the insulation close to the conductor shield. 

Table 3 summarizes some of the present state-of-the-art parameters obtained for undoped and doped i -SiH(F) material thus produced. The device-quahty material exhibits semiconductivity because In G vs 10 /Texhibits a straight line with a conductivity activation energy of eV, which is... 

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