Semiconducting property

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... 

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... 

An excellent review of the semiconducting properties of tellurium is available (6). 

Semiconducting Properties. Sihcon carbide is a semiconductor it has a conductivity between that of metals and insulators or dielectrics (4,13,46,47). Because of the thermal stabiUty of its electronic stmcture, sihcon carbide has been studied for uses at high (>500° C) temperature. The Hall mobihty in sihcon carbide is a function of polytype (48,49), temperature (41,42,45—50), impurity, and concentration (49). In n-ty e crystals, activation energy for ioniza tion of nitrogen impurity varies with polytype (50,51). 

The specific electrical conductivity of dry coals is very low, specific resistance 10 ° - ohm-cm, although it increases with rank. Coal has semiconducting properties. The conductivity tends to increase exponentially with increasing temperatures (4,6). As coals are heated to above ca 600°C the conductivity rises especially rapidly owing to rearrangements in the carbon stmcture, although thermal decomposition contributes somewhat below this temperature. Moisture increases conductivity of coal samples through the water film. 

Table 5.4 Intrinsic semiconducting properties of eeramie oxides...

We did not observe any voltage dependent variation of the tube images. Also, the measured heights of the tubes were comparable with their diameters. Both of these observations indicate that the tubes have rather metallic than semiconducting properties. 

The literature of polyimines is extensive [164-173]. A number of researchers have tried to synthesize high molecular weight polymers but failed due to poor solubility in organic solvents. Polyimines are of great interest because of their high thermal stability [174-176], ability to form metal chelates [174-177], and their semiconducting properties [178-181]. Due to insolubility and infusibility, which impeded characterization of the molecular structure, the application of these polymers is very limited and of little commercial importance. 

Polyfarylene vinylene)s form an important class of conducting polymers. Two representative examples of this class of materials will be discussed in some detail here. There are poly(l,4-phenylene vinylcne) (PPV) 1, poly(l,4-thienylene viny-lenc) (PTV) 2 and their derivatives. The polymers are conceptually similar PTV may be considered as a heterocyclic analog of PPV, but has a considerably lowci band gap and exhibits higher conductivities in both its doped and undoped stales. The semiconducting properties of PPV have been shown to be useful in the manufacture of electroluminescent devices, whereas the potential utility of PTV has yet to be fully exploited. This account will provide a review of synthetic approaches to arylene vinylene derivatives and will give details an how the structure of the materials relate to their performance in real devices. 

These considerations lead to the assumption that the practical aspects of the problem lie in the possibility of obtaining PCS-based thermally resistant materials, catalysts for some chemical reactions, antioxidants, stabilizers, photochromic substances, and materials combining valuable mechanical properties with special electrical (particularly semiconductive) properties. 

This review tries to analyze and evaluate the new phenomena disclosed in this fresh field of polymer chemistry and to outline directions of its further development. The properties of polymeric materials and the methods of their synthesis will be considered only so far as is required for the fulfilment of that task. The semiconductive properties of PCSs will not be discussed in detail, since this problem is dealt with in a number of special reviews2-8 and monographs9-11 including those written with the participation of the present authors4,8 10. Apart from the literature data, the results obtained during the last 15 years by a research team headed by the authors will be considered. The studies have been carried out mainly in the following directions ... 

Bearing in mind the semiconductive properties of PCSs one might expect that these substances, being p-type semiconductors in air, possess photosensitizing activity. We, indeed, have demonstrated40 that PCSs, such as poly(schiff base)s, salts of poly(propynoic acid), or polyquinoline, are active photosensitizers of... 

Varying the semiconducting properties of the catalyst crystal affects the rate of ammonium perchlorate decomposition. 

The semiconducting properties of the compounds of the SbSI type (see Table XXVIII) were predicted by Mooser and Pearson in 1958 228). They were first confirmed for SbSI, for which photoconductivity was found in 1960 243). The breakthrough was the observation of fer-roelectricity in this material 117) and other SbSI type compounds 244 see Table XXIX), in addition to phase transitions 184), nonlinear optical behavior 156), piezoelectric behavior 44), and electromechanical 183) and other properties. These photoconductors exhibit abnormally large temperature-coefficients for their band gaps they are strongly piezoelectric. Some are ferroelectric (see Table XXIX). They have anomalous electrooptic and optomechanical properties, namely, elongation or contraction under illumination. As already mentioned, these fields cannot be treated in any detail in this review for those interested in ferroelectricity, review articles 224, 352) are mentioned. The heat capacity of SbSI has been measured from - 180 to -l- 40°C and, from these data, the excess entropy of the ferro-paraelectric transition... 

The M(dioxime-BR2)2 class of complexes 120-122 with four-coordinate metal ions in a square-planar environment has attracted attention in view of possible columnar M M interactions that may result in interesting semiconducting properties in the solid state [182]. Therefore, a series of nickel(II) complexes... 

The structure of CaB contains bonding bands typical of the boron sublattice and capable of accommodating 20 electrons per CaB formula, and separated from antibonding bands by a relatively narrow gap (from 1.5 to 4.4 eV) . The B atoms of the B(, octahedron yield only 18 electrons thus a transfer of two electrons from the metal to the boron sublattice is necessary to stabilize the crystalline framework. The semiconducting properties of M B phases (M = Ca, Sr ", Ba, Eu, Yb ) and the metallic ones of M B or M B5 phases (Y, La, Ce, Pr, Nd ", Gd , Tb , Dy and Th ) are directly explained by this model . The validity of these models may be questionable because of the existence and stability of Na,Ba, Bft solid solutions and of KB, since they prove that the CaB -type structure is still stable when the electron contribution of the inserted atom is less than two . A detailed description of physical properties of hexaborides involves not only the bonding and antibonding B bands, but also bonds originating in the atomic orbitals of the inserted metal . ... 

In contrast to chloride compounds, niobium oxides have a VEC of 14 electrons, due to an overall anti-bonding character of the a2u state, caused by a stronger Nb-O anti-bonding contribution. In some cases, the VEC cannot be determined unambiguously due to the uncertainty in the electron distribution between the clusters and additional niobium atoms present in the majority of the structures. The 14-electron compounds exhibit semiconducting properties and weak temperature-independent paramagnetism. Unlike niobium chlorides, the oxides do not exhibit a correlation between the electronic configuration and intra-cluster bond distances. 

Trigonal, metallic selenium has been investigated as photoelectrode for solar energy conversion, due to its semiconducting properties. The photoelectrochemistry of the element has been studied in some detail by Gissler [35], A photodecomposition reaction of Se into hydrogen selenide was observed in acidic solutions. Only redox couples with a relatively anodic standard potential could prevent dissolution of Se crystal. 

Rare earth sulfides, selenides, and tellurides show semiconducting properties and have potential for application in thermoelectric generation. Thin film chalcogenides of various rare earths have been prepared by multisource evaporator systems [233]. 

In addition to the stoichiometry of the anodic oxide the knowledge about electronic and band structure properties is of importance for the understanding of electrochemical reactions and in situ optical data. As has been described above, valence band spectroscopy, preferably performed using UPS, provides information about the distribution of the density of electronic states close to the Fermi level and about the position of the valence band with respect to the Fermi level in the case of semiconductors. The UPS data for an anodic oxide film on a gold electrode in Fig. 17 clearly proves the semiconducting properties of the oxide with a band gap of roughly 1.6 eV (assuming n-type behaviour). 

The bis(phthalocyaninato) lanthanide (III) complexes have generated a great deal of interest because they are found to show electrochromic and semiconducting properties [87, 203]. They have been characterized by a range of spectroscopic methods, but much research interest has been directed towards understanding the f-electronic structures and their magnetic properties. In order to understand... 

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