Type. Impurity Levels

2 Conduction Mechanism Conduction Type. Impurity Levels 

Nonstoichiometric Eu-rich samples (and EuSe doped with Gd ) are n-type conductors [5], Shapira etal. [8]. The p-type/n-type border lies between x = 10 and 10 Q -cm at room temperature [1] and at lower temperatures in a magnetic field [5]. The carrier sources are donors, most probably Se vacancies caused by excess Eu. An annealing decreases the donor concentration (increases stoichiometry) which is seen in the decreases of the d.c. and a.c. conductivities, p. 235, and of the dieletric constant, p. 261, and also in the change of the 

At low temperatures the conductivity is dominated by impurity conduction, i.e., thermally activated hopping motion of charge carriers between the localized states of the impurity centers, at high temperatures by ordinary band conduction. A magnetic field favors the band conduction, see the following. 

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Figure 9.9 Impurity levels I in (a) an n-type and (b) a p-type semiconductor C is the conduction band and V the valence band...

Type of process Operating temperature, °C Extraction Crystal conversion Acid concentration, % P3O3 Acid impurity level vs dihydrate acid P3O3 recovery, %... 

Specifications and Standards, Shipping. Commercial iodine has a minimum purity of 99.8%. The Committee of Analytical reagents of the American Chemical Society (67) and the U.S. Pharmacopoeia XXII (68) specify an iodine content not less than 99.8%, a maximum nonvolatile residue of 0.01%, and chlorine—bromine (expressed as chlorine) of 0.005% (ACS) and 0.028% (USP), respectively. In the past these requirements were attained basicaHy only by sublimation, but with processing changes these specifications can be met by direct production of iodine. Previously the impurities of the Chilean product were chiefly water, sulfuric acid, and insoluble materials. Improvements in the production process, and especiaHy in the refining step, aHow the direct obtainment of ACS-type iodine. Also, because of its origin and production process, the Chilean iodine has a chlorine—bromine impurity level of no more than 0.002%. 

The impurity atoms used to form the p—n junction form well-defined energy levels within the band gap. These levels are shallow in the sense that the donor levels He close to the conduction band (Fig. lb) and the acceptor levels are close to the valence band (Fig. Ic). The thermal energy at room temperature is large enough for most of the dopant atoms contributing to the impurity levels to become ionized. Thus, in the -type region, some electrons in the valence band have sufficient thermal energy to be excited into the acceptor level and leave mobile holes in the valence band. Similar excitation occurs for electrons from the donor to conduction bands of the n-ty e material. The electrons in the conduction band of the n-ty e semiconductor and the holes in the valence band of the -type semiconductor are called majority carriers. Likewise, holes in the -type, and electrons in the -type semiconductor are called minority carriers. 

Increase adhesion tension. Maximize surface tension. Minimize contact angle. Alter surfactant concentration or type to maximize adhesion tension and minimize Marangoni effects. Precoat powder with wettahle monolayers, e.g., coatings or steam. Control impurity levels in particle formation. Alter crystal hahit in particle formation. Minimize surface roughness in milhng. 

Feeds to hydrotreatment units vary widely they could he any petroleum fraction, from naphtha to crude residues. The process is relatively simple choosing the desulfurization process depends largely on the feed type, the level of impurities present, and the extent of treatment needed to suit the market requirement. Table 3-12 shows the feed and product properties from a hydro treatment unit. ... 

The unique electronic properties of semiconductor devices arise at the regions where p-typc and ra-typc materials ate in close proximity, as in p-n junctions. Typical impurity levels ate about 0.0001 at %, and their inclusion and distribution need to be very strictly controlled during preparation. Without these deliberately introduced point defects, semiconductor devices of the type now commonly available would not be possible. 

These types of carbon blacks are characferized by particularly high surface area (50-1,500 m g i)-fo-volume rafios and have much lower impurity levels. However, despite the many black products available, only a small number have been reported for fuel cell use (XC72, BP2000, Ketjen EC300J). Currently,... 

The usual picture of cationic chemisorption on a p-type semiconductor is that an electron is transferred from the foreign atom to an impurity level in the solid. The foreign atom is converted to the cation, and a depletion layer is built up in the solid. This occurs in the and C(PAfR regions of Fig. 7, provided that the SJl level lies above the impurity levels. In this case, the ASJl level is vacant and a hydrogen-like foreign atom exists on the surface as the cation. 

An unusual type of cationic chemisorption occurs in the C(9 and Cimpurity levels. In this case an electron is lost from an impurity level for each foreign atom adsorbed, and two electrons are trapped in the C(P level. Now the wave functions for these electrons are small on the foreign atom, which exists therefore on the surface as the cation. This chemisorption is depletive. If the C<9 level lies between the bottom of the conduction band and the impurity levels, the chemisorption is still cationic, but the electrons in the impurity levels play no part in the process, and only one electron is trapped in the C(P level in the neighborhood of the first crystal atom. This chemisorption is cumulative. 

Figure 6.15 Temperature dependence of charge carrier concentration of -type extrinsic germanium with two different As impurity levels. From K. M. Ralls, T. H. Courtney, and J. Wulff, Introduction to Materials Science and Engineering. Copyright 1976 by lohn Wiley Sons, Inc. This material is used by permission lohn Wiley Sons, Inc.

Chapter 4 discussed semiconductivity in terms of band theory. An intrinsic semiconductor has an empty conduction band lying close above the filled valence band. Electrons can be promoted into this conduction band by heating, leaving positive holes in the valence band the current is carried by both the electrons in the conduction band and by the positive holes in the valence band. Semiconductors, such as silicon, can also be doped with impurities to enhance their conductivity. For instance, if a small amount of phosphorus is incorporated into the lattice the extra electrons form impurity levels near the empty conduction band and are easily excited into it. The current is now carried by the electrons in the conduction band and the semiconductor is known as fl-type n for negative). Correspondingly, doping with Ga increases the conductivity by creating positive holes in the valence band and such semiconductors are called / -type (p for positive). 

Extrinsic Semiconductors. Impurity levels can be either donor levels near the empty zone (normal or n-type), or acceptor levels near the filled band (abnormal or p-type). Conductivity in n-type conductors will be due to electrons in the empty band donated by the impurity levels, and in p-type conductors, to positive holes in the previously filled band, arising from the transition of electrons to the impurity acceptor levels. 

Nonstoichiometric composition producing impurity levels can arise in two ways, either (1) excess atoms in interstitial positions or (2) holes in the lattice. Both methods [(1) and (2)] are theoretically possible in n- and p-type semiconductors. 

Bachmann s products were known as Type B RDX and contained a constant impurity level of 8-12%. The explosive properties of this impurity were later utilized and the explosive HMX, also known as Octogen, was developed. The Bachmann process was adopted in Canada during World War II, and later in the USA by the Tennes-see-Eastman Company. This manufacturing process was more economical and also led to the discovery of several new explosives. A manufacturing route for the synthesis of pure RDX (no impurities) was developed by Brockman, and this became known as Type A RDX. 

It was found that the radical type catalytic sites, which crack isohexane via the radical mechanism and adsorb NH3 dissociatively to form NH2 species, are generated on mesoporous silica by the calcination at high temperature. Although the sites, tentatively assigned to the strained siloxane bridges, are generated from purely siliceous structure, the incorporation of aluminum of impurity level enhances the generation of such active sites. 

The presence of the region of weak dependence of the conductivity of alloyed semiconductors on temperature can be explained by tunneling of electrons from one impurity centre to another, unoccupied centre. The necessary condition of the impurity conductivity is the partial filling of the impurity levels. At low temperatures this conduction can be maintained only by semiconductor compensation, i.e. by the simultaneous presence of donor and acceptor impurities. In the case, for instance, of the n-type semiconduc-... 

Figure 18.13 Schematic diagram of the energy levels of silicon with a p-type impurity. (From Wang et al., 1975.)...

Petroleum can be fractionated into four generic types of materials representing general chemical properties. These include saturated hydrocarbons, aromatic hydrocarbons, resins, and asphaltenes. The standard ASTM separation procedure (D2007) for isolating the asphaltenes and the other components in petroleum is based on solubility behavior and chromatography, as shown in Fig. 5. Commerically, many refineries utilize solvent separations to produce a solvent deasphalted oil which has lower impurity levels. 

Semiconductors may also be made from a material which is normally an insulator by introducing an impurity, a process known as doping. Figure 9.9 shows two ways in which an impurity may promote semiconducting properties. In Figure 9.9(a) the dopant has one more valence electron per atom than the host and contributes a band of filled impurity levels I close to the conduction band of the host. This characterizes an n-type semiconductor. An example is silicon (KL3s23p2) doped with phosphorus KL3s13pi), which reduces the band gap to about 0.05 eV Since kT at room temperature is about 0.025 cV, the phosphorus... 

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