Doping impurity

Fig. 6. Calculated siUcon resistivity vs temperature for the impurity (doping) levels shown, where (—) is -type, (------), n-ty e. Left of the peaks is the...

This kind of microstructure also influences other kinds of conductors, especially those with positive (PTC) or negative (NTC) temperature coefficients of resistivity. For instance, PTC materials (Kulwicki 1981) have to be impurity-doped polycrystalline ferroelectrics, usually barium titanate (single crystals do not work) and depend on a ferroelectric-to-paraelectric transition in the dopant-rich grain boundaries, which lead to enormous increases in resistivity. Such a ceramic can be used to prevent temperature excursions (surges) in electronic devices. 

Nonstoichiometry of the oxides can be due to a number of reasons, such as hydration,159 incomplete oxidation,158 and the generation of defects at interfaces.157 An important factor affecting the chemical composition of the oxides is the incorporation of electrolyte species into the growing alumina. There have even been suggestions to use this for impurity doping of oxides and modifying their properties.161 Various kinds of anion distributions and mechanisms of anion incorporation and their influence on oxide properties have been reported. The problems attracting attention are ... 

The spinel family of oxides with composition AB2O4 has the A and B cations distributed in octahedral and tetrahedral sites in a close-packed oxygen structure (Supplementary Material SI). Impurity doping can take place by the addition of a dopant to octahedral or tetrahedral sites. In this, the spinel family of compounds is quite different from the A2B04 perovskite-related phases of the previous section in that both cation sites are similar in size and can take the same cations. 

Negative temperature coefficient thermistors are made from impurity-doped transition-metal oxides. Donor doping, for example, Fe203 doped with Ti02, produces n-type thermistors. The favored mechanism is the formation of electrons ... 

Intentional impurities, doping After the comments previously reported on impurity effects and on the path towards higher and higher purity, a few remarks may be noteworthy on intentional addition of Impurities . To underline this point we quote... 

We now turn to photoconductive and photodiode detectors, both of which are semiconductor devices. The difference is that in the photoconductive detector there is simply a slab of semiconductor material, normally intrinsic to minimize the detector dark current, though impurity doped materials (such as B doped Ge) can be used for special applications, whereas by contrast, the photodiode detector uses a doped semiconductor fabricated into a p-n junction. 

By far, the most common use for germanium is in the semiconductor and electronics industries. As a semiconductor, germanium can be used to make transistors, diodes, and numerous types of computer chips. It was the first element that could be designed to act as different types of semiconductors for a variety of applications just by adding variable amounts of impurities (doping) to the germanium crystals. 

As shown in Fig. 3.6, for intrinsic (undoped) semiconductors the number of holes equals the number of electrons and the Fermi energy level > lies in the middle of the band gap. Impurity doped semiconductors in which the majority charge carriers are electrons and holes, respectively, are referred to as n-type and p-type semiconductors. For n-type semiconductors the Fermi level lies just below the conduction band, whereas for p-type semiconductors it lies just above the valence band. In an intrinsic semiconductor tbe equilibrium electron and bole concentrations, no and po respectively, in tbe conduction and valence bands are given by ... 

Ions like S2, SeJ and SSe are found to align along the <110> directions in most alkali halides, while in Nal, KBr and KI, the S—bond of S2 is oriented along the <100> direction. In the case of Oj, the p orbitals are parallel to the <100> direction in sodium salts but are parallel to the direction in rubidium and potassium salts. Extensive spectroscopic studies have been reported on molecular ions such as NO, NO3, Cr04 and MnO . Several reviews(Corish et al, 1977 Bridges, 1975 Grimes, 1976) are available on such impurity-doped solids. 

Figure 16. Measured and calculated values of boron and phosphorus diffusiv-ities as a function of total impurity doping. Data are divided into contributions to substitutional impurity diffusion under nonoxidizing conditions, DSj, and the enhanced contribution due to oxidation, AD0. Data are from Taniguchi et al. (44). (Reproduced with permission from reference 45. Copyright 1981 The Electrochemical Society, Inc.)...

Fig. 31 The logarithmic derivative of the resistivity with respect to T 1 versus temperature as a function of impurity doping for samples of TSeF, TTF/TCNQ 0> x = 0 , x = 0.003 A, x = 0.0125 0, x = 0.025. The maxima at ca. 28 K and ca. 36 K correspond to the phase transitions. (After Craven etal., 1977)...

Semi-conductors It refers to those elements whose conductivity increases with the increase in temperature, e.g., impurity doped silicon and germanium. 

Systematic investigations on the dependence of the PPC properties on different growth conditions are still needed to elucidate the nature of the deep level defects which are responsible for PPC. Needless to say, the future development of GaN devices depends critically on the improvements in impurity doping, which would rely heavily on the full understanding of the physics of doped impurities. For many device applications, it is important to eliminate (or minimise) effects of deep level impurities through improved crystal growth processes and device designs. 

This section focuses on a-Si H image pickup tubes and presents a-Si H property requirements and fabrication and impurity doping techniques, as well as the structure of photoconductive targets. Pickup tube characteristics that have been attained and some of their applications will be described. 

For bonding outer s and p electrons there is always (the case of impurity doping in semiconductors that is discussed by Mott is an obvious exception and special case) the relation R < Rc n,l) 2R, but for outer d and f electrons the relation may be reversed since the interatomic distance R is determined by bonding s and p electrons of higher principal quantum number n, i.e. of larger mean radial extension see eq. 5). 

Impurity doping with group III elements (Al, Ga, In, B) or with group IV elements (Si,Ge, Ti Zr, or Hf) incorporated substitutionally on Zn-sites. Alternatively also group VII elements have been used such as F incorporated on an 0-site. The lowest values achieved today range at 1.5 - 2x 10 " Gem rather independent of the method of preparation. 

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