Field-effect

The earliest pictures of the density of states in a-Si H were obtained from field-effect (FE) measurements (Spear and LeComber, 1972 Madan et ai, 1976 Madan and LeComber, 1977). These results indicated a surprisingly low g E), particularly near midgap, and disclosed two distinct defect bands within about 0.4 and 0.6 eV of the conduction- and valence-band mobility edges, respectively, as well as a fairly sharp band tail near E. A major factor in these results, of course, was that hydrogenated amorphous silicon was a new and distinct material compared to other forms of amorphous silicon and that indeed g E) was very different and much lower than for the other kinds of material people had studied. The ability to perform field-effect measure- 

Most of the earliest analyses (Spear and LeComber, 1972) required certain approximations to deduce g(E) from the FE data. These included a definite form of the potential band-bending function (parabolic or exponential), zero-temperature statistics, and restrictive assumptions about the charge density. Later work (Madan and LeComber, 1977 Hirose et al, 1979) removed the latter two assumptions and some recent work (Goodman and Fritzsche, 1981 Weisfield et al, 1981 Goodman, 1982) removes all of them. In addition, several groups have presented theoretical modeling of the detailed correspondence between FE data and g E) (Goodman and Fritzsche, 1980 Powell, 1981 Weisfield and Anderson, 1981). 

In the following sections we shall describe the field-effect technique in some detail and present a sample of the experimentally determined densities of states. Some of the issues related to the overall sensitivity and reliability of the FE method to determine a bulk density of states in a-Si H will be examined. We shall also mention some of the recent applications of the FE technique to the study of a great variety of phenomena in a-Si H and related materials. Indeed, in spite of their shortcomings, FE measurements continue to be widely applied in the study of amorphous semiconductors and hence still qualify as one of the primary techniques to determine g(E) in these materials. 

The field-effect method measures the surface sheet conductance as a function of applied voltage at the semiconductor-insulator interface in MOS device structures. This surface conductance depends on the degree of carrier accumulation or depletion at the interface, which in turn depends on the band-bending function for a given applied gate voltage. This band bending is 

Barrier region for MOS device structure shown in accumulation. The semiconductor- insulator interface is at x — 0 and the metal gate contact is at x——. Other parameters are defined in the text. 

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The development of Remote Field Eddy Current probes requires experience and expensive experiments. The numerical simulation of electromagnetic fields can be used not only for a better understanding of the Remote Field effect but also for the probe lay out. Geometrical parameters of the prohe can be derived from calculation results as well as inspection parameters. An important requirement for a realistic prediction of the probe performance is the consideration of material properties of the tube for which the probe is designed. The experimental determination of magnetization curves is necessary and can be satisfactory done with a simple experimental setup. 

Baumert T, Engel V, Meier Ch and Gerber G 1992 High laser field effects in multiphoton ionization of Na2 -experiment and quantum calculations Chem. Rhys. Lett. 200 488... 

Here the ijk coordinate system represents the laboratory reference frame the primed coordinate system i j k corresponds to coordinates in the molecular system. The quantities Tj, are the matrices describing the coordinate transfomiation between the molecular and laboratory systems. In this relationship, we have neglected local-field effects and expressed the in a fomi equivalent to simnning the molecular response over all the molecules in a unit surface area (with surface density N. (For simplicity, we have omitted any contribution to not attributable to the dipolar response of the molecules. In many cases, however, it is important to measure and account for the background nonlinear response not arising from the dipolar contributions from the molecules of interest.) In equation B 1.5.44, we allow for a distribution of molecular orientations and have denoted by () the corresponding ensemble average ... 

Figure Bl.22.4. Differential IR absorption spectra from a metal-oxide silicon field-effect transistor (MOSFET) as a fiinction of gate voltage (or inversion layer density, n, which is the parameter reported in the figure). Clear peaks are seen in these spectra for the 0-1, 0-2 and 0-3 inter-electric-field subband transitions that develop for charge carriers when confined to a narrow (<100 A) region near the oxide-semiconductor interface. The inset shows a schematic representation of the attenuated total reflection (ATR) arrangement used in these experiments. These data provide an example of the use of ATR IR spectroscopy for the probing of electronic states in semiconductor surfaces [44]-...

Leventl-Peetz A, Krasovskll E E and Schattke W 1995 Dielectric function and local field effects of TISe2 Phys. Rev. B 51 17 965... 

Electron tunnelling tlirough monolayers of long-chain carboxylic acids is one aspect of interest since it was assumed tliat such films could be used as gate electrodes in field-effect transistors or even in devices depending on electron tunnelling [24, 26, 35, 36, 37 and 38]- It was found, however, tliat tlie whole subject depends critically on... 

Decades of work have led to a profusion of LEERs for a variety of reactions, for both equilibrium constants and reaction rates. LEERs were also established for other observations such as spectral data. Furthermore, various different scales of substituent constants have been proposed to model these different chemical systems. Attempts were then made to come up with a few fundamental substituent constants, such as those for the inductive, resonance, steric, or field effects. These fundamental constants have then to be combined linearly to different extents to model the various real-world systems. However, for each chemical system investigated, it had to be established which effects are operative and with which weighting factors the frmdamental constants would have to be combined. Much of this work has been summarized in two books and has also been outlined in a more recent review [9-11]. 

Three aspects of the inductive effect have to be considered the inductive effect, the inducto-electromeric or rr-inductive effect, and the direct field effect. The first of these is the one most frequently... 

Excluding the phenomenon of hyperconjugation, the only other means by which electronic effects can be transmitted within saturated molecules, or exerted by inductive substituents in aromatic molecules, is by direct electrostatic interaction, the direct field effect. In early discussions of substitution this was usually neglected for qualitative purposes since it would operate in the same direction (though it would be expected to diminish in the order ortho > meta > para) as the cr-inductive effect and assessment of the relative importance of each is difficult however, the field effect was recognised as having quantitative significance. ... 

In contrast, equilibrium properties have been successfully discussed in terms of the field effect. Notable instances are those of the ionisation constants of saturated dibasic acids, - and of carboxyl groups held in... 

The combined inductive and field effects of these poles do not produce strong discrimination between the m- and /i-positions in nitration m p for. NMe3+, and smaller for the protonated poles). This situation is in marked contrast to that produced by, say, the nitro group ( 9.1.3), and suggests that the —M effect is more discriminating between m- and -positions than is the — I effect. 

Earlier in this book ( 7.2.1) the direct field effect was mentioned in rather general terms. The foregoing discussion brings out the fact that there are three aspects of the effect which have to be considered ... 

Ingold introduces the terms substrate field effect and reagent field effect to describe those aspects of the direct field effect numbered (z) and (3) in 9.1.2. His description of the substituent effect of the trimethylammonio group is thus given substantially in terms of the substrate field effect and the TT-inductive effect, i.e. it is an isolated molecule description. The reagent field effect is seen to be significant in nitration and to produce qualitatively the same 226... 

The first three of a series of papers by Ridd and co-workers on Inductive and Field effects in Aromatic Substitution have appeared. Results of studies of the nitration of 4-phenylp5nidine and of 4-benzylpyridine in aqueous sulphuric acid were reported and use of the usual criteria (para 8.2) showed that in each case the conjugate acid was the species undergoing nitration. The values of where fm refers to the corresponding homocyclic compound (biphenyl or diphenylmethane) when plotted against r, the distance between the... 

The correlation is difficult in the case of thiazole and substituted thiazoles because of different effects field effect and anisotropy of hetero-atoms (110), which are very difficult to describe and calculate. When the importance of these two effeas is determined it is then possible to have a good correlation between tt electron densities and corrected chemical... 

Closely related to the inductive effect and operating in the same direction is the field effect In the field effect the electronegativity of a substituent is communicated not by successive polarization of bonds but via the medium usually the solvent A substituent m a molecule polarizes surrounding solvent molecules and this polarization is transmit ted through other solvent molecules to the remote site... 

Fatty acid synthetase (Section 26 3) Complex of enzymes that catalyzes the biosynthesis of fatty acids from acetate Field effect (Section 19 6) An electronic effect in a molecule that IS transmitted from a substituent to a reaction site via the medium (e g solvent)... 

DCFET. See Doped channel field-effect transistor. 

MESFET. See Metal semiconductor field-effect transistor. 

Metal oxide semiconductor field-effect transistor pOSFEQ... 

MODFET. See Modulation doped field-effect transistor. 

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