Unbalanced Charges Research Articles

Schottky barrier depletion modification-a source of output conductance in submicron GaAs MESFETs

This investigation offers a new explanation for the output conductance in submicron GaAs MESFET characteristics. Prior to the avalanche breakdown a sharp rise in the reverse Schottky barrier current, I/sub gs/ is observed at a potential where drain-to-source current, I/sub ds/ saturates. This could be attributed to the fact that after the onset of current saturation there is an increase in the effective channel height of the device as a function of drain-to-source voltage, V/sub ds/. Experimental data suggest that by increasing V/sub ds/, there are more unbalanced positive ionic charges in the gate depletion toward the drain-side of the Schottky barrier. The electric field lines originated by these uncompensated charges induce an opposite charge density in the gate electrode. This modifies the gate biasing and hence the Schottky barrier depletion. As a result there is a wider available channel crossection for the flow of I/sub ds/(V/sub ds/) and consequently the current-voltage (I-V) characteristics exhibit a positive slope after V/sub ds/ saturation.

Electrostatic charging effects in fast H + interactions with thin Ar films

We have studied electron emission, luminescence and sputtering from thin Ar films excited by 2 MeV protons. We varied the voltage, V a, of the anode surrounding the target, which extracts electrons from the film and results in unbalanced positive charges. The resulting large internal electric fields alter sputtering and luminescence. At the beginning of irradiation of a freshly deposited film, we observe that a positive anode voltage of a few hundred volts produces a large, catastrophic increase in sputtering which gradually disappears as irradiation progresses. We discuss the results in terms of dielectric breakdown induced by macroscopic charging associated with deep charged defect sites.

Helping students to understand that outward currents depolarize cells.

The physiology of excitable membranes is a fundamental topic in neuroscience and physiology courses at graduate and undergraduate levels. From the building blocks of ionic gradients and membrane channels whose permeability is selective and variable, we build the concepts of resting potential, action potential, and propagation in neurons and muscle fibers. Many students have an intuitive understanding of the movements of ions and the associated changes in membrane potential. For example, potassium ions leaving a cell through potassium-selective channels become unbalanced positive charges on the outside of the cell (and leave unbalanced negative charges on the inside), thus producing a potential across the membrane with the inside negative with respect to the outside. Later, when we discuss the local circuit currents that underlie propagation or the basis for extracellular stimulation, we make the general statement that "outward currents depolarize cells." Students respond with utter disbelief. Two simple additions to a discussion of membranes are suggested that permit the formulation of a consistent set of rules that apply to everything from the resting and action potentials of nerve and muscle through synaptic potentials and stimulation techniques.

Chemical and compositional changes induced by ion implantation in SiC and resulting hydrogen permeation properties

Crystalline SiC films are of technological interest because of potential applications in optoelectronics, in high-temperature semiconducting devices, as a fusion first-wall material (coated onto steel), and as a material with high wear resistance (again coated onto steel). Much of the usefulness of crystalline SiC is preserved in the amorphous state, a-SiC. Many investigations have been directed towards the study of the structure and mechanical properties of this kind of amorphous film; nevertheless, little is known about bombardment-induced chemical and compositional changes. Here we present an overview of observations and interpretations relevant to bombardment-induced structural and compositional changes in SiC and other carbides. Particular emphasis is placed on N +-implantation, which leads to SiC x N y compounds with C being gradually substituted with N. It is shown that N-implantation strongly enhances the a-SiC-stainless steel adhesion by promoting the formation of new chemical bonds at the ceramicmetal interface. By examining data from the literature, we find more or less clear evidence for a number of radiation-induced transport processes. These include (a) Gibbsian segregation, (b) vaporization, (c) long-range forces such as that provided by unbalanced charges, (d) defect-induced transport, and (e) preferential loss of C from the surface. These phenomena help to explain composition changes variously at or beneath the surface of SiC. When low to medium energy hydrogen collides with a carbide surface both chemical and transport-related processes occur. In the future generation of fusion reactors very high levels of tritium could be present in either a trapped or mobile form in walls and other internal parts. Thus tritium permeation even through thick wall materials may present problems. The permeation of hydrogen through a-SiC films is discussed showing that it can be strongly affected by surface changes.

Composition changes in Ar + and e −-bombarded SiC: an attempt to distinguish ballistic and chemically guided effects

We study with Auger electron spectroscopy (AES) the composition changes in amorphous SiC films with excess (i.e. free) C induced by bombardments either with 30 keV Ar + on with 3 KeV e −. In Ar +-bombarded films the C accumulates at the surface without apparent depletion in the bombarded region, while the Si accumulates at the surface accompanied by major depletion near the Ar + range. Significantly movement of C to the surface occurs also due to e − bombardment. The C profile modifications are explained by assuming that a long range force (especially that provided by unbalanced surface charges) operates on the free C present in the SiC films, whereas the Si transport towards the surface is promoted by the bombardment-induced defects. Whether the direction of the Si motion is determined by the C enrichment at the surface (which traps the Si chemically), or by an interstitial flux (as has been established with such systems as NiSi) cannot be determined, but it is still possible to assert that the composition changes are more nearly due to defects (whence showing chemical guidance than ballistic (whence random). We finally note that the C at the surface will tend to be sputtered away.

Oxygen-vacancy complex in silicon. II. 17O electron-nuclear double resonance.

An electron-nuclear double-resonance (ENDOR) study was performed on the negatively charged oxygen-vacancy complex in silicon. By introducing the isotope $^{17}\mathrm{O}$ (nuclear spin I=(5/2) to an enrichment of about 40%, it was possible to detect ENDOR transitions of this nucleus. In the experiment the magnetic hyperfine interaction with the unpaired defect electron and the nuclear quadrupole interaction were measured. The anisotropic hyperfine interaction could be described by a small allowed admixture of p orbitals. In such a simple one-electron description the Fermi-contact interaction could not satisfactorily be understood. The quadrupole interaction could be explained as arising mainly from unbalanced charges in the oxygen p orbital that points in the direction of the silicon neighbors with which a Si-O-Si ``molecule'' is formed. The technique of field-swept ENDOR was used to determine the relative signs of the hyperfine- and quadrupole-interaction constants. The strong interference of the two interactions gives rise to the occurrence of forbidden EPR transitions and a very complicated angular dependence of the ENDOR frequencies upon rotation of the magnetic field. As a consequence, for some orientations no ENDOR was observed at all.

On the B to A conformation change of the double helix.

AbstractWe have investigated the B to A conformation change of DNA double helices by a new method “soft‐mode analysis.” We find theoretically that a mode does soften when the vibration normal modes are perturbed by increasing the electrostatic interaction between the unbalanced charges on atoms in the double helix. The same mode also softens for enhanced van der Waals interactions. The mode softening indicates the onset of conformation change. The enhancing of the electrostatic and van der Waals interaction mimic the effect of decreasing the polar nature of the solvent or water of hydration associated with the B conformation DNA. We discuss qualitatively the concept of soft modes and their relation to conformation change as well as their applicability to macromolecules. We discuss previous work in which the normal vibrational modes have been calculated. We also discuss the displacement which comes from the soft mode and show that it correlates very well with that expected for the B to A conformation charge.

Lattice-Dynamics Approach to the Theory of Elastic Dielectrics with Polarization Gradient

A continuum theory of elastic dielectrics including polarization gradient is obtained as the long-wave approximation of a theory of lattice dynamics for the shell model of cubic ionic crystals. The additional energy associated with the formation and relaxation of a free surface is obtained by considering the unbalanced charges and dipole moments on the surface. The material coefficients of the continuum theory are related to the lattice properties, and their numerical values are calculated for NaI, NaCl, KI, and KCl. The surface energy density of a free half-space predicted by the continuum theory is compared with experimental results as well as other theoretical results based on discrete models.

ION EXCHANGE IN CLAYS AND OTHER MINERALS

Ion exchange in clays and other minerals is dependent on the crystalline structure of the mineral and on the chemical composition of any solution in contact with the mineral. The structures of clay minerals and zeolites are briefly described to provide a background for the discussion of their ion-exchange reactions. Ion exchange in these minerals is a reversible chemical reaction that takes place between ions held near a mineral surface by unbalanced electrical charges within the mineral framework and ions in a solution in contact with the mineral. Generally the excess charge on the mineral is negative, and it attracts cations from the solution to neutralize this charge. The chemical reactions in ion exchange follow the law of mass action, but the reactions are restricted by the number of exchange sites on the mineral and by the strength of the bonding of the exchangeable cations to the mineral surface. Titration of H-clays with bases shows that montmorillonites and “illites” behave like a mixture of two or three different acids, whereas kaolinite, with an indefinite number of exchange sites, behaves like an indefinite number of acids. Ion-exchange capacity is measured in chemical equivalents of base adsorbed at pH 7. Each clay mineral has a range of exchange capacities because of differences in structure and in chemical composition. The ranges (in milliequivalents per 100 grams) are kaolinite, 3–15; halloysite (2H2O), 5–10; halloysite (4H2O), 40–50; montmorillonite, 70–100; “illite,” 10–40; vermiculite, 100–150; glauconite, 11–20; attapulgite, 20–30; and allophane, 70. The common metallic cations found in exchange positions in clay minerals are Ca+2, Mg+2, Na+, and K+. At low pH values H+ replaces other cations. The order of replaceability of the common cations has been found to be: ![Formula][1] Bivalent cations enter the exchange sites preferentially to univalent cations. The common exchangeable cation in most clay minerals in soils is Ca+2. Other exchange phenomena discussed are anion exchange, fixation of cations and anions by clay minerals, effect of environment on cation exchange, and the exchange capacity of zeolites, of rocks, of other minerals, of organic matter and organic complexes, and of amorphous mineral material. [1]: /embed/graphic-1.gif

Effect of Meterization on Revenues and Consumption

When a drought period, several of which we have had in the last ten years, prevails in a community in which the water services are not metered, there is usually a shortage of water or at least a reduction in pressure. Considerable complaint is registered by the people living on the higher elevations when there is insufficient pressure to supply their domestic needs, while the people on the lower elevations are sprinkling lawns, washing cars, etc. Such a complaint is justified as the people living on the higher elevations pay just as much for water service under a flat rate system as those living on the lower levels. Such is a typical case of unbalanced consumption and charges. Metering will correct it and sometimes under conditions like this people will urge metering. As soon as metering is discussed in a community, however, considerable opposition generally arises thereto, with reasons put forth that it will force people to use less water than is necessary in order to keep the water charges from becoming high and burdensome. In reality it is the first step to keep water rates at a minimum. Thanks to the fortitude of water works and thinking municipal officials the installation of water meters is on the increase. In spite of considerable opposition in many cases before meters are installed, the writer knows of no case where meters, once installed, have been discarded for political or other reasons. This experience alone should indicate, in a broad sense, that the installation of meters is economically sound. It does not necessarily follow that as soon as meters are installed in the service lines of a water system total revenues will be on the increase, but it is invariably true that wastage is cut down and net revenue will increase, affecting parts of the plant remote from the meters. There are some cases where it might not be apparent at once that