Showing papers on "Charge density published in 1985"

Electroosmosis through pores with nonuniformly charged walls

Abstract: The movement of fluid through a charged capillary by the action of an electric field is analyzed in the case where the charge density on the capillary wall varies with axial position. The capillary's radius is assumed much greater than the Debye screening length of the fluid. Our results show that the mean fluid velocity within the capillary is given exactly by the classical Helmholtz equation with the local zeta potential replaced by the average zeta potential, determined by integrating the local value over the length of the capillary. If the magnitude of the local zeta potential exceeds the thermal potential (kT/e), the relationship between zeta potential and wall charge is nonlinear, so that the Helmholtz equation cannot be used to calculate the average charge from a measured electroosmotic flow or streaming potential. Although the mean electroosmotic velocity depends only on the average zeta potential, the fluid velocity field within the capillary is a strong function of the distribution of zeta poten...

Correlation of Radiation Effects in Transistors and Integrated Circuits

Abstract: The effects of ionizing radiation on discrete MOS n- and p-channel transistors are correlated with performance degradation of CMOS integrated circuits. The individual components of radiation induced charge, oxide-trapped charge and interface-state charge, are separated using a subthreshold current technique. Processing splits and post-irradiation biased anneals are used to vary the ratio of oxide-trapped charge to interface-state charge. It is shown that the effective channel mobility depends to first order on the interface-state charge density. Static power supply current is correlated with the n-channel leakage at zero gate voltage while output drive currents are a function of both threshold voltage and channel mobility. Changes in propagation delay of signals through integrated circuits can be understood when both mobility and threshold voltage are considered as a function of the bias dependent charge buildup. A new transistor switching time figure of merit, t/C, which measures the drain to source drive over a full logic level voltage swing at the drain node, is introduced. This index is then shown to correlate with propagation delay in an IC. Finally, performance changes in an IC are modeled using only the measured buildup of oxide-trapped and interface-state charges from transistors as a function of radiation.

Hypernetted chain approximation for the distribution of ions around a cylindrical electrode. II. Numerical solution for a model cylindrical polyelectrolyte

Abstract: The structure of the electrical double layer associated with a cylindrical polyelectrolyte is studied through a simple charged hard sphere/charged hard cylinder model in which the diameter of the ions in the solution is considered. The hypernetted chain (HNC) equation (HNC/MSA version) is established and solved numerically giving the ionic distribution around the polyelectrolyte. Calculations are made for 1‐1 and 2‐2 electrolytes for various values of the concentration, ionic diameter, and polyelectrolyte radius and electrical charge. Using these ionic distributions, excess charge adsorption isotherms, zeta potentials, and mobilities are calculated. These quantities are compared with results of the nonlinear Poisson–Boltzmann (PB) equation and, in the case of the mobility, with electrophoresis measurements for double‐stranded DNA. Important quantitative and qualitative differences between the PB and HNC results are found. The HNC zeta potential is found to be nonmontonic function of various parameters, for example, the charge density. The HNC mobilities are in better agreement with experiment than are the PB results. For the concentrations considered here, no significance is found for any value of the reduced linear charge density parameter.

Ion-aerosol attachment coefficients, ion depletion, and the charge distribution on aerosols

Abstract: Calculations of the ion-aerosol attachment coefficients are carried out for particles with radii between 0.001 and 4 μm in a bipolar ion environment by using the theory (Hoppel, 1977) that includes both image capture and three-body trapping of ions. These attachment coefficients are used to calculate the equilibrium charge distribution on aerosols. The results are compared to Fuchs' theory and to several recent measurements of the equilibrium charge distribution. The experimentally measured charge distributions agree quite well with theory. Recent measurements of aerosol size distributions made with the differential mobility size spectrometer in both continental and oceanic atmospheres are used together with the calculated attachment coefficients to study the sensitivity of the atmospheric ion concentration on the aerosol size distribution.

Comparison of fully relativistic energy bands and cohesive energies of MoSi 2 and WSi 2

Abstract: After correcting small errors in our previous ${\mathrm{WSi}}_{2}$ calculations, we compare the equilibrium bond length and symmetric optical phonon frequency of ${\mathrm{MoSi}}_{2}$ and ${\mathrm{WSi}}_{2}$ calculated self-consistently using the semirelativistic norm-conserving pseudopotential. At the equilibrium bond length, fully relativistic calculations of the energy bands, projected densities of states, cohesive energy, bonding charge density, and Fermi surfaces are compared.

Bond index: relation to second-order density matrix and charge fluctuations

Abstract: It is shown that, in the same way as the atomic charge is an invariant built from the first-order density matrix, the closed-shell generalized bond index is an invariant associated with the second-order reduced density matrix. The active charge of an atom (sum of bond indices) is shown to be the sum of all density-density correlation functions between it and the other atoms in the molecule; similarly, the self-charge is the fluctuation of its total charge.

Quantum theory of local perturbation of the charge-density wave by an impurity: Friedel oscillations

Abstract: The interaction of a single impurity with a charge-density wave (CDW) cannot be described by Ginzburg-Landau theory. In the present paper a one-dimensional microscopic quantum theory is presented considering only the backward scattering of the electrons by the impurity at zero temperature. This theory considers the strong perturbation of the CDW inside the amplitude coherence length, which perturbation is dominated by the Friedel oscillations at short distances.It treats the CDW within the framework of the mean-field approximation, and sums up the backward scattering to all orders in perturbation theory. The main features of a self-consistent treatment of the mean field is briefly outlined and the modification can be embodied into the renormalization of the impurity scattering. The results obtained are sensitive to the impurity-scattering strength. In first order, the results of the rigid CDW are reproduced; in second order, the previous results by Barnes and Zawadowski are obtained. The largest effects are in the strong-scattering region. The following physical quantities are calculated: electron density, ground-state energy, density of states, and the force exerted by the impurity on the CDW as a function of the relative position of the impurity with respect to the CDW, and a solution of the equation of motion is found. Considering the electron density in the intermediate-coupling-strength case, the Friedel oscillations dominate at short distances well inside the amplitude coherence length.In the charge density, the Friedel oscillations and the CDW are additive to a good approximation. Outside the amplitude coherence length, the Friedel oscillations tunnel into the CDW gap. In the density of states at the impurity site, the singularity at the gap edge is smeared out and a pair of bound states appears in the gap if the CDW and the Friedel oscillations are out of phase. Further bound states appear also outside the conduction band. The effective potential describing the interaction of the CDW with the impurity is very nonsinusoidal in the intermediate-coupling-strength region, but becomes more sinusoidal for very weak and very strong coupling. The effect of this nonsinusoidal potential in the equation of motion is in the enhancement of higher harmonics appearing in the narrow-band noise, but their intensities remain monotonically decreasing. Among the observable effects predicted are the following: the temperature dependence of the ratios of the intensities of the harmonics in the narrow-band noise, the effect of the nonsinusoidal potential in the Shapiro steps, and the appearance of Friedel oscillations in NMR and diffraction experiments.

A microscopic model of metal–semiconductor contacts

Abstract: A one‐dimensional local‐density model of the charge density,electrostatic potential, and energetics of metal–semiconductor contacts is constructed. This model is an extension of analogous models of bimetallic junctions to include the gap in the semiconductor excitation spectrum, space charge effects in the semiconductor, and fabrication‐induced charges near the interface. Self‐consistent analysis of the valence‐electron charge redistribution at the model metal–semiconductor interface relative to the corresponding vacuum surfaces reveals a cancellation which makes the height of the rectifying Schottky barrier directly proportional to the observed vacuum work function of the metal in the absence of additional charges induced by atomic relaxations or chemical reactions which occur during the fabrication of the interface. The experimentally observed stability of the Schottky barrier heights against changes is applied bias and semiconductor doping are predicted correctly. The occurrence of interfacial atomic rearrangements and/or chemical reactions is incorporated into the model via the inclusion of charge centers near the interface. Four types of centers are considered: donors, acceptors, and three‐state centers with both positive (U>0) and negative (U<0) effective electron–electron interactions. Applications of the model to describe measured barrier heights on GaSe(0001) and GaAs(110) require approximately 101 4 negative‐U centers per cm2 except in the case of noble metals and Sn on GaSe(0001).

Low‐resistance nonalloyed ohmic contacts to Si‐doped molecular beam epitaxial GaAs

Abstract: We have found evidence that the surface depletion charge density in molecular beam epitaxial n‐GaAs doped heavily with Si approaches the Si concentration. In situ metallization of the as‐grown surface of GaAs uniformly doped with Si at 1×1020 cm−3 yields a specific contact resistivity of 1.3 μΩ cm2, indicating a space‐charge density about equal to the silicon density despite a measured bulk electron density of 4×1018 cm−3. This contact resistivity is among the lowest for nonalloyed ohmic contacts to n‐GaAs. We attribute the large discrepancy between surface space‐charge density and bulk electron density to the amphoteric behavior of silicon in GaAs. Surface Fermi‐level pinning and arsenic stabilization create a surface depletion region where donor site selection predominates, whereas the extrinsic electron density in the bulk causes self‐compensation.

Investigation of Space Charge in High-Density Polyethylene Using Thermal-Pulse Response

Abstract: The thermal-pulse technique for the direct probing of space charge in dielectrics was modified for measurement under short-circuit conditions. The new technique was applied to polyethylene (PE) doped with impurity (antistatic agent), and proved to be effective in outlining the behaviour of the space charge in PE. A positive hetero space charge is formed near the cathode on the application of voltage. With increasing applied field, the total amount of space charge increases and its centroid shifts to the cathode. The electric field at the cathode becomes 1.6–1.7 times as large as the average field. The steep increase of current in the current-voltage characteristics is thought to be closely related to this field enhancement.

Structural and dynamic properties of lithium sulphate in its solid electrolyte form

Abstract: The properties of solid lithium sulphate have been studied by computer simulation. At sufficiently high temperatures, the simulated crystal behaves as a solid electrolyte with lithium ion (jump) diffusion and sulphate group rotation. The atomic radial distribution functions in the rotator phase are discussed in relation to the low temperature, fully ordered, monoclinic structure and the nature of the orientational disorder of the sulphate groups is characterized in terms of tetrahedral rotor functions. The crystal structure factor is found to be sensitive to the model adopted for the charge distribution of the anions; good agreement with experimental neutron diffraction data is obtained when a charge distribution consistent with ab initio quantum mechanical calculations is used. The phase transition whereby the low temperature monoclinic structure transforms to the disordered cubic phase has been investigated by the constant pressure molecular dynamics method. The nature of the lithium ion diffusive motion and its coupling to the anion reorientation, the relaxation of the orientational order and the lattice vibrations are all briefly discussed.

Charge-density excitation on a lateral surface of a semiconductor superlattice and edge plasmons of a two-dimensional electron gas.

Abstract: On the lateral surfaces of a semiconductor superlattice, the structure along the direction of the superlattice axis displays the superlattice periodicity while along the perpendicular direction it is translationally invariant. We investigate the surface-plasma and magnetoplasma modes of such a surface as a function of the components of the wave vector along these two dissimilar directions.

Capacitance-voltage characteristics of Semiconductor-Insulator-Semiconductor (SIS) structure

Abstract: The ideal low- and high-frequency capacitance-voltage curves of a semiconductor(2)-insulator-semiconductor(1) (SIS) structure were first calculated with the insulator thickness, conductivity types and doping concentrations in semiconductor(1) and semiconductor(2) as parameters. The effects of fixed oxide charge and interface trap charge on the low and high frequency capacitance-voltage curves were also calculated. It was found that the fixed oxide sheet charge density with its centroid and the order estimation of the interface trap charge density with its effective type in addition to the insulator thickness, conductivity types and doping concentrations in semiconductor(1) and semiconductor(2) could be estimated from measured low and high frequency capacitance-voltage curves of an SIS structure.