Showing papers on "Diffusion current published in 2001"

Drift without flux: Brownian walker with a space dependent diffusion coefficient

Abstract: Space-dependent diffusion of micrometer-sized particles has been directly observed using digital video microscopy. The particles were trapped between two nearly parallel walls making their confinement position dependent. Consequently, not only did we measure a diffusion coefficient which depended on the particles' position, but also report and explain a new effect: a drift of the particles' individual positions in the direction of the diffusion coefficient gradient, in the absence of any external force or concentration gradient.

Two-dimensional analysis of double-layer heterojunction HgCdTe photodiodes

Abstract: In this paper, the performance of P-on-n double-layer heterojunction (DLHJ) HgCdTe photodiodes at temperature 77 K is analyzed theoretically. Calculation has been performed for the backside-illuminated configuration. The effect of photodiode base layer geometry on quantum efficiency and R/sub o/A product is analyzed. Also, the effect of lateral collection of diffusion current and photocurrent on photodiode parameters has been shown. Moreover the dependence of the p-n junction position within heterostructure on the band-gap energy profiles and photodiode performance is presented. Finally, the influence of the composition gradient and p-side doping concentration on photodiode parameters is described briefly.

Diffusion in stationary flow from mesoscopic nonequilibrium thermodynamics.

Abstract: We analyze the diffusion of a Brownian particle in a fluid under stationary flow. By using the scheme of nonequilibrium thermodynamics in phase space, we obtain the Fokker-Planck equation that is compared with others derived from the kinetic theory and projector operator techniques. This equation exhibits violation of the fluctuation-dissipation theorem. By implementing the hydrodynamic regime described by the first moments of the nonequilibrium distribution, we find relaxation equations for the diffusion current and pressure tensor, allowing us to arrive at a complete description of the system in the inertial and diffusion regimes. The simplicity and generality of the method we propose makes it applicable to more complex situations, often encountered in problems of soft-condensed matter, in which not only one but more degrees of freedom are coupled to a nonequilibrium bath.

Diffusion Length Measurements in p-HgCdTe Using Laser Beam Induced Current

Abstract: The minority carrier diffusion length in p-HgCdTe is a key indicator of material quality and gives an indication of n-on-p diode performance when the zero bias resistance is diffusion limited. We present results of a temperature dependent study of diffusion length in p-HgCdTe using laser beam induced current (LBIC). Carriers are collected by a p-n junction formed using standard diode junction formation conditions, and thus not necessarily extending to the substrate. Two-dimensional modeling is used to examine the validity of results obtained using this geometry, as compared to the more standard diffusion length test structure geometries, which are harder to fabricate. The temperature dependence of the diffusion length can be compared with theoretical models to determine the dominant recombination mechanisms.

Diffusivity and conductivity of a primitive model electrolyte in a nanopore

Abstract: Equilibrium and non-equilibrium molecular dynamics simulations are applied to obtain the diffusion coefficient and electric conductivity of ions in dilute electrolytes confined in neutral cylindrical pores. The electrolyte is described with the restricted primitive model and the wall of the pore is modelled as a soft wall. The equilibrium molecular dynamics simulations show that the axial diffusion coefficient of ions decreases with increasing confinement. For a fixed pore radius the diffusion coefficient decreases with increasing number density of the ions. The current response of the system to an applied electric field is maintained at constant temperature by Gaussian isokinetic equations of motion, and at constant concentration by periodic boundary conditions with recycling of ions in the axial direction. The electric conductivity is calculated from the current density and the electric field applied for different pore sizes. In contrast to the trend in diffusivity, conductivity increases slightly in sm...

The initial time layer problem and the quasineutral limit in a nonlinear drift diffusion model for semiconductors

Abstract: The limit of vanishing Debye length in a nonlinear bipolar drift diffusion model for semiconductors is studied. The limit is performed on both the initial time layer and the original time scale. In both cases the limiting problems are identified. A main tool in the analysis are entropy methods.

Improved extraction of the activation energy of the leakage current in silicon p–n junction diodes

Abstract: An accurate method is proposed for the extraction of the activation energy ET from the volume generation current density JgA in silicon p–n junctions. It combines temperature-dependent current–voltage (I–V) and capacitance–voltage measurements on an array of diodes with different geometry, in order to separate the peripheral from the volume components. The JgA can be found from the volume leakage current by subtraction of the volume diffusion current JdA, which is calculated from the forward I–V characteristic. To derive the correct slope from an Arrhenius plot of the JgA, several additional corrections have been applied. One is the temperature dependence of the depletion width, which is derived from the corrected volume capacitance. The most important ET change is shown to come from the temperature dependence of the recombination lifetime.

Built-in longitudinal field effects in sub-100-nm graded Si/sub 1-x/Ge/sub x/ channel PMOSFETs

Abstract: Simulations and experiments have been conducted to study the built-in longitudinal field effects in sub-100-nm graded Si/sub 1-x/Ge/sub x/ channel vertical PMOSFETs. Results are compared for two operation modes with the source/drain (S/D) contacts interchanged, i.e., the "normal" mode, in which the built-in field is favorable to the drift, and the "reverse" mode, in which the built-in field is a retarding one. The linear drain current at V/sub DS/=-0.1 V is found to he very similar between the two modes. Furthermore, if the body is grounded, the saturation drain current, as simulated, in the normal mode is 20% lower than that of the reverse mode. With floating body effects, however, experimentally it is observed that the saturation drain current in the normal mode is 14% higher than in the reverse mode. The simulation reveals that in the linear condition, the benefits of the accelerating built-in field in the channel is negated by a weaker source injection resulting from a larger source-side built-in potential at the Si source junction, and also by a reduced or even reversed diffusion current in the normal mode; in the saturation condition, the built-in field is overshadowed by the large external field due to high drain biases. The measured off-state leakage current at T/sub DS/=-1.6 V in the normal mode is 100 times higher than in the reverse mode due to the floating body effects.

Lateral current spreading in unipolar semiconductor lasers

Abstract: Lateral current spreading in shallow ridge processed unipolar semiconductor lasers is described using a two-dimensional flow model. In these devices, contrary to bipolar diode lasers, the density of carriers can be considered constant also in the active region. Therefore electron diffusion is a negligible effect and the spatial distribution of the current can be obtained by solving a two-dimensional differential equation for the electric potential. Our calculations prove that the major contribution to the current spreading takes place right before electrons enter the active region and is caused by the discontinuity of the conductivity at the cladding–active region interface.

Diffusion in Two-Temperature Partially Ionized Gases

Abstract: To date, some confusion exists in the literature concerning the employment of the diffusion driving force and the calculation of the diffusion velocity for the gas species in a two-temperature (2-T) plasma. In this letter, a derivation is presented concerning the species diffusion driving force and diffusion velocity under 2-T plasma conditions. Expressions for the ambipolar diffusion coefficients and the electric conductivity of the 2-T plasma are also given.

Contribution of injection in current noise due to generation and recombination of carriers in p–n junctions

Abstract: A theory for the current noise associated with carrier generation and recombination in the space-charge region of p–n junctions is presented We propose a noise model based on the response of the electric field to fluctuations of the trapped charge in this region, and make use of a collective transport-noise theory The effects of the fluctuations of the space-charge region borders due to the fluctuations of the trapped charge are now taken into account This new contribution is negligible when generation–recombination current governs the diode current However, it is significant when diffusion current dominates, allowing the analytical study of generation–recombination noise to be extended to wider ranges of bias and temperature Experimental results at low and high temperatures are explained with our theory Empirical formulas of current noise density are also explained according to this complete theory of current noise calculation

Two-dimensional analysis of double-layer heterojunction HgCdTe photodiodes

Abstract: In the paper the performance of P-on-n double-layer heterojunction HgCdTe photodiodes are temperature 77 K is analyzed theoretically. Calculation has been performed for the backside-illuminated configuration. The effect of photodiode base layer geometry on quantum efficiency and R0A product is analyzed. The effect of lateral collection of diffusion current and photocurrent on photodiode parameters is also shown. Moreover the dependence of the p-n junction position within heterostructure on the band-gap energy profiles and photodiode performance is presented. Finally, the influence of the composition gradient and p- side doping concentration on photodiode parameters is described briefly.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Space and Time Correlations in a Dissipative Structure Emerging in an Electrochemical System with a Cation-Exchange Membrane

Abstract: The mechanism of electromasstransfer in a system containing a cation-exchange membrane at a current density exceeding the limiting diffusion current is studied by analyzing the electric potential fluctuations. An experimental setup is designed for measuring fluctuations simultaneously at 16 points in a galvanostatic mode. The fluctuations are found to form in a near-membrane diffusion layer of electrolyte depleted of carriers. As the current density increases, spectra of resonance fluctuations turn into flicker-noise spectra; this process is studied in great detail. Use of space and time difference moments for processing noise signals allowed us to estimate such parameters as the correlation time, space correlation length, and components of the propagation velocity for perturbations in the dissipative structure forming in the membrane system. The data point to the formation of convective flows thermally induced in a diffusion layer of electrolyte under the action of Joule heating at a current density approaching a limiting value.

Method for isolating semiconductor device

Abstract: PURPOSE: A method for isolating a semiconductor device is provided to reduce a leakage current by increasing an absolute path of a diffusion current, and to guarantee a sufficient process margin by etching a trench broader than a design rule. CONSTITUTION: A substrate(21) on a field region is eliminated by a predetermined depth to form a trench on a semiconductor substrate wherein an active region and the field region are defined. The surface of the trench is made irregular to increase a surface area. The trench is filled with an insulating material(25).

Diffusion of current into conductors

Abstract: Currents are established on the surface of conductors by the propagation of electromagnetic waves in the insulating material between them. If the load is less than the characteristic impedance of the insulating material of the line, multiple reflections and retransmissions eventually build up the line current to that required by the load. The currents are initially established on the surface of the conductors before diffusing relatively slowly into the interior and gives rise to the skin effect. The diffusion velocity depends on the conductivity, permeability, thickness of the conductor and frequency of the excitation, and such effects of the diffusion process are difficult to conceptually appreciate. Fortunately the diffusion of heat into solids is very similar, and will be used as an analogy to aid understanding. This diffusion is the means whereby current moves into conductors and flux into of magnetic cores.

Current mechanisms in VLWIR Hg1−xCdxTe photodiodes

Abstract: VLWIR (c∼15 m to 17 m at 78 K) detectors have been characterized as a function of temperature to determine the dominant current mechanisms impacting detector performance Id−Vd curves indicate that VLWIR detectors are diffusion limited in reverse and near zero bias voltages down to temperatures in the 40 K range At 30 K the detectors are limited by tunneling currents in reverse bias Since the detectors are diffusion limited near zero bias down to 40 K, the R0Aimp versus temperature data represents the diffusion current performance of the detector as a function of temperature The detector spectral response measurement and active layer thickness are utilized to calculate the HgCdTe layer x value and the optical activation energy Ea optical The activation energy, Ea electrical, obtained from the measured diffusion limited R0Aimp versus temperature data is not equal to the activation energy, Ea optical, obtained from the spectral response measurement for all x values measured Ea electrical=*Ea optical, where ranges between 064 and 10 For cutoff wavelengths in the 9 m at 78 K, Ea electrical=Ea optical Ea electrical=065* Ea optical have been measured forc=17 m at 78 K detectors As the band gap energy decreases to values in the range of 70 meV and lower, it is reasonable to expect a more dominant role of band tailing effects on the transport properties of the material system In such a picture, one would expect the optical band gap to be unmodified, whereas the intrinsic concentration could be enhanced from its value for the ideal semiconductor Such a picture could explain the observed behavior Further probing experiments and modeling efforts will help clarify the physics of this behavior

A comparison between Monte Carlo and extended drift-diffusion models for abrupt InP/InGaAs HBTs

Abstract: In this paper, a comparison between the simulation results of abrupt InP/InGaAs n-p-n heterojunction bipolar transistors (HBTs) obtained from both an extended drift-diffusion (EDD) model and from a Monte Carlo (MC) HBT simulator is carried out. The EDD approach is based on Grinberg's model for the electron current through the interfacial emitter-base spike-like potential barrier. The MC procedure includes the effects of quantum transmission/reflection through the abrupt heterojunction. The transmission coefficient is obtained from a numerical solution of Schrodinger's equation (self-consistently embedded in the general MC procedure) that takes into account the nonparabolicity of the bands and the spatial change in the effective mass. The differences between both models and their physical causes are also analyzed in this paper. It Is shown that, for the HBT under consideration, the collector current is determined by the transmission of electrons through the spike, while the numerical values of the parameters associated with the transport mechanisms in the base region have a very small influence on the collector current. An alternative equation to the conventional diffusion current equation in the neutral base region has also been developed, which includes the hot electron effects.

Experimental study of the hot-carrier impact ionization photovoltaic effect in pulsed-CO2-laser-excited silicon junctions

Abstract: An experimental study has been carried out on the dependence of the hot-carrier highly nonlinear photovoltaic effect induced in silicon n+/p junctions by 10.6 µm laser pulses on the doping level and the carrier density profile. The measured response is optimized by using narrow step junctions and a high doping level n+≈1020 cm-3. The main features of the obtained results can be qualitatively interpreted by taking the following into account: conditions for a thermalized hot-carrier plasma, generation rate by impact ionization processes, generated minority diffusion current and recombination and absorption losses.

Generation-Recombination and Diffusion Currents in HgMnTe n + -p Junctions

Abstract: Special features of the distributions of the space charge, the electric-field strength, and potential in the p-n junction in the narrow-gap HgMnTe semiconductor were considered using the Poisson equation. It is shown that, as the band gap narrows, the effect of free charge carriers induces the coordinate dependence of electric-field strength to deviate from the linear dependence and that of the potential to deviate from the quadratic dependence. As a result of this, and also because of an appreciable increase in the diffusion potential in the n+-p junction with the degenerate n+ region, the mechanisms of the charge transport become peculiar: the voltage dependence of the recombination current deviates from those following from the conventionally used analytical expressions, whereas for higher bias voltages, the diffusion current of holes from the lightly doped p region into the n+ region is prevalent.

Self-Magnetic Field Effects on Electron Emission as the Critical Current is Approached

Abstract: The self-magnetic field associated with the current in a planar diode is shown to reduce electron emission below the Child-Langmuir current density As the magnetic field increases, the diode current is limited to the critical current Here, a ID analysis is carried out to calculate the suppressed current density in the presence of a transverse magnetic field The problem is shown to be similar to that of the limiting current (ie, Hull current) calculated in a crossed field gap, in which a constant transverse magnetic field is applied across the gap to insulate the electron flow In the case considered here, the magnetic field is produced by the diode current itself and this self-magnetic field decreases with distance along the gap It is shown that the emitted current density is only modestly reduced from the Child-Langmuir current density The 1-D analysis remains valid until critical current is approached, at which point orbit crossing occurs and a 2-D kinetic analysis is required The minimum diode length required to reach critical current is also derived

Electrical characterization of shallow cobalt-silicided junctions

Abstract: The impact of the thickness of the cobalt-silicide layer on the electrical diode characteristics will be reported, with particular emphasis on the current-voltage (I-V characteristics, the surface and bulk carrier recombination lifetime and the low-frequency noise. It is shown that for thicker silicides the generation-recombination of holes in the top n+-layer is enhanced, giving rise to a higher diffusion current, an increase in the noise and a larger surface recombination velocity. It is believed that these parameters are closely linked, as the underlying physical phenomenon is the minority carrier recombination. In addition, it is shown by transmission electron microscopy that the silicide-silicon interface roughness is increased for larger cobalt thickness, which may explain some of the observations.

Limiting current densities in a system with cation-exchange membrane MK-100 rotating in a glycine-HCl solution

Abstract: Limiting diffusion current densities in an electromembrane system (EMS) comprising cation-exchange membrane MK-100 and a glycine solution containing hydrochloric acid are studied on a rotating membrane disk. The dependence of the limiting current density at a given rotation rate of the disk on the equilibrium volume concentrations of glycine cations and protons in solution is described by a regression model. The diffusion fluxes of these cations in the EMS are shown to be independent.

Process for making energy available during the cathodic protection of steel structures comprises detecting a corrosion current caused by natural corrosion and a diffusion current, and selecting a corrosion protection current as compensation

Abstract: Process for making energy available during the cathodic protection of steel structures (1) comprises detecting a corrosion current caused by the natural corrosion of reinforcing elements (2) and a diffusion current caused by anions; and selecting the corrosion protection current so that it compensates for the corrosion and diffusion streams over pre-selected time period. An independent claim is also included for a device for carrying out the process. Preferred Features: The pre-selected time period corresponds to a maintenance period or the service life of the structure. A photovoltaic energy source is used.