Showing papers on "Diffusion current published in 2009"

Mesa-isolated InGaAs photodetectors with low dark current

Abstract: We demonstrate InGaAs photodiodes with an epitaxial heterostructure that allows simple mesa isolation of individual devices with low dark current and high responsivity. An undoped InAlAs barrier and passivation layer enables isolation of detectors without exposing the InGaAs active region, while simultaneously reducing electron diffusion current. Photodetectors with mesa sizes as small as 25×25 μm2 exhibit dark current densities of 10 nA/cm2 at 295 K and responsivities of 0.62 A/W at 1550 nm.

Study of the Transit-Time Limitations of the Impulse Response in Mid-Wave Infrared HgCdTe Avalanche Photodiodes

Abstract: The impulse response in frontside-illuminated mid-wave infrared HgCdTe electron avalanche photodiodes (APDs) has been measured with localized photoexcitation at varying positions in the depletion layer. Gain measurements have shown an exponential gain, with a maximum value of M = 5000 for the diffusion current at a reverse bias of V b = 12 V. When the light was injected in the depletion layer, the gain was reduced as the injection approached the N+ edge of the junction. The impulse response was limited by the diode series resistance–capacitance product, RC, due to the large capacitance of the diode metallization. Hence, the fall time is given by the RC constant, estimated as RC = 270 ps, and the rise time is due to the charging of the diode capacitance via the transit and multiplication of carriers in the depletion layer. The latter varies between t 10–90 = 20 ps (at intermediate gains M < 500) and t 10–90 = 70 ps (at M = 3500). The corresponding RC-limited bandwidth is BW = 600 MHz, which yields a new absolute record in gain–bandwidth product of GBW = 2.1 THz. The increase in rise time at high gains indicates the existence of a limit in the transit-time-limited gain–bandwidth product, GBW = 19 THz. The impulse response was modeled using a one-dimensional deterministic model, which allowed a quantitative analysis of the data in terms of the average velocity of electrons and holes. The fitting of the data yielded a saturation of the electron and hole velocity of v e = 2.3 × 107 cm/s and v h = 1.0 × 107 cm/s at electric fields E > 1.5 kV/cm. The increase in rise time at high bias is consistent with the results of Monte Carlo simulations and can be partly explained by a reduction of the electron saturation velocity due to frequent impact ionization. Finally, the model was used to predict the bandwidth in diodes with shorter RC = 5 ps, giving BW = 16 GHz and BW = 21 GHz for x j = 4 μm and x j = 2 μm, respectively, for a gain of M = 100.

On the Temperature and Field Dependence of Trap-Assisted Tunneling Current in Ge $\hbox{p}^{+}\hbox{n}$ Junctions

Abstract: The temperature dependence of the trap-assisted tunneling (TAT) current component in Ge p+n junctions has been studied between 25degC and 140degC. It is shown that the impact of TAT reduces significantly due to the combination of the negative thermal activation of the TAT-enhancement factor and the exponential increase of the diffusion current with temperature. It is shown that the experimental data can be well described in the frame of the Hurkx analytical model, which allows a fairly easy assessment of the TAT current contribution to the junction leakage current at realistic operation temperatures of Ge CMOS circuits.

Universal 1/f noise model for reverse biased diodes

Abstract: A 1/f noise model is developed for reverse biased diodes based on McWhorter’s concept of charge tunneling into semiconductor states at passivation layer interfaces [A. L. McWhorter, in Semiconductor Surface Physics, edited by R. H. Kingston (University of Pennsylvania Press, Philadelphia, 1957), pp. 207–228]. The charge modulates the width of semiconductor surface charge layers on either side of the junction, resulting in fluctuations in dark current from these volumes due to the net difference in depletion and diffusion current generation rates per unit volume in the semiconductor. The 1/f spectrum associated with the fluctuating surface charge translates into a 1/f spectrum in thermally generated diode dark current. The model is applied to midwavelength infrared HgCdTe N+/P diode architectures.

Numerical Simulation of Ionic Mass-Transfer Rates with Natural Convection in CuSO4 – H2SO4 Solution I. Numerical Study on the Developments of Secondary Flow and Electrolyte Stratification Phenomena

Abstract: The ionic mass-transfer rates accompanying natural convective electrolyte flow in a CuSO 4 aqueous electrolyte solution acidified with an excess amount of H 2 SO 4 are numerically analyzed. The effects of a supporting electrolyte and an interaction behavior between both cathodic upward and anodic downward natural convections are examined. Both anodic and cathodic current density distributions along the vertical height are also calculated. A mathematical model is extended by incorporating an additional boundary condition at the limiting current. A measure of ionic migration effect e, a ratio of limiting current to limiting diffusion current, involving the transference number of a discharging metallic ion is introduced for this purpose. The present calculation predicts the oscillation behaviors in transient variations in both electrode surface concentration and maximal natural convective velocity, which are deeply related to the periodic fluctuating electrolyte flow patterns distorted by secondary flow. The addition of H 2 SO 4 maintains an e value around 1 and prevents the further development to the transition or turbulent natural convection.

High-Operating-Temperature HgCdTe Avalanche Photodiodes

Abstract: In this communication we report the first results of electro-optical characterization of planar heterostructure HgCdTe avalanche photodiodes (APDs), which enables the operation of APDs at high gain, at low bias, and with low dark current and/or at high operating temperature (HOT). The APD is based on a heterostructure in which the photons are detected in a wide-band-gap layer, and the photoelectrons are amplified in a vertical junction in a confined narrow-gap layer. The dark diffusion current and thermal background sensitivity of the device are limited by using a thin narrow-band-gap amplification layer. In addition, the defect-limited dark current is also expected to be reduced due to the reduced volume of the narrow-band-gap depletion layer. The electro-optical performance was characterized at T = 80 K and T = 200 K for two devices with a nominal thickness of the amplification layer of w = 100 nm and 500 nm, realized in x Cd = 0.3 Hg-vacancy-doped layers grown by molecular-beam epitaxy (MBE). The measurements show an average gain of 〈M〈 = 10 at a reverse bias of 5 V, which is slightly reduced compared with a conventional APD with x Cd = 0.3. The thermal diffusion current measured at low reverse bias, V b = 0.1 V, and at T = 200 K is about 0.1 mA/cm2 to 0.3 mA/cm2, which is a factor of 50 lower than standard x Cd = 0.3 n-on-p APDs. The quantum efficiency due to absorption in the gain layer is high (QEpeak > 30%), although no antireflecting coating was used, indicating that the device can also be used for high-operating-temperature thermal detection.

Cavity-controlled radiative recombination of excitons in thin-film solar cells

Abstract: We study the performance of photovoltaic devices when controlling the exciton radiative recombination time. We demonstrate that when high-quantum-yield fluorescent photovoltaic materials are placed within an optical cavity, the spontaneous emission of the radiative exciton is partially inhibited. The corresponding increase of the exciton lifetime results in an increase of the effective diffusion length and diffusion current. This performance maximizes when the thickness of the cell is comparable to the absorption length. We show that when typical parameter values of thin solar-cell devices are used, the efficiency may improve by as much as three times.

Problems and paradoxes of the Lifshitz theory

Abstract: The problems and paradoxes of the Lifshitz theory in application to real dielectric and semiconductor materials are reviewed. It is shown that the inclusion of drift current of conduction electrons into the model of dielectric response results in contradictions with both thermodynamics and experimental data of different experimental groups. Physical reasons why the problems and paradoxes arise are analyzed and found to be connected with the violation of basic applicability condition of the Lifshitz theory, the thermal equilibrium. A recent alternative approach to the resolution of the problems based on the inclusion of screening effects and diffusion current is considered and demonstrated to be thermodynamically and experimentally inconsistent. It is argued that the inclusion of the diffusion current leads to an even deeper violation of thermal equilibrium. Phenomenologically, the Lifshitz theory with role of drift and diffusion currents neglected is shown to be free of problems and in agreement with both thermodynamics and all available experimental data.

Effect of electric field on diffusion in disordered materials

Abstract: The effect of electric field on diffusion of charge carriers in disordered materials is studied by Monte Carlo computer simulations and analytical calculations. It is shown how an electric field enhances the diffusion coefficient in the hopping transport mode. The enhancement essentially depends on the temperature and on the energy scale of the disorder potential. It is shown that in one-dimensional hopping the diffusion coefficient depends linearly on the electric field, while for hopping in three dimensions the dependence is quadratic.

Analytical drift-current threshold voltage model of long-channel double-gate MOSFETs

Abstract: This paper presents a new, physical threshold voltage model to solve the ambiguity in determining the threshold voltage of double-gate (DG) MOSFETs. To avoid the difficulties of the conventional 2?B model in nearly undoped DG MOSFETs, this study proposes to define the on?off switching based on the actual roles of the drift and diffusion components in the total drain current. The drift current strongly enhances beyond the threshold voltage, while the diffusion current plays a major role in the subthreshold. The threshold voltage is defined as the drift component that exceeds the diffusion counterpart. From the solutions of Poisson's equation, the drift and diffusion currents of DG MOSFETs are separately formulated to derive the analytical expressions of the threshold voltage and associated threshold current. This model provides a comprehensive description of the switching behavior of DG MOSFET devices, and offers a physical onset threshold current to determine the threshold voltage in practical extraction.

Ion Diffusion in the Time-Dependent Potential of the Dynamic Electric Double Layer

Abstract: The description of ion diffusion in the electric field set up by the electric double layer (EDL) is an important issue in many scientific fields because of its close relevance to diffusion-controlled chemical kinetics and ion transport occurring in the environmental, biological, and other systems with multiphase chemical reactions and charged particle transports. When considering ion diffusion in the EDL, the change of ion density in space leads to the change of potential with time. Currently, the static distribution of electric potential in the EDL at equilibrium is described by the nonlinear Poisson−Boltzmann equation. However, describing a time-dependent potential during a diffusion process in the EDL still remains a challenge. In this study, a dynamic Poisson−Boltzmann equation that describes the time-dependent potential was suggested for a slow diffusion problem. By combining the generalized nonsteady state diffusion equation (the linearized Fokker−Planck equation) with the time-dependent potential (...

Infrared light emitting device

Abstract: Provided is an infrared light emitting device wherein dark current and diffusion current caused by thermally excited holes are suppressed. Thermally excited carriers (holes) generated in a first n-type compound semiconductor layer (102) try to diffuse in the direction of a π layer (105), but since an n-type wide band gap layer (103) having a band gap larger than those of the first n-type compound semiconductor layer (102) and the π layer (105) and suppressing diffusion thereof is provided between the first n-type compound semiconductor layer (102) and the π layer (105), the dark current caused by holes is reduced. The n-type wide band gap layer (103) has a band gap shifted relatively to the direction of valence band by n-type doping, and thereby functions as a diffusion barrier of thermally excited holes more effectively. In other words, the band gap and n-type doping of the n-type wide band gap layer (103) are adjusted to suppress diffusion of thermally excited carriers.

Experimental Performance and Monte Carlo Modeling of Long Wavelength Infrared Mercury Cadmium Telluride Avalanche Photodiodes

Abstract: We evaluated the performance of long-wavelength infrared (LWIR, λ c = 9.0 μm at 80 K) mercury cadmium telluride electron-injected avalanche photodiodes (e-APDs) in terms of gain, excess noise factor, and dark current, and also spectral and spatial response at zero bias. We found an exponential gain curve up to 23 at 100 K and a low excess noise factor close to unity (F = 1–1.25). These properties are indicative of a single carrier multiplication process, which is electron impact ionization. The dark current is prevailed by a diffusion current at low reverse bias. However, tunneling currents at higher reverse bias limited the usable gain. The measurements of the pixel spatial response showed that the collection width, and, especially, the amplitude of the response peak, increased with temperature. Furthermore, we developed a Monte Carlo model to understand the multiplication process in HgCdTe APDs. The first simulation results corroborated experimental measurements of gain and excess noise factor in mid-wavelength infrared (MWIR, x = 0.3) and LWIR (x = 0.235) e-APDs at 80 K. This model makes it possible for phenomenological studies to be performed to identify the main physical effects and technological parameters that influence the gain and excess noise. The study of the effect of the n −-layer thickness on APD performance demonstrated the existence of an optimum value in terms of gain.

The Atomistic Simulation of Thermal Diffusion and Coulomb Drift in Semiconductor Detectors

Abstract: In order to enhance the imaging resolution of gamma cameras over standard Compton camera and coded aperture designs, one can add momentum information to the kinematics by tracking the recoil electrons that result from gamma-ray interactions. The initial direction of the recoil electron can be discerned from its meandering trajectory, as measured via the initial electron-hole charges' spatial distribution, which itself is extracted by measuring the induced current signal on the bounding electrodes of the detector. In principle, the extraction of the recoil electron direction is ultimately limited by those stochastic effects that significantly contribute to the charge motion; most notably, thermal diffusion, although for existing systems, electronic noise can contribute or even dominate the position uncertainty. Nevertheless, we neglect the effects of electronic noise in this paper in order to gauge the intrinsic uncertainty in the charges' positions. We model diffusion using two techniques: one found in the literature and based on the diffusion coefficient, the other based on the underlying physics in which the probability density function describing the random thermal motion is sampled to determine the random contribution to each charge's motion, which depends on its drift state as well as the surrounding crystallographic environment. As is shown, the effect of diffusion is always significant, but its effect can be mitigated if the charges drift with alacrity. Coulomb drift, which refers to the dynamic charge motion due to the electromagnetic forces of the space charge created during the radiation event, is usually neglected; however, it can also be important for highly ionizing particles. We thus quantify the effect of Coulomb drift and suggest methods by which its impact can be extracted from the overall charge-track reconstruction.

Anomalous Diffusion at Edge and Core of a Magnetized Cold Plasma

Abstract: Progress in the theory of anomalous diffusion in weakly turbulent cold magnetized plasmas is explained. Several proposed models advanced in the literature are discussed. Emphasis is put on a new proposed mechanism for anomalous diffusion transport mechanism based on the coupled action of conductive walls (excluding electrodes) bounding the plasma drain current (edge diffusion) together with the magnetic field flux "cutting" the area traced by the charged particles in their orbital motion. The same reasoning is shown to apply to the plasma core anomalous diffusion. The proposed mechanism is expected to be valid in regimes when plasma diffusion scales as Bohm diffusion and at high $B/N$, when collisions are of secondary importance.

Bipolar junction transistor

Abstract: The invention relates to a bipolar junction transistor, and belongs to the technical field of high-power semiconductors Two heterojunction surfaces are formed through bonding of silicon carbide and silicon Specifically, a P-type base region and an N + emitting region use the bonding technology to form the heterojunction The P-type base region and an N- collecting region use the bonding technology to form the heterojunction The heteroemitting junction has high electron injection efficiency because the reverse injection of holes is almost completely blocked by an additional hole barrier andthe hole diffusion current value injected into the emitting region from the base region is extremely low and thus the injection efficiency of the emitting junction is extremely high and the current gain of the device is greatly improved In addition, the collecting region uses the silicon carbide material, most of the voltage drop is on the collecting junction and the critical breakdown electric field of the silicon carbide is ten times that of the silicon so that the breakdown voltage of the device can be greatly improved

A drift-diffusion model for semiconductors with temperature effects

Abstract: We establish an existence theorem for a stationary semiconductor model which takes into account the current generated by the gradient of the temperature.

Disturbance of the ionospheric D region by the electric current of the atmospheric-ionospheric electric circuit

Abstract: The mechanism by which electron and ion densities change in the ionospheric D region due to the electric current flowing in the atmospheric-ionospheric electric circuit is studied. The current disturbance in this circuit exists over the regions of increased seismic, meteorological, and thunderstorm activity. In the framework of the model, the influence of the electron and ion transportation under the action of the electric field on the formation of a disturbance in the D region and heating of the plasma electron component by the field are considered. The calculation results show that the densities of electrons and ions can change by an order of magnitude at an increase in the current density up to ∼(10−9–10−8) A m−2, the sign of the disturbance depending on the current direction.

The effect of the concentration of the reacting ion on the control of the electrodeposition process

Abstract: The effect of the concentration of the reacting ion on the nature of the control of the electrodeposition process was investigated by digital simulation of the polarization curve using the Newman form of the polarization curve equation and the Levich dependence of the limiting diffusion current density under natural convection conditions. A simple method for the determination of the exchange current density from polarization measurements is also proposed. The agreement with experiments was correct.

Spin drift–diffusion transport and its applications in semiconductors

Abstract: We study theoretically the propagation and distribution of electron spin density in semiconductors within the drift–diffusion model in an external electric field. From the solution of the spin drift–diffusion equation, we derive the expressions for spin currents in the down-stream ( DS ) and up-stream ( US ) directions. We find that drift and diffusion currents contribute to the spin current and there is an electric field, called the drift–diffusion crossover field, where the drift and diffusion mechanisms contribute equally to the spin current in the DS direction, and that the spin current in the US direction vanishes when the electric field is very large. We calculate the drift–diffusion crossover field and show that the intrinsic spin diffusion length in a semiconductor can be determined directly from it if the temperature, electron density and both the temperature and electron density, respectively, are known for nondegenerate, highly degenerate and degenerate systems. The results will be useful in obtaining transport properties of the electron’s spin in semiconductors, the essential information for spintronic technology.

Actual activation energy of electrode process under mixed kinetics conditions

Abstract: In the case of a single-electron reaction with account for slow diffusion of reagents, equations for actual (experimentally determined) activation energies of two types were derived and analyzed: real energy A f, i.e., the energy measured at a constant electrode polarization value η = const) and formal energy (Ωf, i.e., the value measured at a constant value of potential vs. an ambiguously chosen reference electrode E = const). It is found that under the conditions of a sufficiently significant deviation from equilibrium, the actual activation energy Af is the weighted arithmetic mean of the diffusion activation energy and the sum of A0 + αFη (where A0 is the real activation energy of the discharge stage at polarization of η = 0); herewith, the weighting coefficients are the corresponding values of the current of the discharge stage and the limiting diffusion current. A similar relationship is also obtained for Ωf. It is found that the Af, η- and Ωf, E-curves can in a number of cases feature regions with the negative Af and Ωf values in the mixed kinetics range.

Current Model of Fully Depleted Nanoscale Surrounding-Gate Metal–Oxide–Semiconductor Field-Effect Transistors with Doped Channel in All Operation Regions

Abstract: The diffusion current of fully depleted (FD) nanoscale surrounding-gate (SG) metal–oxide–semiconductor field effect transistors (MOSFETs) with a doped channel was physically modeled with a simple closed form based on the surface potential. The potential distribution [\varPhix(z)] of doped SG MOSFETs was derived using Young's simple approximation from a two-dimensional (2D) Poisson's equation. In the diffusion current model, to consider the dependence on gate bias (VGS) and drain bias (VDS), parameters (DG and DD) were introduced. In the saturation region, the drift current modeling of doped FD SG MOSFETs was easily performed from the derived current model in the linear region. The current of the devices in the transition region was also modeled with the simple closed form using the diffusion and drift current model. Our simple compact model accurately predicted the current behaviors of the devices with a channel length up to 20 nm and shown good agreement with three-dimensional (3D) simulation.

Residual chlorine meter

Abstract: PROBLEM TO BE SOLVED: To provide a residual chlorine meter for accurately measuring chlorine concentration of liquid to be measured, taking into consideration the conductivity variations in the PH dependence of diffusion current. SOLUTION: In the residual chlorine meter applies an applied voltage Ed forming a plateau current between a working electrode 3 and a counter electrode 2 arranged in the liquid L to be measured, and measures the chlorine concentration of the liquid L to be measured, based on the diffusion current Ii flowing between these electrodes, there are included a PH sensor 5 for measuring the PH of the liquid L to be measured, and a correcting section 20 that receives output of the PH sensor 5 and the diffusion current Ii and corrects the value of the diffusion current Ii, by using a concentration correction table showing the relationship between the PH value and diffusion current value. An arithmetic section 20 has a plurality of concentration correction tables, in response to the conductivity and selects the optimal concentration correction table, in response to the conductivity of the liquid L to be measured. COPYRIGHT: (C)2010,JPO&INPIT

Simulations of anomalous ion diffusion in experimentally measured turbulent potential

Abstract: The diffusion of plasma impurities in tokamak-edge-plasma turbulence is investigated numerically. The time-dependent potential governing particle motion was measured by 2D array of 8×8 Langmuir probes in edge region of CASTOR tokamak. The diffusion of particles is found to be classical in the radial direction, but it can be of an anomalous Levy-walk type in the poloidal direction. The diffusion is found to be dependent on the ratio of particles’ mass and charge. When this ratio grows, the diffusion coefficient in radial direction grows as well, whereas poloidal diffusion coefficient drops down. Moreover, movement of particles in the time-frozen snapshot of this potential is investigated showing that also the time-independent potential is much more favorable for the particle diffusion in poloidal direction than in radial one. In the case of single ionized carbon ions the poloidal diffusion in time-independent potential transits to the Levy-walk type for temperatures greater than 25 eV, for radial diffusion Levy-walk was not observed even for 500 eV.

Free charge carrier diffusion response transducer for sensing gradients

Abstract: Devices for sensing gradients are constructed from material whose properties change in response to gradients. One embodiment of the device is a transducer (200) for sensing gradients that includes the material (210) and two or more electrodes (240, 270) coupled to the material. In one embodiment, gradients in a surrounding medium (110) modify the energy gap of the material in the transducer (130) producing a diffusion current density (150). The material is configured to connect to a current or voltage measurement device (520, 530, 540) where a measurement is used to determine the gradient in the medium (160). The devices can be used to measure pressure, temperature, and/or other properties. The transducer can be built on the same substrate as complementary circuitry. A transducer made of Indium Antimonide is used in marine seismology to measure pressure gradients.

Current-voltage characteristics of planar-type InGaAs infrared detectors

Abstract: The planar-type InGaAs infrared detectors are fabricated on the NIN-type InP/In0.53Ga0.47As/InP wafers by using the sealed-ampoule method with SiO2 and Si3N4 respectively as the diffusion masks.The forward current-voltage characteristics at room temperature for the detectors with different diffusion area and the relationship of the reverse dark current density versus perimeter-to-area ratio characteristics of the two-type InGaAs detectors are analyzed.It is indicated that the passivation for the edge of the diffusion area is one of the key points in planar-type InGaAs detector fabrication,and Si3N4 layer has a better passivation effect than SiO2 layer.At room temperature and -0.1 V reverse bias,the dark current density of the detector using Si3N4 layer as diffusion mask is 20 nA/cm2.