Showing papers on "Diffusion current published in 2012"

Spin and charge transport in materials with spin-dependent conductivity

Abstract: The spin and charge transport in materials with spin-dependent conductivity has been studied. It has been shown that there is a charge accumulation along spin diffusion in a ferromagnetic metal, which causes a shortening of the spin diffusion length. It has been shown that there is a substantial interaction between the drift and diffusion currents in semiconductors. The effects of gain/damping of a spin current by a charge current and the existence of a threshold spin current in a semiconductor have been described. Because of the substantial magnitude, these new spintronics effects might be used for new designs of efficient spintronic devices. The influence of a spin drain on spin transport has been studied.

Diffusion current in a system of coupled Josephson junctions

Abstract: The role of a diffusion current in the phase dynamics of a system of coupled Josephson junctions (JJs) has been analyzed. It is shown that, by studying the temporal dependences of the superconducting, quasi-particle, diffusion, and displacement currents and the dependences of average values of these currents on the total current, it is possible to explain the main features of the current-voltage characteristic (CVC) of the system. The effect of a diffusion current on the character of CVC branching in the vicinity of a critical current and in the region of hysteresis, as well as on the part of CVC branch corresponding to a parametric resonance in the system is demonstrated. A clear interpretation of the differences in the character of CVC branching in a model of capacitively coupled JJs (CCJJ model) and a model of capacitive coupling with diffusion current (CCJJ+DC model) is proposed. It is shown that a decrease in the diffusion current in a JJ leads to the switching of this junction to an oscillating state. The results of model calculations are qualitatively consistent with the experimental data.

Drift velocity in non-isothermal inhomogeneous systems.

Abstract: Drift velocity and driving force are not directly proportional in the case of inhomogeneous suspensions, where a space dependent mobility induces an additional contribution to the drift velocity. Similarly, particle flux and drift velocity are related not only by the gradient of density but also by an additional contribution given by the gradient of the self-diffusion coefficient. We provide quantitative support to this scenario in a non-equilibrium system by means of computer simulations with a temperature gradient. Moreover, our simulation results demonstrate that the temperature gradient-induced mass transport coefficient, namely thermal diffusion coefficient, is not directly proportional to the drift velocity so that the well-accepted relation of proportionality is just an approximation.

Influence of the spatial photocarrier generation profile on the performance of organic solar cells

Abstract: We investigate the impact of the interplay of charge carrier drift and diffusion on the fill factor of organic solar cells. Thin film interferences lead to strong gradients in the photocarrier generation profile. By means of numerical simulations, we show that the shape of the absorption profile is crucial for the efficiency of organic solar cells. High absorption in the peripheral areas of the active layer advantages an unfavorable diffusion current which leads to a reduction of the fill factor. Our work suggests design rules for the optical optimization of organic solar cells.

One-Dimensional Physical Model to Predict the Internal Quantum Efficiency of Si-Based LEDs

Abstract: A simple analytical model for p-i-n light-emitting diodes is presented to give insight into the device physics. The 1-D model describes the dc electrical characteristics and internal quantum efficiency (ηIQE) as a function of the applied bias and is in good agreement with TCAD simulations. An optimization scheme, based on the same model, shows improved ηIQE for engineered heterojunctions by reducing the diffusion current contribution. The results show that the use of heterojunctions increases the light intensity inside a narrow-bandgap material, akin to the experimentally observed results. The bandgap of the active region determines the voltage at which the maximum efficiency occurs. It is also shown that maximum ηIQE occurs at a lower bias than that typically used for studying the maximum light intensity. The effect of injection dependence of recombination coefficients on the efficiency is also studied. For the first time, the electrical performance of a multilayer active region is modeled.

Analysis for rapid tail current decay in IGBTs with low dose p-emitter

Abstract: A new theory is developed to correctly describe the turn-off mechanism in IGBTs. We found that the p-emitter efficiency (α) in IGBTs dramatically reduces during the fall-time. The hole current component of the tail current in IGBTs with low α instantly decays to zero at the onset of the fall-time, because α becomes negative during the fall-time and the stored holes are removed towards the collector electrodes. An abnormal hole diffusion current flows in the n-drift toward the p-emitter because the decreasing carrier density gradient toward the p-emitter is created. The magnitude of the stored carriers (p n ) at the n-drift near the p-emitter is universally determined by the forward bias (V Fp ) between the p-emitter and the n-drift even in the turn-off transient. It is found that the initial V Fp is decided only by the p-emitter dose (d P ). Consequently, the turn-off tail current in IGBTs is decided by dP because V Fp in the fall-time and the stored carriers in the n-drift in the fall-time are decided by d P . It is difficult to correctly describe the phenomenon with conventional IGBT models because the models assume that the α does not change in the fall-time period.

Performance of low dark current InGaAs shortwave infrared detector

Abstract: In x Ga 1-x As ternary compound is suitable for detector applications in the shortwave infrared (1-3 μm) band. The alloy In 0.53 Ga 0.47 As is lattice-matched to InP substrate, which leads to high quality epitaxial layers. Consistently the In 0.53 Ga 0.47 As detector shows low dark current density and high detectivity at room temperature with wavelength response between 0.9 and 1.7 μm. In this paper, planar-type 24×1 linear InGaAs detector arrays with guard-ring structure were designed and fabricated based on n-i-n + type InP/In 0.53 Ga 0.47 As/InP epitaxial structure by sealed-ampoule diffusion method. At first the dark current density is about 30~60 nA/cm 2 at -0.1 V at room temperature. After modifications to the detector design and processing, the dark current density reduces to 2~9 nA/cm 2 at -0.1 V at 293 K. The ideality factors simulated from I-V curves come close to 1 and less than the factors of previous detectors, which indicates that the dark current is dominated by diffusion current, while the generation-recombination current exhibits in the previous detectors. At the temperature of 293 K, the R 0 A of the detector reaches more than 1×10 7 Ω·cm 2 , the relative spectral response is in the range of 0.9 μm to 1.68 μm, the mean peak responsivity is 1.2 A/W and the mean peak detectivity is more than 3.0×10 12 cm·Hz 1/2 /W.

Noise in Submicron Metal-Oxide-Semiconductor Field Effect Transistors: Lateral Electron Density Distribution and Active Trap Position

Abstract: Experiments were carried out for the n-channel devices, processed in a 0.3 µm spacer less complementary metal–oxide–semiconductor technology. Random-telegraph-signal measurements were performed for the constant gate voltage. It is supposed that electron concentration in the channel decreases from the source to the drain contact. Lateral component of the electric field is inhomogeneous in the channel and it has a minimum value near the source and reaching the maximum value near the drain electrode. Drain current is given by two components – diffusion and drift ones. Diffusion current component is independent on the x-coordinate and it is equal to the drift current component for the low electric field. The model explaining the experimentally observed capture time constant dependence on the lateral electric field and the trap position is given. From the dependence of the capture time constant τc on the drain current could be calculated longitudinal coordinate of the trap position.

Electroluminescent element and manufacturing method therefor

Abstract: PROBLEM TO BE SOLVED: To provide an electroluminescent element which can manufacture an element having a sensitivity to a specific wavelength easily without selecting the material.SOLUTION: The operation of generating a diffusion current at a junction of a p layer and an n layer by applying a forward bias voltage, and changing the surface shape and/or the dopant distribution of any layer based on the Joule heat generated by the diffusion current is repeated. The surface shape and/or the dopant distribution is fixed by induction discharge in multiple steps by a non-adiabatic process based on the population inversion at a position where near-field light is generated, thereby reducing the diffusion current and the Joule heat.

Control of the ion diffusion coefficient in a nanochannel

Abstract: In this study, we first propose a simple yet novel method to measure the diffusion coefficient of ions through a nanochannel. Back-side track etching is used for the fabrication of a nanochannel on an n-type silicon substrate. A metal-oxide semiconductor field-effect transistor (MOSFET) like device named the metal-semiconductor-solution field-effect transistor (MSSFET) is implemented to control the ion diffusion current. When a negative gate voltage is applied, positive ions that travel along the nanochannel are confined to the central zone of the nanochannel allowing the radial Brownian movements to be reduced. The effect is equivalent to an increase of the diffusion coefficient. However, a positive gate voltage can produce an opposite Zeta potential on the nanochannel surface. The cations in the nanochannel are dragged to the channel surface. This condition can be regarded as a decrease of the diffusion coefficient. Experimental results illustrate that the transfer characteristics of the MSSFET are similar to those of a p-channel depletion-type MOSFET. The ion diffusion coefficient in a nanochannel can be controlled when the the initial ion concentration difference across a nanochannel is larger than a certain threshold.

Semiconductor laser diode and manufacturing method thereof

Abstract: PROBLEM TO BE SOLVED: To improve luminous efficiency in a semiconductor laser diode which uses an indirect type semiconductor such as Si, Ge or Gap as a material to produce the diode.SOLUTION: A diffusion current is generated in an active layer 12 by applying a forward biased voltage, and the surface shape and/or the dopant distribution on one or more of layers 11, 12 and 13 is repeatedly changed based on Joule heat produced by the generated diffusion current, while a population inversion is generated in conduction and valence bands in the active layer 12 by the forward biased voltage. In places where proximity light is generated based on the changed surface shape and/or dopant distribution, electrons in the conduction band constituting the population inversion are made to emit light by stimulation in plural steps based on a non-adiabatic process, whereby the diffusion current is reduced to lower the Joule heat. That way, the surface shape and/or the dopant distribution is fixed, and also the light generated by stimulated emission is resonated between resonance surfaces, giving rise to further stimulated emission.

Electrical and electroluminescent properties of InAsSb-Based LEDs (λ = 3.85–3.95 μm) in the temperature interval 20–200°C

Abstract: The temperature dependences of the electrical and electroluminescent properties of InAsSbP/InAsSb/InAsSbP heterostructure LEDs (λ ≈ 3.8−4.0 μm) are studied in the temperature interval 20–200°C. It is shown that the radiation power decreases with increasing temperature in a superexponential manner and that this decrease is associated primarily with a rise in the rate of Auger recombination. The position of the maximum in the radiation spectrum varies with temperature nonmonotonically, since radiative recombination is observed both in the active region and in the wide-gap layer. At room temperature, current through the heterostructure is tunneling current irrespective of the applied voltage polarity. As the temperature rises, either the thermal emission of charge carriers appears (direct bias) or the diffusion current becomes significant (reverse bias).

Redox sorption of oxygen on a layered cathode-polarized nanocomposite metal-ion exchanger

Abstract: The redox sorption of molecular oxygen from a flow of deionized water onto a cathode-polarized granular layer of nanocomposite copper-ion exchanger is considered. A mathematical description of it in terms of external diffusion is given. In contrast to better-known approaches, conditions are created that are as close as possible to the limiting diffusion current; this effect can be achieved by dividing the granular layer into shallow layers, each of which is then polarized with a near-limiting current. This allows water to be obtained with a particular value of deoxygenation close to the theoretically calculated value in stationary sorption membrane electrolyzers equipped with a unit containing a nanocomposite copper-ion exchanger. It is established that the lower deoxygenation value relative to the one calculated from the limiting current is associated with the additional reduction of oxygen with copper nanoparticles.

Simulation of electron diffusion effect on plasma formation in silicon trapatt diodes

Abstract: Plasma formation and extraction processes in silicon n+np+ TRAPATT (TRApped Plasma Avalanche Triggered Transit) diodes were simulated. The drift-diffusion model was chosen for the simulation of the processes. This model is adequate for diodes under consideration with a total thickness of 6.5 µm. Two approximations of carrier diffusion coefficient dependence on the electric field above 20 kV/cm were used. A strong dependence of plasma density and oscillation period on n+n junction steepness was found in the case of a constant electron diffusion coefficient in the electric field range above 20 kV/cm. This behaviour depends on the impact ionization model in silicon. Two models were used. In one of them we included drift and diffusion current in the impact ionisation process. In the other model we included only the drift current in the impact ionisation process. In the second case the influence of the n+n junction steepness on the plasma formation process is much stronger. In the diodes with a highly abrupt n+n junction the TRAPATT mode is impossible. These results explain our experiments on TRAPATT diodes with an abrupt n+n junction.

Dynamics of copper electrodeposition onto fibrous carbon electrodes

Abstract: Distribution of copper electrodeposited from a sulfuric acid solution onto fibrous carbon electrodes, copper deposition rate, and current efficiency by the metal were studied in relation to the electrolysis duration, electrical conductivity of the electrode, geometric current density, and solution flow rate. The variation of the electrode thickness on which copper ions discharge at the limiting diffusion current at various solution flow rates and the electrode thickness on which the whole amount of oxygen dissolved in the electrolyte is reduced were calculated in relation to the solution flow rate and geometric current density. The main factors governing the distribution of copper across the electrode thickness and the electrolysis parameters from the beginning of the process till “clogging” of a part of the electrode by the metal were determined.

Electrical properties of strained Si p-n junctions

Abstract: p-n junctions are of great importance for both modern Si complementary metal oxide semiconductors (CMOS) devices and other semiconductor devices In this study, we experimentally examined the strain induced modification of the current-voltage characteristics of Si p-n junctions The strain was applied to the forward biased p+-n and n+-p junctions though a wafer bending method It is observed that, under the uniaxial tensile stress, the ideality factor in the diffusion current region of a forward p-n junction decreases with the increase in the applied stress Meanwhile, the junction current increases with the increase in the applied stress It is also found that the applied uniaxial tensile stress causes a significant junction-current increase in the large forward biases region and a relative small current increase in the diffusion current region

Investigation and modeling of impact ionization spatial-transient effects in silicon devices

Abstract: Impact ionization (II) has played an important role in semiconductor devices; yet the understanding of II has not been mature. Abnormal behaviors related to II in deep sub-micrometer devices were observed and have not been fully explained. Existing models are not rigorously applicable to predicting II in different device structures and different operational regimes. Monte Carlo (MC) programs simulating transport of both electrons and holes are developed to investigate II in homogeneous electric field and in scaled devices. The programs' accuracy is verified by accurately producing many different transport parameters obtained from both experiments and previous MC simulations' results. Impact ionization, for the first time, is modeled as a positive feedback loop in which electrons create holes, and the secondary holes feed back secondary electrons. This model is analytically proven to be valid for short devices due to the existence of the II dead-space. This model is also numerically proven to be accurate by producing a good fit to the experimental data. It is easy to conclude from the positive feedback model that the breakdown voltage is the same for both the electron-initiating and hole-initiating II processes in a high field region. In addition, the positive feedback model also shows that the current gain from the electron-initiating II process is always higher than the current gain from the hole-initiating II process within the same high field region. More importantly, the positive feedback loop enables successful simulations of the II process in which both electrons and holes participate simultaneously. This is particularly important at high current gain. An efficient algorithm is also developed to speed up spatial transient simulations by implementing temporal meshes rather than the traditional spatial meshes. The II current gain in short p-i-n diodes is studied. The calculated results fit well to the experimental data of diodes with different lengths. Various physical insights are learned from the simulations. The minimum breakdown voltage for highly doped junctions is extrapolated to be at least 4.41V. Franz-Keldysh effect plays a significant role at low bias, especially for short devices. For the first time, Franz-Keldysh effect is invoked to explain the experimental current gain. II threshold energy is not constant with respect to the electric field, which partly explains various values of the reported threshold energy. II threshold energy is higher for holes than for electrons. Both electron and hole II coefficients come to equilibrium with the electric field after a dead-space distance. This spatial transient effect is a major cause for the disagreements among the experimentally-extracted values of the II coefficients. The values extracted from the double drift p-n junction experiments are more reliable in terms of accounting for the II spatial-transient effect. The II spatial-transient effect is identified to be the main cause for the failures of different well-known II models for semiconductor devices. A pseudo-local electric field model and the positive feedback model are proposed and proven to be sufficient in predicting the II current gain in short devices.MC simulations are conducted to study mixed tunneling and II process in short p-n diodes, which are potential terahertz source devices. Tunneling current is treated as generation current, which is also subject to the tunneling dead-space distance. Another gain stage is added on top of the positive feedback model to account for the tunneling dead-space. II is less important for more heavily doped p-n junctions. The contribution of the diffusion current and its II is negligible compared to the tunneling counterparts. Abnormal behaviors of II in deep sub-micrometer MOSFETs are investigated and explained. Channel carrier distribution functions are generated by MC simulations employing the rare-state algorithm. The thermal tail of the distribution function is Maxwellian with the lattice temperature as the effective temperature. By formulating the thermal tails as functions of position and bias voltage, an analytical formula of the substrate current is successfully derived for the first time. The formula is then used to explain experimental results of the substrate current in a sub-micrometer pMOSFET. The newly-developed formula is able to explain different abnormal behaviors of the substrate current that cannot be explained by the conventional formulas.

Numerical simulation study of barrier inhomogeneities at Schoktty contacts

Abstract: The Poisson equation and the drift diffusion equations have been used to simulate the current-voltage characteristics of inhomogeneous Schottky contact having discrete distribution of different barrier height patches. The potential variation inside the bulk semiconductor near the metal-semiconductor contact was estimated first and then the current as a function of bias through the Schottky diode using silicon parameters were calculated for various discrete distribution patterns of barrier heights. From the simulated current-voltage characteristics the apparent diode parameters were extracted by fitting of current-voltage data into thermionic emission diffusion current equation. The derived barrier parameters are analyzed to study the effect of barrier inhomogeneities on the current-voltage characteristics of Schottky contact.

Analysis of dark current in long-wavelength HgCdTe junction diodes at low temperature and an approximate method to calculate the trap density of depletion region

Abstract: This paper analyzes the dark current char acteristic of two different sizes at different temperature. The wavelength is 12 A m. We find that the main mechanism of the dark current is band-to-band tunnel current at 20 k-30 k. By fitting the ideal factor, we find that at temperature of 80 k-100 k, the dark current is a mixture of diffusion current, g-r current. We find the large size is better by comparing the R 0 A-V curves. At 45 k-60 k, the dark current is a mixture of trap-assisted tunnel current and band-to-band tunnel current. An interesting thing is that in this temperature area, the different R 0 -V curves have a same sharp cross which does not exist in the 20 k-30 k area or 80 k-100 k area. At this point the trap-assisted tunnel is equal to the band-to-band tunnel. We calculated the trap density of depletion region which is about 1.38 H10 11 cm -3 at 55 k. Keywords: long-wavelength, dark current, R 0 A, trap density, HgCdTe 1. INTRODUCTION In recent years, due to the strong demand for strategic and tactical applications, the long-wavelength infrared detector is increasingly valued

Diffusion Losses in a Gretzel Solar Cell

Abstract: The activation and diffusion losses in the redox reaction of iodide electrolyte of the Gretzel battery at the anode and cathode were examined. It was shown that the limiting diffusion current of the battery is limited by the diffusion mode of ion transport at the cathode or the anode depending on the composition of the electrolyte. In the case of a porous anode in accordance with the Zeldovich theory, activation losses at the anode are reduced by half compared with a flat anode, and diffusion limitations of the current density are absent. The operational conditions for the battery that reduce the losses were considered.