Showing papers on "Diffusion current published in 2008"

XBn Barrier Photodetectors for High Sensitivity and High Operating Temperature Infrared Sensors

Abstract: A barrier photodetector is a device in which the light is absorbed in a narrow bandgap semiconductor layer whose bands remain essentially flat or accumulated at the operating bias so that all carrier depletion is excluded. In a conventional photodiode below a threshold temperature T0, typically 130-150K for MWIR devices, the dark current is due to Generation-Recombination (G-R) centres in the depletion layer. In a barrier detector, the absence of depletion in the narrow bandgap semiconductor ensures that the G-R contribution to the dark current is negligible. The dark current in the barrier detector is thus dominated by the diffusion component, both above and below T0. Therefore, at a given temperature below T0, a barrier detector will exhibit a lower dark current than a conventional photodiode with the same cut-off wavelength. Alternatively, for a given dark current, a barrier detector will operate at a higher temperature than a conventional photodiode, provided that this temperature is below T0. Device architectures are presented for barrier detectors with photon absorbing layers based on InAs1-xSbx alloys and type-II InAs/GaSb superlattices (T2SL). The thermionic and tunneling components of the dark current are analyzed and shown to be negligible for typical device parameters. An operating temperature of ~150K is estimated for a MWIR barrier detector with f/3 optics and a cut-off wavelength of 4.2μ.

Analysis of graded band gap solar cells with scaps

Abstract: To s imulate the complicated, graded structure of modern thin film CIGS solar cells, we followed a 'material driven' approach. Each layer is considered as a compound A1-yB B y; the desired composition grading y(x) over a layer is set; all materials properties are specified for the pure materials A and B; finally, the local materials properties are derived from the local composition. This strategy was implemented in our thin film solar cell simulation software SCAPS. The new facility was used to study CIGS based thin film solar cells with a graded absorber layer. We showed that a grading strategy with a Ga-poor bulk and narrow Ga-rich layers at both ends of the absorber can indeed lead to a more favourable trade-off between Jsc and Voc than can be obtained with uniform CIGS absorbers. However, this result depends on the specific assumptions on recombination. density NC(x) and NA(x), transport properties μn(x) and μp(x), recombination properties Nt(x), σn(x), σp(x),… A simulation tool including grading thus should take everywhere the position dependent value of the materials parameters. It is not trivial at all to implement this in a way that is convenient for the user and efficient for the internal operation of the programme. A more scientific consequence of grading is that it is modifying the semiconductor equations governing the problem. In a problem with uniform layers, the driving forces for electrical current are the electrostatic potential gradient ∇Φ (drift current) and the concentration gradients ∇n and ∇p (diffusion current). When grading is present, extra driving terms should be added: the gradient of the electron affinity ∇χ, the gradient of the band gap ∇Eg, and the gradients of the effective density of states in the conduction and valence bands: ∇(log NC) and ∇(log NV). Also, the electron and hole continuity equations are modified by the presence of a mobility gradient ∇μn or ∇μp, and the Poisson equation is modified by a gradient ∇e in dielectric constant. These modified equations have been described in the literature (4)(5)(7)(8) and are now im plemented and solved in SCAPS.

Heterostructure Barriers in Wrap Gated Nanowire FETs

Abstract: We present results on the effects of inserting a heterostructure barrier along the channel of vertical wrapped insulator-gate field-effect transistors (WIGFETs). Two sets of devices were fabricated, one InAs WIGFET and one with a 50-nm-long InAs0.8P0.2 segment in the channel. This addition of P induces a barrier in the conduction band of 130 mV, measured from the Fermi-level. The barrier blocks the diffusion current through the channel and reduces the feedback gating of holes created from band-to-band tunneling, resulting in improvements in on/off current ratio, and subthreshold characteristics. The heterosegment also induces a shift in the threshold voltage and provides an additional parameter for threshold voltage control in nanowire III-V MOSFETs.

Charge-Carrier Plasma Dynamics During the Reverse-Recovery Period in $\hbox{p}^{+}$ - $\hbox{n}^{-}$ - $\hbox{n}^{+}$ Diodes

Abstract: This brief describes the influence of switching conditions and a dynamic avalanche on the extraction of the charge-carrier plasma during the reverse-recovery period of p+-n--n+ diodes. The influence of the drift components and the diffusion components of the electron- and hole-current densities on the evolution of the plasma layer was investigated. The analysis shows that diffusion currents influence the plasma extraction process and can induce a change in the direction of the particle flux. Such a change in the particle flux may result in a longitudinal displacement of the plasma layer and significantly modify the reverse-recovery behavior. The results explain how a low on -state current density, a high circuit inductance, and the appearance of a dynamic avalanche will modify the reverse-recovery behavior.

Drift-diffusion crossover and the intrinsic spin diffusion lengths in semiconductors

Abstract: We study the propagation of electron spin density polarization and spin currents in n-doped semiconductors within the two-component drift-diffusion model in an applied electric field (E). The drift and diffusion contributions to the spin currents are examined, which shows how the spin current could be enhanced. We find that there is a crossover field (Ex), where the drift and diffusion contribute equally to the spin current in the downstream direction. This suggests a possible way to identify whether the process for a given E would be in the drift or diffusion regime. We derive the expression for Ex and show that the intrinsic spin diffusion length in a semiconductor can be calculated directly from Ex. The results will be useful in obtaining transport properties of the carriers’ spin in semiconductors. This investigation, however, highlights the need for further experiments to be conducted to measure Ex in semiconductors.

Current carriers in the bifurcated tail current sheet: Ions or electrons?

Abstract: [1] We have studied statistical distributions of the current density in the geomagnetic tail current sheet for different sets of local plasma parameters using Cluster data. It is shown that the electric current density calculated from 4-point magnetic field measurements exhibits no correlation with the number flux of ions. We conclude that main current carriers in the magnetospheric system of reference are electrons.

Drift instabilities in current sheets with guide field

Abstract: Drift instabilities in current sheets with or without the guide field are investigated with a newly developed improved electrostatic dispersion relation. Traditional (local) theories of lower-hybrid drift instability typically assumes small electron drift speed, and expand the electron distribution function in Taylor series. This approximate treatment is removed in this paper. The resulting formalism is uniformly valid for an arbitrary magnitude of relative ion and electron drift speeds, and is valid for an arbitrary strength of the guide field.

Negative density of states: Screening, Einstein relation, and negative diffusion

Abstract: In strongly interacting electron systems with low density the thermodynamic density of states is negative at low temperatures. This creates difficulties with understanding of the Einstein relation between conductivity and diffusion coefficient. Using the expression for electrochemical potential that takes into account the long-range part of the Coulomb interaction it is shown that at negative density of states the Einstein relation gives a negative sign of the diffusion coefficient $D$, but under this condition there is no thermodynamic limitation on the sign of $D$. That happens because the unipolar relaxation of inhomogeneous electron density is not described by the diffusion equation. The relaxation goes much faster due to electric forces caused by the inhomogeneous electron density. The diffusion coefficient is irrelevant in this case and it is not necessarily positive because the diffusion process does not contribute to the positive production of entropy. In the case of bipolar diffusion, negative $D$ results in a global absolute instability that leads to formation of neutral excitons. Graphene is considered as an example of a system where the density relaxation is expected to be due to electric force rather than diffusion. It may also have a negative density of states.

Theoretical study of terahertz current oscillation in GaAs1-xNx

Abstract: We have theoretically investigated current self-oscillation in doped n+nn+ GaAs1−xNx diodes driven by direct current (dc) electric field The current self-oscillation is associated with negative differential velocity effect in the highly nonparabolic conduction band of this unique material system By solving a time-dependent drift-diffusion model that takes into account the negative differential velocity effect, we provide a detailed analysis of the current oscillations The frequencies of current oscillations are in the gigahertz to terahertz region, depending on the doping concentration and the applied dc electric field The calculated average current density is in qualitative agreement with the measured result

Dynamic investigation of the transport current in YBa2Cu3O7−δ thin films

Abstract: The current density evolution in YBa2Cu3O7?? thin films is studied by time-resolved magneto-optical imaging as a function of the phase of an ac current applied simultaneously with a perpendicular dc magnetic field. We present a new empirical method to separate the total current distribution into a circulating current, which screens the applied field, and the applied transport current. The latter shows an asymmetric profile with pronounced peaks at the edges of the sample and its phase-dependent self-field is contained in the flux region bound by the circulating current. Threading dislocations provide the necessary pinning sites for the observed high local values of the transport current.

Nonlinear Evolution of Lower-Hybrid Drift Instability in Harris Current Sheet

Abstract: We perform 2.5-dimensional particle-in-cell simulations to investigate the nonlinear evolution of the lower hybrid drift instability (LHDI) in Harris current sheet. Due to the drift motion of electrons in the electric field of the excited low hybrid drift (LHD) waves, the electrons accumulate at the outer layer, and therefore there is net positive charge at the inner edge of the current sheet. This redistribution of charge can create an electrostatic field along the z direction, which then modifies the motions of the electrons along the y direction by E × B drift. This effect strongly changes the structure of the current sheet.

Coulomb interaction among transporting charge carriers confined in two dimensions

Abstract: The time evolution of trapped charge carriers in an oxide layer as in a charge-trap nonvolatile memory was studied experimentally with electrostatic force microscopy and Monte Carlo simulation based on Smoluchowski equation for a localized charge system. Experimental results show non-Fickian behavior as suggested by our calculation. In the charge carrier transport in an oxide, the ratio of the drift current driven by the self-induced electric field to the diffusion current was explicitly solved for Gaussian and Lorentzian charge distributions in two dimensions. The ratio is scaled by a quantity of the total charge divided by the characteristic width of the charge distribution. It was found that the drift current is comparable to the diffusion current when charge of 10−16 C is confined within a region of micrometer size.

Diffusion dark current in CCDs and CMOS image sensors

Abstract: Long neglected as unimportant, the dark current that arises due to diffusion from the bulk is assuming a more important role now that CCD and CMOS imagers are finding their way into consumer electronics which must be capable of operating at elevated temperatures. Historically this component has been estimated from the diffusion related current of a diode with an infinite substrate. This paper explores the effect of a substrate of finite extent beneath the collecting volume of the pixel for both a front-illuminated device and a thinned back-illuminated device and develops corrected expressions for the diffusion related dark current. The models show that the diffusion dark current can be much less than that predicted by the standard model

Spin-dependent Hall effect in degenerate semiconductors: A theoretical study

Abstract: The spin-dependent Hall (SDH) effect in degenerate semiconductors is investigated theoretically. Starting from a two-component drift–diffusion equation, an expression for SDH voltage (VSDH) is derived, and drift and diffusive contributions to VSDH are studied. For the possible enhancement of the diffusive part, degenerate and nondegenerate cases are examined. We find that due to an increase in the diffusion coefficient VSDH increases in a degenerate semiconductor, consistent with the experimental observations. The expression for VSDH is reduced in three limiting cases, namely diffusive, drift–diffusion crossover and drift, and is analysed. The results agree with those obtained in recent theoretical investigations.

Coaxial Solar Cell Structure and Continuous Fabrication Method of its Linear Structure

Abstract: A coaxial solar cell forces an exposed annular light receiving layers of a constant thickness to directly receive projection of light excitedly to generate pairs of electrons and holes that are driven by radial built-in electronic field formed on a PN junction to travel by the same distance paths to coaxial inner and outer electrodes. The photons directly enter an exposed drift region. The excited pairs of electrons and holes are separated by the even built-in electronic field to output current. Loss caused by crowding and recombination of diffusion current can be prevented. Photoelectric conversion efficiency improves without losing photon energy of short wavelengths projecting to the surface. The linear coaxial solar cell is fabricated by forming coaxial and annular semiconductor layers or compound films through deposition. Thus, it can be continuously fabricated by extending its length and mass production to reduce costs.

Doebner-Goldin Equation for Electrodynamic Particle. The Implied Applications

Abstract: We set up the Maxwell's equations and the corresponding classical wave equations for the electromagnetic waves which together with the generating source, a traveling oscillatory charge of zero rest mass, comprise a particle traveling in the force field of an usual conservative potential and an additional frictional force $f$. At the de Broglie wavelength scale and in the classic-velocity limit, the total wave equation decomposes into a component equation describing the particle kinetic motion, which for $f=0$ identifies with the usual linear Schr\"odinger equation as previously. The $f$-dependent probability density presents generally an observable diffusion current of a real diffusion constant; this and the particle's usual quantum diffusion current as a whole are under adiabatic condition conserved and obey the Fokker-Planck equation. The corresponding extra, $f$-dependent term in the Hamiltonian operator identifies with that obtained by H.-D. Doebner and G.A. Goldin. The friction produces to the particle's wave amplitude a damping that can describe well the effect due to a radiation (de)polarization field, which is always by-produced by the particle's oscillatory charge in a (nonpolar) dielectric medium. The radiation depolarization field in a dielectric vacuum has two separate significances: it participates to exert on another particle an attractive, depolarization radiation force which resembles in overall respects Newton's universal gravity as we showed earlier, and it exerts on the particle itself an attractive, self depolarization radiation force whose time rate gives directly the frictional force $f$.

Compact Current Modeling of Fully Depleted Symmetric Double-Gate Metal–Oxide–Semiconductor Field Effect Transistors with Doped Short-Channel

Abstract: The current behaviors of fully depleted (FD) symmetric double-gate (DG) metal–oxide–semiconductor field effect transistors (MOSFETs) with doped short-channel were parametrically modeled with the simple closed-form in all operational regions. In the diffusion current model, a physical parameter DG as a function of gate bias (VGS), drain bias (VDS), silicon body width (WB), channel length (L), and channel doping concentration (Nb) was introduced to consider the VGS dependence. Also, the subthreshold slope (SS) of DG MOSFETs with doped channel was modeled accurately with DG. DD which is dependent on VDS was introduced to consider the drain-induced barrier lowering (DIBL) in the diffusion current model. After the strong inversion, the drift current of doped DG MOSFETs was modeled by considering inversion-layer capacitance based on charge-sheet approximation. The channel length modulation by VDS was considered for accuracy in the current modeling of DG MOSFETs with doped short-channel. Our simple compact models predicted accurately DC characteristics of the devices with the channel length to 20 nm and shown good agreement with two-dimensional (2D) simulation.

Macroscopic theory of electrochemical transducer of alternating current, based on two-dimensional model with mesh cathodes

Abstract: Basing on approximate equations describing real processes, a possibility of the presenting in general closed form of the theoretical frequency dependence of diffusion current density in the entire frequency range is considered.

Diffusion of charged defects in Tellurium-rich CdTe

Abstract: Diffusion of charged point defects is studied theoretically in shallow-donor-doped tellurium-rich CdTe, which is typically used for a preparation of radiation detectors. Diffusion model involves complete charge defect statistics including formation of associates and internal electric field induced by the charged defect gradient. We show how extrinsic doping influences the rate of chemical diffusion, which can be both accelerated and retarded. In case of strongly compensated material at low temperatures the diffusion of compensating Cd vacancies is significantly enhanced in comparison with the undoped case and the defect relaxation is enhanced this way. The possibility to utilize this effect in the detector adjustment is discussed.

Device for measuring oxidation/reduction material concentration

Abstract: PROBLEM TO BE SOLVED: To provide an device for measuring oxidation/reduction material concentration that dispenses with a means for controlling the flow rate of liquid to be measured, or a movable part, such as a motor for making an electrode rotate or vibrate itself. SOLUTION: This device is equipped with a measuring tube 1, through which the liquid to be measured, containing an oxidation or reduction material, flows; a detection electrode 2 and a counter electrode 3 provided in contact with the liquid to be measured in the facing state on the inner wall surface of the measuring tube 1; a power supply circuit 4 for applying a voltage, set beforehand in between the detection electrode 2 and the counter electrode 3; a diffusion current detection circuit 5 for detecting the diffusion current, flowing between the detection electrode 2 and the counter electrode 3; an oxidizing/reducing material content calculating circuit 6 for calculating the content concentration of the oxidizing or reducing material, based on the diffusion current detected by the diffusion current detection circuit 5; and a restriction mechanism 7, provided in the circumscribed state to the inner circumferential wall surface of the measuring tube 1, on the upstream side of the detection electrode 2 and the counter electrode 3, for making the pressure loss of the flow of the liquid to be measured generated. COPYRIGHT: (C)2008,JPO&INPIT

Diffusion current induced 1/f noise in HgCdTe photodiodes

Abstract: Experiments indicate that excess low frequency noise (1/f noise) is related to current components in semiconductor diodes Our measurements on HgCdTe photodiodes support this interpretation We find that for the diffusion current components, the 1/f noise current power spectrum is very nearly proportional to the current squared This disagrees with the predictions of models for 1/f noise in diodes which start from Hooge’s relation for 1/f noise in conductors Although Hooge’s relation also employs a current squared dependence, association of the minority carrier density with N (the number of current carriers in the sample) in Hooge’s relation leads to a predicted linear relation between noise power and current in diodes This predicted linear relationship is not obtained in our experiments

Anomalous dependence on the diffusion coefficients of the ionic relaxation time in electrolytes.

Abstract: We show, by using a numerical analysis, that the dynamic toward equilibrium for an electrolytic cell subject to a step-like external electric field is a multirelaxation process when the diffusion coefficients of positive and negative ions are different. By assuming that the diffusion coefficient of positive ions is constant, we observe that the number of involved relaxation processes increases when the diffusion coefficient of the negative ions diminishes. Furthermore, two of the relaxation times depend nonmonotonically on the ratio of the diffusion coefficients. This result is unexpected, because the ionic drift velocity, by means of which the ions move to reach the equilibrium distribution, increases with increasing ionic mobility.

Infrared sensor, and an infrared sensor ic

Abstract: PROBLEM TO BE SOLVED: To provide an infrared sensor that suppresses a dark current and a diffusion current due to a thermally excited hole SOLUTION: Although a thermally excited carrier (hole) generated in a first compound semiconductor layer 102 is to be diffused toward a third compound semiconductor layer 105, a second compound semiconductor layer 103 which has a larger band gap than the first compound semiconductor layer 102 and a third compound semiconductor layer 105 and suppresses the diffusion thereof is provided between the first compound semiconductor layer 102 and third compound semiconductor layer 105 to reduce the dark current due to the hole The second compound semiconductor layer 103 has its band gap shifted by n-type doping relatively in a valence-band direction to effectively function as a diffusion barrier for the thermally excited hole Namely, the n-type compound semiconductor layer 103 has its band gap and n-type doping adjusted to suppress the diffusion of the thermally excited carrier COPYRIGHT: (C)2010,JPO&INPIT

Modeling aspects of current calculation of 4H-SiC Schottky diode

Abstract: This paper present modeling aspects for 4H-SiC Schottky diode using drift diffusion model. Drift diffusion model consists of the current continuity equations for electrons and holes, Poissonpsilas equation and the equations for electron and hole currents. Using this model physical surface trait, state of art of device has been extracted to understand the current condition of 4H-SiC Schottky barrier diode. In Schottky diodes current transport takes place by three mechanisms, diffusion of carriers from the semiconductor into the metal, thermionic emission of carriers across the Schottky barrier and quantum-mechanical tunneling through the barrier. For these three mechanisms current calculation using drift diffusion model has been done.

In-flight charge loss from electrospinning jets

Abstract: Due to the difficulty of measuring the electric current being drawn from the high voltage power supply to the high voltage electrode in the electrospinning process the electric current typically observed is the current flowing from the opposite electrode to ground. This current can then be used to infer details about the electrostatic environment such as the charge density on the fluid jet. It has been shown in literature that assuming initially the current is conducted in an ohmic mode and a reasonable conductivity for the polymer solution an electric current with the same order of magnitude to that observed. As conductivity is directly related to ionic concentration it was possible to add a known amount of ionic salt and measure the electric current while electrospinning fibre. This measured current was then compared to the expected electric current calculated from the ionic concentration. It was found that as the ionic concentration increased the observed electric current increased but at a slower rate than the calculated electric current increased. This suggests that a significant portion of the charge on the jet may be lost during flight. This further implies that in order to increase the charge density where the bending instability occurs (driving force proportional to charge density) the initial charge density must be drastically increased to compensate for the losses in flight. This may be limited as the effects of increased charge density may be detrimental to the formation of the Taylor cone.

Effect of electron‐electron interaction on the diffusion current of spin‐polarized electrons

Abstract: Electron-electron interaction modifies carrier transport in the spin-polarized system. The effects are investigated in the two-dimensional electron gas in semiconductor heterostructures. We find that the diffusion currents of spin-up and spin-down electrons are reduced, compared to the non-interacting values, by the momentum exchange between spin-up and spin-down electrons through their collisions (Spin Drag), and also by the electron energy renormalization arising from the manyparticle correlations. We numerically calculate the diffusion coefficients of spin-up and spin-down electrons separately in high-quality heterostructures of GaAs at low temperatures, including the effect of finite spin life time. Our calculations show that the diffusion coefficients are reduced down to less than half of their non-interacting values. We also find the negative diffusivity at low temperatures. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Simulation studies of current voltage characteristics of inhomogeneous Schottky diode with discrete distribution of barrier heights

Abstract: The current voltage characteristics of inhomogeneous Schottky contact with discrete Gaussian distribution of barrier heights are simulated considering thermionic emission diffusion theory. The diode parameters are extracted by fitting of simulated current voltage data into thermionic emission diffusion current equation. The effect of discrete Gaussian distribution of barrier heights on barrier parameters like barrier height, ideality factor and activation energy plots are discussed.