Carrier Current Density Research Articles

Development of Surface Hall Potentiometry to Reveal the Variation of Drift Velocity of Carriers in Semiconductor Materials

Surface Hall potentiometry is a method of detecting the Hall effect occurring in a semiconductor material as the surface potentials change. An experimental setup to detect surface Hall potentials by the Kelvin method is introduced. After we reveal the basic characteristics of surface Hall potentials, a silicon sample is prepared to induce the variation of the drift velocity of carriers. Line-profile measurements of surface Hall potentials with this sample demonstrate that our method reveals the distribution of drift velocities of carriers in semiconductors, which reflects electronic or structural properties such as the carrier type (hole or electron), current density, and thickness.

Numerical study of the influence of thermooptic effects on the competition of modes in diode lasers

A software package is developed for numerical analysis of radiation of edge-emitting diode lasers. The optical block of the package uses the three-dimensional diffraction method of counterpropagating beams. The current carrier density distribution in a quantum well is found from the diffusion equation, and the temperature distribution is found from the heat conduction equation. The software package can be used to analyse the competition of optical modes and to find the critical current at which single-mode lasing is quenched. The operation of a semiconductor laser having a built-in system of waveguides with antiresonance reflection is analysed. The conditions of single-mode generation with the output power above 1 W are found for this laser.

Optical gain analysis of strain-compensated InGaN–AlGaN quantum well active regions for lasers emitting at 420–500 nm

Strain-compensated InGaN quantum wells with tensile AlGaN barriers are analyzed as improved gain media for laser diodes emitting at 420–500 nm. The band structure is calculated using the 6-band k ·p formalism, taking into account valence band mixing, strain effect, and spontaneous and piezoelectric polarizations. The optical gain analysis exhibits significant improvement in the peak optical gain and differential gain for the strain-compensated structures. The calculation also shows a significant reduction of threshold carrier density and current density for diode lasers employing the strain-compensated InGaN–AlGaN QW active regions.

Use of epitaxial gallium arsenide in detectors

The results of investigations of semiconductor detectors on the basis of epitaxial layers of gallium arsenide for detecting x rays and low-energy radiation are examined. It is shown that epitaxial layers ranging in thickness from 60 to 300 µm with current carrier density ≤5·1013 cm−3 and electron mobility ≥6000 cm2/(V·sec) at 300 K hold promise for such detectors.

Latent spatial instability of current in high-power bipolar switches

A theory describing the injection instability in high-power bipolar semiconductor switches at a current density of J ∼ 1 kA/cm2 and a collector field strength of E > 5 × 104 V/cm is proposed. It is established that, if the efficiency of cathode injection source increases with J and E, fluctuations in the current distribution over wafer area with a wavelength on the order of λ ∼ 100 μm (not related to the wafer transverse size) can develop in such devices. The corresponding volume fluctuations in the charge carrier densities, potentials, field strength, and current density are localized in the bulk, at a depth that is much smaller than the plate thickness. This latent instability is not related to the sign of the external differential resistance, so that internal damping factors are necessary in order to prevent its development.

Effects of intradot electron–electron interaction on the photon-assisted Andreev tunneling through a finite-sized carbon-nanotube system

Abstract The effects of intradot electron–electron interaction on the photon-assisted Andreev tunneling of a superconductor/carbon-nanotube/superconductor system are studied by using nonequilibrium Green's function technique. The inverse supercurrent reflecting the π -junction transition emerges in the spin-split energy-levels regime polarized by the Coulomb interaction. For the positive tunneling case, the supercurrent reaches its maximum when the spin-degenerate energy-levels are nearest to the Fermi surface. Conversely, for the negative tunneling case, the supercurrent reaches its maximum when two split energy-levels are symmetric with respect of the Fermi surface. The sign and the amplitude of the Andreev tunneling depend distinctly on the energy-level spacing tuned by photon-assisted tunneling. In order to fully understand the transport characteristics, the current-carrying density of states are investigated, which clearly shows the enhancement, suppression or even reversion of the supercurrent.

Phase Coexistence and Resistivity near the Ferromagnetic Transition of Manganites

Pairing of oxygen holes into heavy bipolarons in the paramagnetic phase and their magnetic pair breaking in the ferromagnetic phase (the so-called current-carrier density collapse) has accounted for the first-order ferromagnetic-phase transition, colossal magnetoresistance, isotope effect, and pseudogap in doped manganites. Here we propose an explanation of the phase coexistence and describe the magnetization and resistivity of manganites near the ferromagnetic transition in the framework of the current-carrier density collapse. The present quantitative description of resistivity is obtained without any fitting parameters, by using the experimental resistivities far away from the transition and the experimental magnetization, and is essentially model-independent.

Analytical model for base transit time of a bipolar transistor with Gaussian-doped base

The present analysis for base transit time τ b of a modern high-speed npn bipolar transistor is done for Gaussian-doped base considering doping dependence of mobility, bandgap narrowing effect and carrier velocity saturation effect at base–collector junction. First the minority carrier current density and electron density equations incorporating all the effects are analytically solved. The collector current density J c and base stored charge Q b are then separately expressed as a function of the injected electron density n(0) in the base in order to find base transit time. The modeling of J c, Q b and τ b are essential for the design of high-speed bipolar transistor. The equations are applicable for low level of injection. The base transit time calculated analytically is compared with numerical results in order to demonstrate the validity of the assumptions made in deriving the equations. The closed form equations for collector current density and base transit time offer a physical insight into device operation and are a useful tool in device design and optimization.

Impact of N on the lasing characteristics of GaInNAs∕GaAs quantum well lasers emitting from 1.29 to 1.52μm

The origin of the degradation with N of the threshold current density (Jth) and external differential quantum efficiency (ηd) of 1.29 to 1.52μmGaInNAs∕GaAs laser diodes is analyzed. Adding N to InGaAs leads to a ∼25% reduction of the carrier injection efficiency and thus to an increase of Jth and a decrease of ηd. This effect is likely related to carrier recombination losses in the barriers and is independent of the N content. The optical absorption losses and the internal transparency current density are found to increase with N content, accounting for the rest of the degradation in Jth. Modeling of the transparency carrier and radiative current densities identifies the increase of the defect-related recombination coefficient in GaInNAs as the dominant effect leading to the N dependence of Jtr.

Small‐signal characteristics of organic semiconductors with continuous energy distribution of traps

AbstractThe electrical characteristics of organic light emitting devices containing a continuous distribution of trap states in the forbidden gap are obtained by numerically solving the basic semiconductor equations for the steady state and under small‐signal conditions. The spatially‐dependent occupied trap states, which are described by an explicit density of states function, modify the charge distribution within the structure and the shape of the electric field and the carrier current densities. The effect of the modulation frequency, the applied voltage and the device temperature are studied for a hole conducting layer with a trap density of states consisting of a double gaussian profile and for a given set of microscopic parameters including the carrier mobilities and thermal velocities, the capture cross sections, and the residual shallow impurity concentration. The frequency‐dependent loss and capacitance curves are obtained for various experimental conditions, like temperature and applied steady‐state voltage. Effects of parameters describing electrical contacts and trap density of states are shown. Such results are particularly useful for the analysis of experimental electrical characteristics obtained by thermal admittance spectroscopy. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Minority carrier transport equation for bipolar transistors with polysilicon emitter contact

In order to improve the current gain via a reduction of the emitter base junction saturation current density, a perpendicular ordering of the grain boundaries (columnar grain boundaries) in the polysilicon emitter-region of a bipolar transistor is proposed. The complex polysilicon current component is obtained by a two-dimensional resolution of the current transport equation along the polysilicon region. The minority carrier current density component in the non-uniformly heavily doped single-crystal emitter is calculated by an average value approach. The influence of the two-dimensional structure of the polysilicon region on the current gain is analysed, and it is shown that the current gain decreases with increasing grain number. The evolution of the normalised current gain in terms of physical parameters is studied. The results show that the current gain improves with polysilicon thickness for a thin oxide layer, while it is insensitive for a thick oxide layer. In addition, the results show that the bulk recombination in a single-crystal emitter cannot be neglected in devices with a deliberately grown interfacial oxide.

超高感度交流磁界計測のための高周波キャリア型薄膜磁界センサの設計

We designed high-frequency-carrier type thin film sensors to obtain high impedance change. We considered the relationships between carrier current density and noise level, carrier current density and sideband level, and optimum impedance of sensor element. The meander type sensor element fabricated had an impedance change of 30 Ω/Oe at 500 MHz. Sensitivity of 4.5×10-8 Oe was demonstrated at 501 kHz using a carrier-suppression technique.

Conductive mechanism in manganite materials

We describe a model in which f(T)=M(T)/M m a x represents both the fraction of the itinerant electron density in the double-exchange (DE) theory and the magnetization σ in the current carrier density collapse (CCDC) theory. With this model, we have checked the DE and CCDC theories with our experimental results of the transport behavior. The DE theory yields agreement with the experimental resistivity excellently, in which the conductivity is the sum of the polaronic and itinerant electronic conductivity for the insulator-metal transition regime. The fitting curves of the resistivity by the CCDC theory deviate from the experiment seriously. This might be caused by the improper assumption of the temperature-dependent carrier density and the temperature-independent carrier mobility. Therefore, it is concluded that the DE theory is more suitable to explain the conductive mechanism in perovskite manganites.

Fundamental connection between hydrogen/deuterium desorption at silicon surfaces in ultrahigh vacuum and at oxide/silicon interfaces in metal–oxide–semiconductor devices

The fundamental connection between electron stimulated desorption (ESD) of hydrogen (H)/deuterium (D) at silicon surfaces in ultrahigh vacuum and hot-carrier-stimulated desorption of H/D at the oxide/silicon interfaces in complementary metal–oxide–semiconductor (CMOS) devices is presented. The dependences of device degradation on carrier energy and current density were studied on two generations of CMOS devices. The results suggest that the interface degradation in long channel devices is primarily due to the desorption of H/D by high energy electrons through the direct electronic desorption mechanism, while the multiple vibrational heating mechanism becomes important for ESD of H/D in deep submicron devices. By measuring interface trap generation at various stressing conditions in large time scales, we also provide experimental evidence to show that, unlike the uniform energy distribution of Si–H on silicon surfaces, the disordered interface environment introduces a variation of Si–H bond strength at the interface.

Photogalvanic effect in aqueous Methylene blue nickel mesh systems: Conversion of light into electricity

The photogalvanic effect in electrochemical cells, employing aqueous Methylene blue and Fe(II)/Fe(III) couple electrolyte and nickel-mesh electrodes, were experimentally investigated. Five different standard H-cell configurations were set-up by modifying the electrolyte. Long-term open-circuit voltage measurements were conducted in order to test the stability of the cells. Light on–off reproducibility experiments were also carried out during lengthy cell operations. By comparing experimental quantum yield with theoretical predictions, it was found that the cells operate on differential electrode kinetics. Oxidation of the illuminated electrode was detected. This affected the current–voltage characteristics of the cells after a sufficiently long cell operation. Schottky junction treatment was used to model the electrolyte–electrode junction. After calculating the ratio between the majority carrier (electron) current density and minority carrier (hole) current density, we concluded that the oxidation of the electrodes contributes positively to the cell performance since the electrode–electrolyte interface shows unipolar Schottky diode characteristics. Copyright © 2001 John Wiley & Sons, Ltd.

Massive spectral weight transfer and colossal magneto-optical effect in doped manganites

We calculate the value of the Fröhlich electron-phonon interaction in manganites, cuprates, and some other charge-transfer insulators and show that this interaction (∼1 eV) is much stronger than any relevant magnetic interaction. This suggests that carriers in those systems are small (bi)polarons at all temperatures and doping levels, in agreement with the oxygen isotope effect and other data. As these materials are cooled below the Curie temperature, the colossal magnetoresistance (CMR) is accompanied by a massive transfer of the spectral weight of the optical conductivity to lower frequencies. As with the CMR itself, this change in the optical conductivity is explained by the dissociation of bipolarons into small polarons by the exchange interaction with localized Mn spins during the transition. The corresponding current carrier density collapse in doped manganites leads to a colossal change of the optical conductivity in an external magnetic field at temperatures close to the ferromagnetic transition, in agreement with available experimental observations.

Colossal magneto-optical conductivity in doped manganites

We show that the current carrier density collapse in doped manganites, which results from bipolaron formation in the paramagnetic phase, leads to a colossal change of the optical conductivity in an external magnetic field at temperatures close to the ferromagnetic transition. As with the colossal magnetoresistance itself, the corresponding magneto-optical effect is explained by the dissociation of bipolarons into polarons owing to the exchange interaction with the localized Mn spins in the ferromagnetic phase. The effect is positive at low frequencies and negative in the high-frequency region. The present results agree with available experimental observations.

An asymptotic analysis of a unipolar junction model

Mock in 1983 and Kerkhoven in 1991 have introduced and studied the questions related to the existence and uniqueness of solutions of a unipolar junction model for semiconductor devices. In this model one of the carrier current density is assumed to vanish, so that the original bipolar model reduces to a system of two coupled differential equations. We perform a singular perturbation analysis of this system in the two dimensional case as the singular perturbation parameter goes to zero. We obtain L2 asymptotic error estimates and uniform convergence results for the difference between the solution of the system and the first order term of the asymptotic expansion under any bias conditions. We use a new uniqueness result for the associated reduced unipolar system together with perturbation arguments with respect to the reduced solution combined with variational techniques. These results extend in a certain way the results of Alabau and Moussaoui [5] which where valid for a more complex model (the bipolar one) but only for sufficiently small biases. This is due to the fact that the obtention of asymptotic estimates is related to uniqueness of solutions of the reduced equations which does not hold in general in the bipolar case (unless the applied biases are sufficiently small).

Electron spin resonance study of carbon nanotubes

The electron spin resonance of multiwall carbon nanotubes has been measured in the temperature range 100–550K. The spin paramagnetic susceptibility and g-shift are connected with 2-D current carriers and low density of localized centers. The temperature dependence of the g-factor and spin paramagnetic susceptibility are well described in terms of the band model of quasi-two-dimensional graphite. The density of current carriers and localized centers are evaluated.

A network approach to analysis of nonsteady-state facilitated ionic diffusion processes

The network approach has been used to analyze transient behavior of facilitated transport systems. A network model based on the time-dependent version of the Nernst-Planck and Poisson equations is proposed for the electrically induced carrier transport through a liquid membrane. The model is illustrated and tested for a common situation: facilitated transport in the nitric-oxide-ferrous-ion membrane system. Two special situations, namely when the external partial pressures of nitric oxide on the two sides of the membrane are not equal and when they are equal (electrically induced carrier transport system), have been simulated for obtaining the transient behavior of the system at varying partial pressure differences across the membrane, as well as for different carrier concentrations and current densities.