Showing papers on "Drift velocity published in 1995"

Electronic transport studies of bulk zincblende and wurtzite phases of GaN based on an ensemble Monte Carlo calculation including a full zone band structure

Abstract: The ensemble Monte Carlo technique including the details of the first four conduction bands within the full Brillouin zone is used to calculate the basic electronic transport properties for both zincblende and wurtzite crystal phases of bulk gallium nitride. The band structure throughout the Brillouin zone is determined using the empirical pseudopotential method. Calculations of the electron steady‐state drift velocity, average energy, valley occupancy and band occupancy in the range of electric fields up to 500 kV/cm are presented. It is found that the threshold electric field for intervalley transfer is greater and that the second conduction band is more readily occupied in wurtzite than in zincblende GaN over the range of electric fields examined here.

Electron drift mobility of oxadiazole derivatives doped in polycarbonate

Abstract: Charge drift mobilities of five oxadiazole derivatives doped in polycarbonate (PC) were evaluated with the time‐of‐flight technique. It is demonstrated that oxadiazoles incline to having electron‐transport characteristics. In particular, an oxadiazole with naphthyl substituent (BND) was found to possess high potential of electron transport; the electron drift mobility of 50 wt % BND doped PC was 2.2×10−5 cm2 V−1 s−1 at an electric field of 7.5×105 V cm−1 at room temperature. In addition, incorporating strong electron‐releasing substituents into oxadiazoles was demonstrated to add hole transport characteristics to oxadiazoles.

Theoretical study of electron transport in gallium nitride

Abstract: This paper describes characteristic electron transport properties for GaN in bulk and quantum well structures. First, ensemble Monte Carlo calculations of steady‐state electron drift velocity in bulk GaN are presented as a function of applied electric field for different lattice temperatures. At 300 K, the calculated peak steady‐state drift velocity is 2.8×107 cm/s and the threshold field is 160 kV/cm. It is found that the peak steady‐state electron drift velocity decreases only slightly by about 20% as the temperature increases from 300 to 600 K while the threshold field increases slightly by about 20%. Therefore, in addition to its high temperature stability, GaN has a low temperature coefficient making it ideal for high temperature applications. For electron transport in heterostructures, quantum mechanical calculations of the electron capture rate in GaN‐based quantum wells as a function of well thickness are also presented. An oscillatory behavior of the electron capture rate as a function of quantum...

Comparison of plasma flow velocities determined by the ionosonde Doppler drift technique, SuperDARN radars, and patch motion

Abstract: We compare measurements of polar cap ionospheric plasma flow over Resolute Bay, Canada, made by a digital ionosonde using the Doppler drift technique with simultaneous measurements at the same location made by the first operational pair of SuperDARN HF radars. During the 3-hour comparison interval the flow varied widely in direction and from 100 to 600 m/s in speed. The two measurement techniques show very good agreement for both the speed and direction of flow for nearly all of the samples in the interval. The difference between the velocities determined by the two techniques has a scatter of about ±35° in direction and ±30% in speed, with no systematic difference above the level of the scatter. The few samples which strongly disagreed were usually associated with strong spatial structure in the flow pattern measured by SuperDARN in the vicinity of the comparison point. The drift speed measured by the ionosonde was independently verified by observing the time taken for polar cap F layer ionization patches to drift between ionosondes sited at Eureka and Resolute Bay. These results confirm that the speed and direction of the polar cap ionospheric convection can be reliably monitored by the ionosonde Doppler drift technique.

A CO(3--2) Survey of Nearby Mira Variables

Abstract: A survey of CO(3-2) emission from optically visible oxygen-rich Mira variable stars within 500 pc of the sun was conducted. A molecular envelope was detected surrounding 36 of the 66 stars examined. Some of these stars have lower outflow velocities than any Miras previously detected in CO. The average terminal velocity of the ejected material was 7.0 km/sec, about half the value found in Miras selected by infrared criteria. None of the stars with spectral types earlier than M 5.5 were detected. The terminal velocity increases as the temperature of the stellar photosphere decreases, as would be expected for a radiation driven wind. Mass loss rates for the detected objects were calculated, and it was found that there is no correlation between the infrared color of a Mira variable, and its mass loss rate. The mass loss rate is correlated with the far infrared luminosity, although a few stars appear to have extensive dust envelopes without any detectable molecular wind. A power-law relationship is found to hold between the mass loss rate and the terminal velocity of the ejected material. This relationship indicates that the dust envelope should be optically thick in the near infrared and visible regions of the spectrum when the outflow velocity is > 17 km/sec. At the low end of the range of outflow velocities seen, the dust drift velocity may be high enough to lead to the destruction of the grains via sputtering. Half of the stars which were detected were re-observed in the CO(4-3) transition. A comparison of the outflow velocities obtained from these observations with those obtained by other investigators at lower frequencies shows no evidence for gradual acceleration of the outer molecular envelope.

An ion-neutral species collision model for particle simulation of glow discharge

Abstract: An ion-neutral species collision model with charge exchange is developed for use in particle simulation of a glow discharge based on an extension of the theory of Langevin (1905) and Hasse (1926). The validity of the model is checked by Monte Carlo calculation of drift velocity for He+-He, Ne+-Ne, Ar+-Ar and Kr+-Kr collisions. The results show good agreement with the experimental data and the solution by the moment method, especially for heavy gases. By proper choice of the cut-off value of the dimensionless impact parameter in the model, it is applicable to a collision of ions of energy 1 keV.

Kinetic theory of the Farley‐Buneman instability in the E region of the ionosphere

Abstract: The linear stage of the Farley-Buneman instability in the ionospheric E region is reexamined. Unlike previous theories, the present analysis is based on a consistent kinetic theory for electrons developed earlier which takes into account such important factors as the non-Maxwellian nature of the perturbed electron distribution, different rates for electron momentum and energy relaxation, and the velocity dependence of the electron-neutral collision frequency. The dispersion relation obtained using this theory is applicable for wave frequencies small compared to the ion-neutral collision frequency and for sufficiently low altitudes where the electron-electron collisions are negligible. The threshold conditions prove to be strongly modified compared to those resulting from the earlier simplified theories. For strong electric fields which can occur in the auroral regions, unstable long-wave waves excited along the bisector between the directions of the electric field E0 and E0×B drift velocity are predicted at low atitudes. The excitation of such waves becomes possible due to a new kinetic mechanism associated with the perturbed electron current caused by the Pedersen conductivity.

Choking of electron flow: A mechanism of current saturation in field-effect transistors

Abstract: We describe a mechanism of the current saturation in a field-effect transistor (FET) caused by choking of electron flow. The choking occurs when the electron velocity at the drain side of the channel reaches the plasma-wave velocity. This effect is quite similar to the choking of a gas flow in a pipe. This mechanism is an alternative to the well-known mechanism of current saturation caused by the drift velocity saturation in the FET channel. We show that the choking mechanism may dominate in a submicrometer ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As/GaAs FET at 10 K and low drain and gate bias voltages.

Electric‐field‐induced exciton transport in coupled quantum well structures

Abstract: We report a conceptionally new approach to achieve electrostatically induced transport and confinement for spatially indirect excitons Experimentally, exciton transport is demonstrated in an electric‐field‐tunable GaAs/AlAs coupled quantum well structure, which is configured as a three‐terminal device In spatially resolved photoluminescence experiments, it is shown that indirect excitons experience a drift field, which is given by an electrostatically induced band‐gap gradient in the plane of the coupled quantum well structure

Design and Characteristics of InGaAsDnP Composite-Channel HFET' s

Abstract: A design for composite-channel structures consisting of an InGaAs channel and an InP subchannel for use as het- erostructure field-effect transistors is presented for the first time. This novel channel structure takes advantage of both the high drift velocity and low impact ionization of InP at high electric fields as well as the high electron mobility of InGaAs at low electric fields. It is shown that the doping density of the InP subchannel is the key parameter to realize the advantages of the composite channel. A very high transconductance of 1.29 S/mm and a current gain cutoff frequency of 68.7 GHz are achieved with 0.6- and 0.7-pm gates, respectively. The average velocity of electrons in the composite channel is 2.9 x lo' cds. The devices have no kink phenomena in their I-V characteristics possibly due to low impact ionization in the InP subchannel.

A CO(3-2) Survey of Nearby Mira Variables

Abstract: A survey of CO(3-2) emission from optically visible oxygen-rich Mira variable stars within 500 pc of the sun was conducted. A molecular envelope was detected surrounding 36 of the 66 stars examined. Some of these stars have lower outflow velocities than any Miras previously detected in CO. The average terminal velocity of the ejected material was 7.0 km/sec, about half the value found in Miras selected by infrared criteria. None of the stars with spectral types earlier than M 5.5 were detected. The terminal velocity increases as the temperature of the stellar photosphere decreases, as would be expected for a radiation driven wind. Mass loss rates for the detected objects were calculated, and it was found that there is no correlation between the infrared color of a Mira variable, and its mass loss rate. The mass loss rate is correlated with the far infrared luminosity, although a few stars appear to have extensive dust envelopes without any detectable molecular wind. A power-law relationship is found to hold between the mass loss rate and the terminal velocity of the ejected material. This relationship indicates that the dust envelope should be optically thick in the near infrared and visible regions of the spectrum when the outflow velocity is > 17 km/sec. At the low end of the range of outflow velocities seen, the dust drift velocity may be high enough to lead to the destruction of the grains via sputtering. Half of the stars which were detected were re-observed in the CO(4-3) transition. A comparison of the outflow velocities obtained from these observations with those obtained by other investigators at lower frequencies shows no evidence for gradual acceleration of the outer molecular envelope.

Design and characteristics of InGaAs/InP composite-channel HFET's

Abstract: A design for composite-channel structures consisting of an InGaAs channel and an InP subchannel for use as heterostructure field-effect transistors is presented for the first time. This novel channel structure takes advantage of both the high drift velocity and low impact ionization of InP at high electric fields as well as the high electron mobility of InGaAs at low electric fields. It is shown that the doping density of the InP subchannel is the key parameter to realize the advantages of the composite channel. A very high transconductance of 1.29 S/mm and a current gain cutoff frequency of 68.7 GHz are achieved with 0.6 and 0.7 /spl mu/m gates, respectively. The average velocity of electrons in the composite channel is 2.9/spl times/10/sup 7/ cm/s. The devices have no kink phenomena in their I-V characteristics possibly due to low impact ionization in the InP subchannel. >

Monte Carlo calculations of the temperature‐ and field‐dependent electron transport parameters for 4H‐SiC

Abstract: Monte Carlo simulation results for the field‐ and temperature‐dependent electronic mobilities, drift velocities, and diffusion coefficients in 4H‐SiC are presented. The calculations include crystal anisotropy, and values are obtained for field orientations both parallel and transverse to the c axis of the hexagonal structures. The simulations are based on electron effective mass data that has only recently become available. Our theoretical predictions of the electron mobilities and their anisotropy ratios compare very well with available experimental data at 300 K. A room‐temperature velocity of 2.7×107 cm/s was obtained in 4H‐SiC for transport parallel to the c axis. This value is found to be larger than both 6H and 3C material. Finally, our calculations for the longitudinal and transverse diffusion coefficients at 300 K indicate that both have appreciable field dependences and exhibit a ‘‘soft’’ threshold.

Comparison of satellite electron density and temperature measurements at low latitudes with a plasmasphere‐ionosphere model

Abstract: Observations made by the Hinotori satellite of the latitude and diurnal variations of electron density and temperature near 600 km altitude in the low-latitude region are studied by comparison with values from the Sheffield University plasmasphere-ionosphere model (SUPIM). The model results show that the observed features of higher electron density in the summer hemisphere and higher electron temperature in the winter hemisphere are caused principally by the difference in the summer and winter hemisphere values of the meridional neutral wind. Closer agreement between the modeled and observed values is obtained when the interhemisphere difference in the meridional wind, as given by the horizontal wind model (HWM) 90, is reduced and when the peak value of the daytime poleward wind is moved to the afternoon sector in the winter hemisphere and to the morning sector in the summer hemisphere. The model results also show that the altitude variation of the vertical E×B drift velocity plays an important role in the development of the ionospheric equatorial anomaly. The latitude and diurnal variations of the modeled electron density and temperature are in good agreement with the observations when the E×B drift velocity used by the model is in accord with the observations made by the AE-E satellite for magnetic field lines with apex altitude less than 400 km and at Arecibo for magnetic field lines with apex altitude greater than 2000 km; linear interpolation of the observed values is used for the intermediate magnetic field lines.

Coordinated digisonde and incoherent scatter radar F region drift measurements at Sondre Stromfjord

Abstract: Comparison of drift measurements made at Sondre Stromfjord show that the apparent velocities measured by the digisonde (DGS) are in good agreement with the drift velocities observed by the collocated incoherent scatter radar (ISR). Data from December 5 to 9, 1991, show the mean DGS velocities to be within 50 m s−1 of the ISR velocities, that is, within the uncertainty levels of each instrument. The analysis highlights the dominance of the electric field in controlling the plasma motion. The measured velocities are generally height independent, as would be expected for an E field mapped along the magnetic field lines from high altitudes to ionospheric heights. In addition, the comparative analysis is used to study an ionospheric event where a large section of ionization was removed from the daytime cusp region by a strong anti-Sunward drift when the interplanetary magnetic field (IMF) Bz component changed orientation.

Regimes of suprathermal electron transport

Abstract: Regimes of the one‐dimensional (1‐D) transport of suprathermal electrons into a cold background plasma are delineated. A well ordered temporal progression is found through eras where J⋅E heating, hot electron–cold electron collisional heating, and diffusive heat flow dominate the cold electron energy equation. Scaling relations for how important quantities such as the width and temperature of the heated layer of cold electrons evolve with time are presented. These scaling relations are extracted from a simple 1‐D model of the transport which can be written in dimensionless form with one free parameter. The parameter is shown to be the suprathermal electron velocity divided by the drift velocity of cold electrons which balances the suprathermal current. Special attention is paid to the assumptions which allow the reduction from the collisional Vlasov equation, using a Fokker–Planck collision operator, to this simple model. These model equations are numerically solved and compared to both the scaling relations and a more complete multigroup electron diffusion transport. Implications of the scaling relations on fast ion generation, magnetic field generation, and electric field inhibition of electron transport are examined as they apply to laser heated plasmas.

Trapped particle precession in advanced tokamaks

Abstract: Plasma equilibrium parameters affect the precessional drift velocities of trapped particles, which can significantly influence plasma stability (eg collisionless trapped electron driftwaves and fishbones) The calculation outlined here computes the bounce-averaged drift velocities of trapped particles for toroidally symmetric plasmas of arbitrary aspect ratio with elliptical and triangular shaping of the poloidal plasma cross section Local equilibrium parameters are specified to describe the geometry and magnetic field structure on a given up down symmetric flux surface Making use of an expansion in the neighbourhood of the flux surface the precessional drift velocities of trapped particles can be calculated The calculation is then applied to flux surfaces with the properties characteristic of JET and also of the START experiment, where it has been postulated that favourable trapped particle drift effects may be responsible for improved confinement Our calculation fails to confirm this idea, and indeed suggests that the drive for instability from toroidal drifts at START is similar to that in JET The picture is complex with the competing effects of aspect ratio, plasma shaping, shear and pressure gradients Desirable features needed in the equilibria (eg high pressure gradients and low shear) are discussed Drifts are also calculated in extreme equilibria where mod B mod does not decrease monotonically around the flux surface to a minimum on the outboard mid-plane

Nonstationary macroparticle charging in an arc plasma jet

Abstract: The charging of liquid metal macroparticles in the rarified part of a vacuum arc plasma jet is studied. The sheath in the vicinity of the macroparticle is collisionless and the problem with different Debye length to macroparticle radius ratios is analyzed. Maxwellian velocity distribution functions with different temperatures for the electrons and ions in an arbitrary ratio are allowed in the model. By solving the equation for the electric field together with the equation for ion and electron flux, the charging time and the near electric field of the macroparticles were calculated. The kinetics of the macroparticle charging are controlled by the ion and electron flux to the macroparticle, which depend on the potential distribution in the sheath. The potential falls off slower than 1/r/sup 2/ in the case of the large Debye length to macroparticle radius ratio, and falls off more rapidly than 1/r/sup 2/ in the other case. The charge which accumulates on a macroparticle at distances of about 10 cm from a 100-/spl Aring/ cathode is about 10/sup -16/ C and the charging time is about 10/sup -5/ s. The influence of the plasma drift velocity on the macroparticle charging is small. The model presented here agrees well with an experimental study of macroparticle repulsion from biased substrates.

Spatiotemporal analysis of the double layer formation in hydrogen radio frequency discharges

Abstract: Radio-frequency discharges at 13.56 MHz in pure H2 and CH4-SiH4 mixtures highly diluted in H2 are investigated by spatially and spatiotemporally resolved emission spectroscopy. Spatial emission profiles of the excited species exhibit double layers near the electrodes in pure hydrogen and hydrogen-controlled discharges. Spatiotemporal analyses give complementary information showing that the emitting layers, which result from atomic and molecular excitations, occur at different times in the radio-frequency cycle. The mechanisms involved in the double layer formation are interpreted by simplified numerical models based on both a fluid model and a particle-in-cell-Monte Carlo model. The results of the models are in good agreement with the spatiotemporal representations of the excitation rate obtained experimentally. The formation of the double layers is analysed as resulting from the high drift velocity of the hydrogen ions. This behaviour seems to be characteristic of radio frequency discharges in hydrogen.

Convection in binary fluid mixtures. II. Localized traveling waves

Abstract: Nonlinear, spatially localized structures of traveling convection rolls that are surrounded by quiescent fluid in horizontal layers of binary fluids heated from below are investigated in quantitative detail as a function of Rayleigh number for two different Soret coupling strengths (separation ratios) with Lewis and Prandtl numbers characterizing ethanol-water mixtures. A finite-difference method was used to solve the full hydrodynamic field equations numerically in a vertical cross section perpendicular to the roll axes subject to realistic horizontal and laterally periodic boundary conditions with different periodicity lengths. Structure and dynamics of these localized traveling waves (LTW's) are dominated by the concentration field. As in the spatially extended convective states that are investigated in an accompanying paper, the Soret-induced concentration variations strongly influence, via density changes, the buoyancy forces that drive convection. The spatiotemporal properties of this feedback mechanism, involving boundary layers and concentration plumes, show that LTW's are strongly nonlinear states. Light intensity distributions are determined that can be observed in side-view shadowgraphs done with horizontal light along the roll axes. Detailed analyses of all fields are made using color-coded isoplots, among others. In the frame comoving with their drift velocity, LTW's display a nontrivial spatiotemporal symmetry consisting of time translation by one-half an oscillation period combined with vertical reflection through the horizontal midplane of the layer. A time-averaged concentration current is driven by a phase difference between the waves of concentration and vertical velocity in the bulk of the LTW state. The associated large-scale concentration redistribution stabilizes the LTW and controls its drift velocity into the quiescent fluid by generating a buoyancy-reducing concentration "barrier" ahead of the leading LTW front. All considered LTW's drift very slowly into the direction of the phase velocity of the pattern. For weak Soret coupling, $\ensuremath{\psi}=\ensuremath{-}0.08$, LTW's have a small selected width and exist in a narrow band of Rayleigh numbers above the stability threshold for growth of TW's. For stronger coupling, $\ensuremath{\psi}=\ensuremath{-}0.25$, LTW's exist below the bifurcation threshold for extended TW's in a narrow band of Rayleigh numbers. In its lower part, LTW's have a small selected width. For somewhat higher Rayleigh numbers, there exist two LTW attractors with two different widths. For yet higher Rayleigh numbers, there is again only one LTW attractor; however, with a broader width. Dynamical properties and the dependence on the system length are analyzed. Comparisons with experiments are presented.

Control of Re-Entrant Activity in a Model of Mammalian Atrial Tissue

Abstract: We evaluate the feasibility of using resonant drift under feedback driven stimulation to control re-entrant excitation in atrial muscle. We simulate a two-dimensional sheet of atrial tissue, where the local kinetics are described by the Earm-Hilgemann-Noble equations for a rabbit atrial cell, and the effects of small amplitude spatially uniform forcing of the whole sheet are computed. Repetitive forcing can induce a drift of a spiral wave in the two-dimensional model, with a drift velocity of up to 10 cm s$^{-1}$. For a 4 cm $\times $ 4 cm atrial surface this resonant drift can move the re-entrant spiral to the inexcitable boundaries, eliminating re-entry in less than 10 s when the amplitude of the repetitive stimulation is 10% that of the single shock defibrillation threshold.

Drift velocity and longitudinal diffusion coefficient of electrons in CO 2 -Ar mixtures and electron collision cross sections for CO 2 molecules

Abstract: The drift velocity and longitudinal diffusion coefficient of electrons in 0·2503% and 1· 97% C02-Ar mixtures were measured for 0·03 ~ E/N ~ 20 Td. The measured electron swarm parameters in the mixtures were used to derive a set of consistent vibrational excitation cross sections for the C02 molecule. Analysis of electron swarms in pure C02 using the present vibrational excitation cross sections was also carried out in order to determine a new momentum transfer cross section for the C02 molecule.

Forced diffusion of impurities in natural diamond and polycrystalline diamond films

Abstract: A method is proposed for the determination of the state of an impurity (donor, acceptor, or deep level) in semiconductor lattice. To demonstrate the method boron was diffused into type Ia natural diamond under a dc electric field. The concentration and diffusion profiles of boron were affected by the applied field. Boron diffuses as a negative ion since it is an acceptor shallow enough to be partially ionized at the temperature of diffusion. The drift velocity of boron ions at the temperature of diffusion was also estimated. The diffusion of lithium and oxygen from a Li2CO3 source in chemical vapor deposited diamond films was performed under bias at 1000 °C in an argon atmosphere. After diffusion, the concentrations of Li, O, and H in the diamond films were found to be around (3–4)×1019 cm−3. No dependence of these concentrations on the applied bias was observed. It was found that the diffusion of Li goes primarily through grain boundaries, which may explain why it does not depend on the applied voltage. ...

Positron mobility in polyethylene in the 60–400 K temperature range

Abstract: We have determined the positron mobility (μ+) in polyethylene samples (67.2% crystalline, glass transition temperatureTg=151 K) in the 64–400 K temperature range by Doppler shift measurements. A method based on the simulataneous observation of two γ lines from133Ba and137Cs radioactive sources together with the positron annihilation γ line, was employed to measure the Doppler shift of the 511 keV γ line as a function of the electric field applied to the samples. With this method we were able to measure at the same time the drift velocity of positrons and theS parameter. This parameter is very important in the interpretation of the mobility trend in samples where the positron states change with temperature. The positron mobility was corrected for positronium formation. μ+ at 64 K is 31.7±0.8 cm2 V−1 s−1 then decreases up to 123 K, increases at 148 K and decreases again up to 170 K (μ+=26.9±0.8 cm2 V−s−). This sharp change in mobility is centred around the glass transition temperature of our samples. Then the mobility remains almost constant up to 230 K. From 250 K to 377 K, μ+ increases and reaches the value of 38.4±1.0 cm2 V−1s−1. The corrected experimental data were well fitted by a simple model taking into account scattering and a thermally activated process (hopping mechanism).

Carrier drift mobility study in neutron irradiated high purity silicon

Abstract: Carrier drift mobility, drift velocity and its saturation value have been studied for silicon p-n junction detectors irradiated to neutron fluence range specific for experiments in the future large colliders. The transient current technique (TCT), which monitors the drift current of externally generated free carriers, has been applied for direct measurements of electron and hole drift velocities in fully depleted p-n junction detectors with resistivity ranging from 300 to 5000 ω cm. It has been found that neutron radiation up to fluences of about 5 × 10 13 n/cm 2 has little effect on the carrier transport properties, including carrier drift mobilities for both carriers and their saturation velocities, at room temperature and at lower temperatures all the way down to 110 K at which the phonon scattering is sufficiently suppressed. This fact has been discussed in terms of efficiency of scattering by radiation induced defects and disorder regions as compared with that by phonon.

Two different types of enhanced ion acoustic fluctuations observed in the upper ionosphere

Abstract: UHF and VHF data for the EISCAT incoherent scatter radar facility in northern Scandinavia is presented. Electron and ion temperatures, electron density, and ion drift velocity were measured from heights of 280 to 1500 km. Enhanced ion acoustic fluctuations are more observable with VHF than UHF radar due to wavelength effects. The fluctuations are usually associated with a large flux of precipitating electrons with energies from 100 ev to 10 kev. The spatial extent of the turbulent regions are determined. 23 refs., 6 figs.