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Results for “two-dimensional electron gas”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records
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NASA NTRS1984

Electron mobility in modulation-doped heterostructures

A model for electron mobility in a two-dimensional electron gas confined in a triangular well was developed. All major scattering processes (deformation potential and piezoelectric acoustic, polar optical, ionized impurity, and alloy disorder) were included, as well as intrasubband and intersubband scattering. The model is applied to two types of modulation-doped heterostructures, namely GaAs-GaAlAs and In(0.53)Ga(0.47)As-Al(0.52)In(0.48)As. In the former case, phonons and remote ionized impurities ultimately limit the mobility, whereas in the latter, alloy disorder is a predominant scattering process at low temperatures. The calculated mobilities are in very good agreement with recently reported experimental characteristics for both GaAs-Ga(1-x)Al(x)As and In(0.53)Ga(0.47)As-Al(0.52)In(0.48)As modulation-doped heterostructures.

Walukiewicz, W.↗
NASA NTRS1986

Characterization of InGaAs/AlGaAs pseudomorphic modulation-doped field-effect transistors

High-performance pseudomorphic In(y)Ga(1-y)As/Al0.15-Ga0.85As y = 0.05-0.2 MODFET's grown by MBE have been characterized at dc (300 and 77 K) and RF frequencies. Transconductances as high 310 and 380 mS/mm and drain currents as high as 290 and 310 mA/mm were obtained at 300 and 77 K, respectively, for 1-micron gate lengths and 3-micron source-drain spacing devices. Lack of persistent trapping effects, I-V collapse, and threshold voltage shifts observed with these devices are attributed to the use of low mole fraction Al(x)Ga(1-x)As while still maintaining two-dimensional electron gas concentrations of about 1.3 x to the 12th per sq cm. Detailed microwave S-parameter measurements indicate a current gain cut-off frequency of 24.5 GHz when y = 0.20, which is as much as 100 percent better than similar GaAs/AlGaAs MODFET structures, and a maximum frequency of oscillation of 40 GHz.

Ketterson, Andrew A.↗
NASA NTRS1987

Pseudomorphic InGaAs/AlGaAs modulation-doped FET's with reduced low-frequency noise and thermally stable performance

A high-performance MODFET structure grown by MBE with the incorporation of a single quantum well In(0.15)Ga(0.85)As layer for the transport of two-dimensional electron gas has been critically examined for its thermal stability at 80 K and low-frequency noise form 0.01 to 10 to the 8th Hz. Experimental results indicate that the behavior of this device in both these respects is much superior when compared with the same behavior of conventional MODFETs. A maximum low-field carrier mobility of 29,000 sq cm/s at 80 K and an average carrier saturation velocity of 2 x 10 to the 7th cm/s at 300 K in a 1-micron gate device clearly indicate that the quality of the pseudomorphic quantum well (InGaAs) layer is either comparable or better than that of the usual GaAs buffer layer. The deep level spectra, obtained through photo-FET measurements, and the low-frequency noise spectra at different temperatures obtained for the new pseudomorphic and conventional MODFET's have clearly indicated that contributions from various deep levels present in the new structure are significantly reduced.

Liu, Shih-Ming↗
NASA NTRS1987

Anisotropic transport in modulation-doped quantum-well structures

Anisotropic electron transport has been observed in GaAs modulation-doped quantum wells grown by molecular-beam epitaxy on a thick (001) Al(0.3)Ga(0.7)As buffer grown at 620 C. Thicker quantum wells (150, 200, and 300 A) show progressively less anisotropy, which vanishes for a 300-A quantum well. The degree of anisotropy is also reduced or eliminated by suspending growth of the Al(0.3)Ga(0.7)As for a period of 300 s prior to growing the GaAs quantum well. Growing the Al(0.3)Ga(0.7)As buffer at higher temperatures (680 C) also reduces the degree of anisotropy. Higher two-dimensional electron gas sheet densities result in less anisotropy.The anisotropy is eliminated by replacing the thick Al(0.3)Ga(0.7)As buffer with a periodic multilayer structure comprising 15 A of GaAs and 200 A of Al(0.3)Ga(0.7)As. The degree of anisotropy is related to the thickness and growth parameters of the Al(0.3)Ga(0.7)As layer grown just prior to the growth of the GaAs.

Radulescu, D. C.↗
NASA NTRS1987

Effects of substrate misorientation and background impurities on electron transport in molecular-beam-epitaxial grown GaAs/AlGaAs modulation-doped quantum-well structures

The effects of substrate misorientation off the (001) plane and of background impurities on electron transport in MBE-grown GaAs/AlGaAs modulation-doped superlattice-buffered quantum-well structures were investigated. Low-field transport data were obtained on GaAs/AlGaAs structures grown on substrates oriented 0, 2, 4, and 6.5 deg off the (001) plane towards either (111)A or (111)B. It is shown that the low-field two-dimensional electron gas (2DEG) mobility is a function of the angle and direction of the substrate orientation, and that the 2DEG mobility is a function of the direction of the applied electric field in the GaAs quantum well. The anisotropy in the 2DEG mobility is also a function of the tilt angle and tilt azimuth direction of the substrate from the (001) plane. In addition, it is shown that the amount of interface scattering from the inverted interface is a sensitive function of the amount of background impurities in the MBE machine.

Radulescu, D. C.↗
NASA NTRS1989

Calculation of the electron wave function in a graded-channel double-heterojunction modulation-doped field-effect transistor

Three double-heterojunction modulation-doped field-effect transistor structures with different channel composition are investigated theoretically. All of these transistors have an In(x)Ga(1-x)As channel sandwiched between two doped Al(0.3)Ga(0.7)As barriers with undoped spacer layers. In one of the structures, x varies from 0 from either heterojunction to 0.15 at the center of the channel quadratically; in the other two, constant values of x of 0 and 0.15 are used. The Poisson and Schroedinger equations are solved self-consistently for the electron wave function in all three cases. The results showed that the two-dimensional electron gas (2DEG) concentration in the channel of the quadratically graded structure is higher than the x = 0 one and slightly lower than the x = 0.15 one, and the mean distance of the 2DEG is closer to the center of the channel for this transistor than the other two. These two effects have important implications on the electron mobility in the channel.

Mui, D. S. L.↗
NASA NTRS1990

Selective equilibration among the current-carrying states in the quantum Hall regime

The Hall resistance of a two-dimensional electron gas is measured with gated probes to determine the extent of equilibration among the N current-carrying states in the quantum Hall regime. After traveling macroscopic distances, current injected into the first state is equilibrated among the N - 1 lowest states but equilibration into the highest state varies strongly across the Hall plateau. This is attributed to a change in the Nth state from being localized within a magnetic length of the edge to substantially extending into the sample.

Alphenaar, B. W.↗
NASA NTRS1990

Surface electrons in inverted layers of p-HgCdTe

Anodic oxide passivation of p-type HgCdTe generates an inversion layer. Extremely high Hall mobility data for electrons in this layer indicated the presence of a two-dimensional electron gas. This is verified by use of the Shubnikov-de Haas effect from 1.45 to 4.15 K. Data are extracted utilizing a numerical second derivative of dc measurement. Three sub-bands are detected. Their relative occupancies are in excellent agreement with theory and with experimental results obtained on anodic oxide as accumulation layers of n-type HgCdTe. The effective mass derived is comparable to what was expected.

Schacham, Samuel E.↗
NASA NTRS1990

GaAs CCD readout for engineered bandgap detectors

GaAs and related heterostructure charge-coupled devices (CCDs) for detector array readout multiplexer applications are described. Features of resistive-gate CCDs are compared with capacitive-gate CCDs for this application. Design examples of GaAs CCD readouts for linear and two-dimensional detector arrays with different detection methods and signal coupling schemes are described. Recent progress in two-dimensional electron gas (2DEG) Al(0.3)Ga(0.7)As/GaAs resistive-gate CCDs is also presented. The 2DEG resistive-gate CCD conventional buried-channel GaAs CCD.

Song, J.-I.↗
NASA NTRS1983

Two-dimensional photometry of planetary nebulae

In connection with the study of planetary nebulae, problems still exist in understanding such basic properties as three-dimensional structure, optical opacity to the central star's ionizing flux, and electron temperature and electron density variations within the nebular gas. To study these properties, two-dimensional images taken in many spectral lines are required. However, such a study presents a formidable problem in data analysis. In the present investigation, an attempt has been made to overcome the difficulties by using an imaging system which encodes the data digitally. Calibrated intensity maps could be constructed to test models of ionization structure and to produce two-dimensional maps of electron temperature and density. Both the results of a uniform-shell test and the nature of the solutions for the volume emissivity were found to support a nebular model in which the bright ring is part of a closed shell of variable density that resembles the torus proposed by Minkowski and Osterbrock (1960).

Kupferman, P. N.↗
NASA NTRS1990

Effects of neutral gas releases on electron beam injection from electrically tethered spacecraft

The presence of high neutral densities at low altitudes and/or during thruster firings is known to modify the spacecraft potential during active electron beam injection. Two-dimensional (three velocity) particle simulations are used to investigate the ionization processes including the neutral density required, the modification of the spacecraft potential, beam profile and spatial distribution of the return current into the spacecraft. Three processes are identified: (1) beam-induced ionization, (2) vehicle-induced ionization, and (3) beam plasma discharge. Only in the first two cases does the beam propagate away with little distortion.

Winglee, R. M.↗
NASA NTRS1989

Calculation of real-gas effects on blunt-body trim angles

The effect of vibrational excitation and dissociation at high temperatures on the trim angle of attack of a blunt lifting body is calculated for a nonequilibrium flow regime in air using a CFD technique. The vibrational-electronic temperature and the species densities are calculated assuming the flow to be in a nonequilibrium state. The forebody flow of a two-dimensional blunt body of the shape of the Apollo Command Module at a finite angle of attack is calculated. The results show that the pitching moment around a reference point is larger and the trim angle of attack is smaller for a reacting gas than for a perfect gas. The calculated shift in the trim angle due to the real-gas effect is of the same order as that seen during the Apollo flights.

Park, Chul↗
NASA NTRS1975

The NATA code: Theory and analysis, volume 1

A computer program for calculating quasi-one-dimensional gas flow in axisymmetric and two-dimensional nozzles and rectangular channels is presented. Flow is assumed to start from a state of thermochemical equilibrium at a high temperature in an upstream reservoir. The program provides solutions based on frozen chemistry, chemical equilibrium, and nonequilibrium flow with finite reaction rates. Electronic nonequilibrium effects can be included using a two-temperature model. An approximate laminar boundary layer calculation is given for the shear and heat flux on the nozzle wall. Boundary layer displacement effects on the inviscid flow are considered also. Chemical equilibrium and transport property calculations are provided by subroutines. The code contains precoded thermochemical, chemical kinetic, and transport cross section data for high-temperature air, CO2-N2-Ar mixtures, helium, and argon. It provides calculations of the stagnation conditions on axisymmetric or two-dimensional models, and of the conditions on the flat surface of a blunt wedge. The primary purpose of the code is to describe the flow conditions and test conditions in electric arc heated wind tunnels.

Bade, W. L.↗
NASA NTRS1988

Plasma contactors for use with electodynamic tethers for power generation

Plasma contactors are proposed as a means of making good electrical contact between biased surfaces such as found at the ends of an electrodynamic tether and the space environment. The plasma contactor emits a plasma cloud which facilitates the electrical connection. The physics of this plasma cloud is investigated for contactors used as electron collectors. The central question addressed is whether the electrons collected by a plasma contactor come from the far field or by ionization of local neutral gas. This question is important because the system implications are different for the two mechanisms. It is shown that contactor clouds in space will consist of a spherical core possibly containing a shock wave. Outside of the core the cloud will expand anisotropically across the magnetic field leading to a turbulent cigar shape structure along the field. This outer region is itself divided into two regions by the ion response to the electric field. A two-dimensional theory for the outer regions of the cloud is developed. The current voltage characteristic of an Argon plasma contactor cloud is estimated for several ion currents in the range of 1 to 100 Amperes. It is suggested that the major source of collected electrons comes by ionization of neutral gas while collection of electrons from the far field is relatively small.

Hastings, D. E.↗
NASA NTRS1988

The electromagnetic effect on the critical ionization velocity process

Electromagnetic effects on the critical ionization velocity (CIV) process become important when the neutral gas velocity V(n) exceeds the local Alfven speed V(A). The electron heating due to unstable lower hybrid waves necessary for CIV still occurs, but the efficiency of the electron heating is significantly reduced when the electromagnetic effect comes into play. This is verified by a series of simulation runs using two-dimensional electromagnetic particle code combined with PANIC. The significance of the electromagnetic effects for the occurrence of CIV in the comet-solar wind interaction and other space phenomena is briefly discussed. It is found that the comet environment is marginal for the excitation of CIV.

Machida, S.↗
NASA NTRS1985

The effect of hydrodynamic turbulence on Langmuir waves in a slightly ionized plasma

In the lower altitude regions of the ionosphere, the gas present is only slightly ionized, and it behaves as a fluid. It is expected that a turbulent wake will form behind a spacecraft. Information is to be obtained regarding the effect of this turbulence on plasma waves measured by the spacecraft. If the turbulence is two-dimensional (2-D) and carries enstrophy, a fluctuating magnetic field and electric fields are produced. The fields induce fluctuations in the electron population which correlate with the magnetic and electric fluctuations which produced them. This correlation causes diffusion which changes the electron distribution and heats the electrons. As a result, the Langmuir frequency increases, and the Landau damping of Langmuir waves in the plasma decreases. Details regarding these processes are discussed, taking into account the results of a mathematical analysis.

Parish, J. L.↗
NASA NTRS1974

Optical measurement of static temperature and hydroxyl radical profiles in a hydrogen-fueled supersonic combustor

Profiles of static temperature and hydroxyl radical concentration were measured in a two-dimensional supersonic combustor test section 22.8 cm downstream of hydrogen injection. A high-pressure gas generator supplied vitiated air to the test section at Mach 2.44, atmospheric pressure, and a total temperature of about 2240 K. Room-temperature hydrogen was injected through a 0.40-cm step slot at Mach 1 and matched pressure. The measurements utilized a noninterfering spectral line absorption technique in which narrow ultraviolet emission lines of the hydroxyl electronic transition are absorbed by the broader absorption lines in the combustion gas. Comparison of the measured temperature profiles with theoretical calculations showed good agreement.

Gaugler, R. E.↗
NASA NTRS1989

A multi-temperature TVD algorithm for relaxing hypersonic flows

In this paper, the extension of a multispecies TVD algorithm, second-order accurate for real-gas flows to a multitemperature formulation is described. The convection algorithm is coupled to internal relaxation processes, and the features of the coupling are examined. The first version consists of a three-temperature model, where translational-rotational, vibrational, and electronic energy modes are separately convected. Although several species are present, there is only one vibrational temperature in this model. The second version generalizes to a vibrational temperature for each molecular specie, with additional couplings between species. The algorithms are applied to a generic two-dimensional flow field, and results are compared with experimental observations.

Cambier, Jean-Luc↗