837 research outputs found
Design of testbed and emulation tools
The research summarized was concerned with the design of testbed and emulation tools suitable to assist in projecting, with reasonable accuracy, the expected performance of highly concurrent computing systems on large, complete applications. Such testbed and emulation tools are intended for the eventual use of those exploring new concurrent system architectures and organizations, either as users or as designers of such systems. While a range of alternatives was considered, a software based set of hierarchical tools was chosen to provide maximum flexibility, to ease in moving to new computers as technology improves and to take advantage of the inherent reliability and availability of commercially available computing systems
Thermal velocity limits to diffusive electron transport in thinâbase np+n GaAs bipolar transistors
We present experimental evidence that minority electron transport across a thin, quasineutral p+ GaAs region is limited by the thermal velocity of the electrons rather than by conventional diffusive transport. A set of GaAs homojunction np+n transistors with base widths of 4000, 2000, 1000, and 500 Ă
was fabricated and characterized. The diffusive modelpredicts that the dc collector current of the 500âĂ
base width transistors should be eight times larger than the collector current of transistors with a 4000âĂ
âwide base. The experimental results, however, show only a factor of ~3.5 increase in collector current. The measured collector current versus base width characteristic agrees well with theoretical treatments of thinâbase transport. These new results present evidence of quasiballistic electron transport in p+ GaAs and have important implications for GaAs transistor design
Basic Studies of III-V High Efficiency Cell Components
The objective of the project is to raise the understanding of dark current mechanisms in GaAs-related solar cells to a level comparable to that of silicon cells. Motivation for this work arises from the observation that much of the progress in crystalline silicon cell performance has occurred as a result of a very deep knowledge of the physics controlling the cellâs dark current. Based on this knowledge, new cell structures evolved to suppress dominant dark current mechanisms. A comparable level of knowledge of GaAs cell device physics does not yet exist, but will be essential if cell performance near the thermodynamic limit is to be achieved. Moreover, knowledge gained from studies of the AlGaAs/GaAs material system, should help identify the key problems to be addressed in other III-V materials
Concentrationâdependent opticalâabsorption coefficient in nâtype GaAs
The doping-dependent, near-band-edge optical-absorption coefficient CY(h v) was deduced from optical transmission measurements in n-type GaAs thin films. The selenium-doped films were grown by metalorganic chemical-vapor deposition and do ed to produce room-temperature electron concentrations from 1.3 x 10â to 3.8X 1018 cm- P . The transmission measurements covered photon energies between 1.35 and 1.7 eV and were performed on double heterostructures with the substrate removed by selective etching. The results show good qualitative agreement with previous studies and good quantitative agreement, except for the heavily doped samples. For na=3.8 X 10â cme3, a( 1.42 eV\u3e is approximately four times that reported by previous workers. Secondary-ion-mass spectrometry measurements on flms grown under differing conditions demonstrate that a(hv) is sensitive to electrically inactive dopants and supports the hypothesis that precipitates or compensation influenced previous measurements. These comprehensive results on high-quality, uncompensated material should prove useful for fundamental studies of optical transitions in n-type GaAs as well as for modeling optoelectronic devices
Orientation-dependent perimeter recombination in GaAs diodes
Perimeter recombination currents affect the performance of GaAs-based devices such as solar cells, heterojunction bipolar transistors, and injection lasers. We report that the n SEf 2 perimeter recombination current has a strong orientation dependence. More than a factor of five variation in the surface recombination current at mesa-etched edges has been observed. These results suggest that with proper device design, perimeter recombination currents could be substantially reduced
Basic Studies of III-IV High Efficiency Cell Components
The objective of the project is to raise the understanding of dark current mechanisms in GaAs-related solar cells to a level comparable to that of silicon cells. Motivation for this work arises from the observation that much of the progress in crystalline silicon cell performance has occurred as a result of a very deep knowledge of the physics controlling the cellâs dark current. Based on this knowledge, new cell structures evolved to suppress dominant dark current mechanisms. A comparable level of knowledge of GaAs cell device physics does not yet exist, but will be essential if cell performance near the thermodynamic limit is to be achieved
Modeling a Schottky-barrier carbon nanotube field-effect transistor with ferromagnetic contacts
In this study, a model of a Schottky-barrier carbon nanotube field- effect
transistor (CNT-FET), with ferromagnetic contacts, has been developed. The
emphasis is put on analysis of current-voltage characteristics as well as shot
(and thermal) noise. The method is based on the tight-binding model and the
non- equilibrium Green's function technique. The calculations show that, at
room temperature, the shot noise of the CNT FET is Poissonian in the
sub-threshold region, whereas in elevated gate and drain/source voltage regions
the Fano factor gets strongly reduced. Moreover, transport properties strongly
depend on relative magnetization orientations in the source and drain contacts.
In particular, one observes quite a large tunnel magnetoresistance, whose
absolute value may exceed 50%.Comment: 8 pages, 4 figure
Recombination-current suppression in GaAs p-n junctions grown on AlGaAs buffer layers by molecular-beam epitaxy
n+pp+GaAs and n+pP+ GaAs/GaAs/Al0.3Ga0.7As mesa diodes have been fabricated from films grown by molecularâbeam epitaxy. The diodes made from films employing an AlGaAs buffer layer show marked improvements (a factor of 5 reduction) in recombination current densities. Deep level transient spectroscopy measurements moreover indicate that deep level concentrations are reduced by the AlGaAs buffer
Decoherence due to contacts in ballistic nanostructures
The active region of a ballistic nanostructure is an open quantum-mechanical
system, whose nonunitary evolution (decoherence) towards a nonequilibrium
steady state is determined by carrier injection from the contacts. The purpose
of this paper is to provide a simple theoretical description of the
contact-induced decoherence in ballistic nanostructures, which is established
within the framework of the open systems theory. The active region's evolution
in the presence of contacts is generally non-Markovian. However, if the
contacts' energy relaxation due to electron-electron scattering is sufficiently
fast, then the contacts can be considered memoryless on timescales coarsened
over their energy relaxation time, and the evolution of the current-limiting
active region can be considered Markovian. Therefore, we first derive a general
Markovian map in the presence of a memoryless environment, by coarse-graining
the exact short-time non-Markovian dynamics of an abstract open system over the
environment memory-loss time, and we give the requirements for the validity of
this map. We then introduce a model contact-active region interaction that
describes carrier injection from the contacts for a generic two-terminal
ballistic nanostructure. Starting from this model interaction and using the
Markovian dynamics derived by coarse-graining over the effective memory-loss
time of the contacts, we derive the formulas for the nonequilibrium
steady-state distribution functions of the forward and backward propagating
states in the nanostructure's active region. On the example of a double-barrier
tunneling structure, the present approach yields an I-V curve with all the
prominent resonant features. The relationship to the Landauer-B\"{u}ttiker
formalism is also discussed, as well as the inclusion of scattering.Comment: Published versio
Microsecond Lifetimes and Low Interface Recombination Velocities in Moderately Doped n-GaAs Thin Films
We have observed lifetimes greater than 1 ps in moderately doped, thin film, n-GaAs/A1a,Gae,As double heterostructure membranes formed by etching away the substrate. We attribute these ultralong lifetimes to enhanced photon recycling caused by the removal of the substrate. Nonradiative recombination in the bulk and at the interfaces is very low; the upper limit of the interface recombination velocity is 25 cm/S.-Such long lifetimes in GaAs doped at N,= 1.3 X 10â cme3 suggest that thin-film solar cells offer a potential option for achieving very high efficiencies
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