334 research outputs found
Direct measurement of doping density and barrier lowering effect with bias in quantum wells
An experimental method for determining the doping density in thin-sheet semiconductor material such as quantum wells (QWs) is demonstrated in GaAs/AlGaAs multiquantum-well infra-red photodetectors. The results agree very well with the conventional Hall measurement method. Barrier lowering effect with bias in QWs is determined experimentally
Study of Phonon Modes in Germanium Nanowires
The observation of pure phonon confinement effect in germanium nanowires is
limited due to the illumination sensitivity of Raman spectra. In this paper we
measured Raman spectra for different size germanium nanowires with different
excitation laser powers and wavelengths. By eliminating the local heating
effect, the phonon confinement effect for small size nanowires was clearly
identified. We have also fitted the Raman feature changes to estimate the size
distribution of nanowires for the first time.Comment: 11 pages,15 figure
1D Phonon BTE Solver (Small Scale Heat Transport Simulation)
In current technology, electronic devices shrink to the size of nanometers. The ability to accurately model heat transport to understand the thermal behavior of these small electronic devices becomes increasingly important. Since heat transport is very difficult to measure directly in small electronic devices, simulation becomes an effective means to model heat transport. A user-interactive simulation tool is created to model heat transport in small electronic devices of different lengths. Heat transport is modeled by solving one-dimensional Boltzmann transport equation (BTE) to obtain the transient temperature profile of a multi-length and multi-timescale thin film under constant temperature boundary condition or under hotspot cooling process. Unlike Fourier Heat equation, BTE can capture the effect of ballistic phonon transport expected at short lengh/time scale. Rapid Application Infrastructure (Rappture), a toolkit used to create user-interactive graphical user interface, is used to create the user interface for this simulation. The inputs to the simulation tool are thin film length and simulation time specified by user. BTE is then solved by using Lattice Boltzmann method (LBM) in MATLAB to obtain the temperature profile plot. The temperature profile plot explains how the temperature changes throughout the entire length of the material. This simulation tool allows users to accurately simulate heat transport in small electronic devices of different lengths that will help them in the thermal design and thermal management of small electronic devices
Ultrafast Temperature Profile Calculation in Ic Chips
One of the crucial steps in the design of an integrated circuit is the
minimization of heating and temperature non-uniformity. Current temperature
calculation methods, such as finite element analysis and resistor networks have
considerable computation times, making them incompatible for use in routing and
placement optimization algorithms. In an effort to reduce the computation time,
we have developed a new method, deemed power blurring, for calculating
temperature distributions using a matrix convolution technique in analogy with
image blurring. For steady state analysis, power blurring was able to predict
hot spot temperatures within 1 degree C with computation times 3 orders of
magnitude faster than FEA. For transient analysis the computation times where
enhanced by a factor of 1000 for a single pulse and around 100 for multiple
frequency application, while predicting hot spot temperature within about 1
degree C. The main strength of the power blurring technique is that it exploits
the dominant heat spreading in the silicon substrate and it uses superposition
principle. With one or two finite element simulations, the temperature point
spread function for a sophisticated package can be calculated. Additional
simulations could be used to improve the accuracy of the point spread function
in different locations on the chip. In this calculation, we considered the
dominant heat transfer path through the back of the IC chip and the heat sink.
Heat transfer from the top of the chip through metallization layers and the
board is usually a small fraction of the total heat dissipation and it is
neglected in this analysis.Comment: Submitted on behalf of TIMA Editions
(http://irevues.inist.fr/tima-editions
Method of Images for the Fast Calculation of Temperature Distributions in Packaged VLSI Chips
Thermal aware routing and placement algorithms are important in industry.
Currently, there are reasonably fast Green's function based algorithms that
calculate the temperature distribution in a chip made from a stack of different
materials. However, the layers are all assumed to have the same size, thus
neglecting the important fact that the thermal mounts which are placed
underneath the chip can be significantly larger than the chip itself. In an
earlier publication, we showed that the image blurring technique can be used to
calculate quickly temperature distribution in realistic packages. For this
method to be effective, temperature distribution for several point heat sources
at the center and at the corner and edges of the chip should be calculated
using finite element analysis (FEA) or measured. In addition, more accurate
results require correction by a weighting function that will need several FEA
simulations. In this paper, we introduce the method of images that take the
symmetry of the thermal boundary conditions into account. Thus with only "two"
finite element simulations, the steady-state temperature distribution for an
arbitrary complex power dissipation profile in a packaged chip can be
calculated. Several simulation results are presented. It is shown that the
power blurring technique together with the method of images can reproduce the
temperature profile with an error less than 0.5%.Comment: Submitted on behalf of TIMA Editions
(http://irevues.inist.fr/tima-editions
Effect of enhanced external counterpulsation and cardiac rehabilitation on quality of life, plasma nitric oxide, endothelin 1 and high sensitive CRP in patients with coronary artery disease: A pilot study
Objective To investigate the effect of enhanced external counterpulsation (EECP) on plasma nitric oxide (NO), Endothelin 1 (ET1), high sensitive C-reactive protein (HSCRP) and quality of life (QoL) in patients with coronary artery disease (CAD).Methods We conducted a pilot randomized clinical trial in order to evaluate plasma NO, ET1, HSCRP and QoL before and after twenty sessions of EECP (group A) and cardiac rehabilitation (CR, group B) in 42 patients with CAD (21 in each group).Results Forty-two patients (33 male and 9 female) were included in the study. The mean age was 58.2±10 years. The mean HSCRP was 1.52±0.7 in the EECP group and it was reduced to 1.27±0.4 after intervention. The reduction in HSCRP was not statistically significant in EECP and CR groups with p=0.33 and p=0.27, respectively. There was not significant improvement of NO, ET1, and QoL in the EECP and CR groups shortly after therapy (p>0.05).Conclusion Although the short-term EECP treatment in CAD patients improved HSCRP, NO, ET1, and QoL compared with the baseline those improvements are not statistically significant. Further studies are necessary with large study groups and more sessions. © 2015 by Korean Academy of Rehabilitation Medicine
Efficiency in nanostructured thermionic and thermoelectric devices
Advances in solid-state device design now allow the spectrum of transmitted
electrons in thermionic and thermoelectric devices to be engineered in ways
that were not previously possible. Here we show that the shape of the electron
energy spectrum in these devices has a significant impact on their performance.
We distinguish between traditional thermionic devices where electron momentum
is filtered in the direction of transport only and a second type, in which the
electron filtering occurs according to total electron momentum. Such 'total
momentum filtered' kr thermionic devices could potentially be implemented in,
for example, quantum dot superlattices. It is shown that whilst total momentum
filtered thermionic devices may achieve efficiency equal to the Carnot value,
traditional thermionic devices are limited to efficiency below this. Our second
main result is that the electronic efficiency of a device is not only improved
by reducing the width of the transmission filter as has previously been shown,
but also strongly depends on whether the transmission probability rises sharply
from zero to full transmission. The benefit of increasing efficiency through a
sharply rising transmission probability is that it can be achieved without
sacrificing device power, in contrast to the use of a narrow transmission
filter which can greatly reduce power. We show that devices which have a
sharply-rising transmission probability significantly outperform those which do
not and it is shown such transmission probabilities may be achieved with
practical single and multibarrier devices. Finally, we comment on the
implications of the effect the shape of the electron energy spectrum on the
efficiency of thermoelectric devices.Comment: 11 pages, 15 figure
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