Influence of Dimensionality on Thermoelectric Device Performance

The role of dimensionality on the electronic performance of thermoelectric devices is clarified using the Landauer formalism, which shows that the thermoelectric coefficients are related to the transmission, T(E), and how the conducing channels, M(E), are distributed in energy. The Landauer formalism applies from the ballistic to diffusive limits and provides a clear way to compare performance in different dimensions. It also provides a physical interpretation of the "transport distribution," a quantity that arises in the Boltzmann transport equation approach. Quantitative comparison of thermoelectric coefficients in one, two, and three dimension shows that the channels may be utilized more effectively in lower-dimensions. To realize the advantage of lower dimensionality, however, the packing density must be very high, so the thicknesses of the quantum wells or wires must be small. The potential benefits of engineering M(E) into a delta-function are also investigated. When compared to a bulk semiconductor, we find the potential for ~50 % improvement in performance. The shape of M(E) improves as dimensionality decreases, but lower dimensionality itself does not guarantee better performance because it is controlled by both the shape and the magnitude of M(E). The benefits of engineering the shape of M(E) appear to be modest, but approaches to increase the magnitude of M(E) could pay large dividends.Comment: 23 pages, 5 figure

Oxygen in the Galactic thin and thick disks

First results from a study into the abundance trends of oxygen in the Galactic thin and thick disks are presented. Oxygen abundances for 21 thick disk and 42 thin disk F and G dwarf stars based on very high resolution spectra (R\sim 215000) and high signal-to-noise (S/N>400) of the faint forbidden oxygen line at 6300 A have been determined. We find that [O/Fe] for the thick disk stars show a turn-down, i.e. the ``knee'', at [Fe/H] between -0.4 and -0.3 dex indicating the onset of SNe type Ia. The thin disk stars on the other hand show a shallow decrease going from [Fe/H] \sim -0.7 to the highest metallicities with no apparent ``knee'' present indicating a slower star formation history.Comment: To be published in "CNO in the Universe", ASP Conference Series, C. Charbonnel, D. Schaerer & G. Meynet (eds.

Properties of the Milky Way stellar disks in the direction of the Draco dSph galaxy

We present the first results of a study where we determine the metallicity distribution function in the Galactic disks as a function of height above the Galactic plane. Observations in the Stromgren photometric system enables us to identify the dwarf stars and derive metallicities for them. The resulting metallicity distribution functions at 0.5 and 2.0 kpc above the Galactic plane are significantly broader and more metal-rich than is anticipated from standard models such as the Besancon model. Our results can be explained by invoking a smaller scale height and larger local normalisation for the thick disk than is commonly used in the models. These results are compatible with recent determinations of the thick disk scale height based e.g. on SDSS data. The age of the stellar populations as a function of height above the Galactic plane is also investigated by studying the turn-off colour and metallicity. We tentatively find that at 2.0 kpc above the Galactic plane there exist an intermediate age population.Comment: 6 pages, 4 figures, IAU symposium 25

On momentum conservation and thermionic emission cooling

The question of whether relaxing momentum conservation can increase the performance of thermionic cooling device is examined. Both homojunctions and heterojunctions are considered. It is shown that for many cases, a non-conserved lateral momentum model overestimates the current. For the case of heterojunctions with a much heavier effective mass in the barrier and with a low barrier height, however, non-conservation of lateral momentum may increase the current. These results may be simply understood from the general principle that the current is limited by the location, well or barrier, with the smallest number of conducting channels. These results also show that within thermionic emission framework, the possibilities of increasing thermionic cooling by relaxing momentum conservation are limited. More generally, however, when the connection to the source is weak or in the presence of scattering, the situation may be different. Issues that deserve further study are identified.Comment: 36 pages, 1 table, 9 figure

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

Simulation of the Spin Field Effect Transistors: Effects of Tunneling and Spin Relaxation on its Performance

A numerical simulation of spin-dependent quantum transport for a spin field effect transistor (spinFET) is implemented in a widely used simulator nanoMOS. This method includes the effect of both spin relaxation in the channel and the tunneling barrier between the source/drain and the channel. Account for these factors permits setting more realistic performance limits for the transistor, especially the magnetoresistance, which is found to be lower compared to earlier predictions. The interplay between tunneling and spin relaxation is elucidated by numerical simulation. Insertion of the tunneling barrier leads to an increased magnetoresistance. Numerical simulations are used to explore the tunneling barrier design issues.Comment: 31 pages, 14 figures, submitted to Journal of Applied Physic

Computational Electronics for the 21st Century: Reflections on the Past, Present, and Future

The author’s career has coincided with the development of numerical simulation into an essential component of semiconductor device technology research and development. We now have a sophisticated suite of simulation capabilities along with new challenges for 21st Century electronics. This talk presents a short history of the field and a description of the current state of the art, but it concentrates on lessons learned and thoughts about how computational electronics can continue to contribute effectively to the development of new electronic device technologies. The author will argue that electronics is changing, and that computational electronics can play a key role in this evolution. In addition to supporting the continuing development of a small suite of physically detailed / first principles tools, he will argue for more emphasis on analytically compact, strongly physical, conceptual models. Such models help guide the development of physically detailed models, connect to circuit and application designers, and advance device science itself