174 research outputs found
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
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
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
Self‐consistent scattering matrix calculation of the distribution function in semiconductor devices
The scattering matrix approach is a new technique for solving the Boltzmann equation in devices. We report a self-consistent application of the technique to realistic silicon devices exhibiting strong nonlocal effects. Simulation of a hot-electron, n-i-n diode demonstrates that the new technique efficiently and accurately reproduces Monte Carlo results without the statistical noise, allowing much tighter convergence with Poisson’s equation
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
A computational study of the thermoelectric performance of ultrathin Bi2Te3 films
The ballistic thermoelectric performance of ultrathin films of Bi2Te3, ranging in thickness from 1 to 6 quintuple layers, is analyzed using density functional theory combined with the Landauer approach. Our results show that the thinnest film, corresponding to a single quintuple layer, has an intrinsic advantage originating from the particular shape of its valence band, leading to a large power factor and figure-of-merit exceeding bulk Bi2Te3. The interaction between the top and bottom topological surface states is key. The thinnest film yields a six-fold increase in power factor compared to bulk
On the role of phonon scattering in Carbon Nanotube Field-Effect Transistors
The role of phonon scattering in carbon nanotube field-effect transistors (CNTFETs) is explored by solving the Boltzmann transport equation using the Monte Carlo method. The results show that elastic scattering in a short-channel CNTFET has a small effect on the source-drain current due to the long elastic mean-free path (mfp) (~1μm)(~1μm). If elastic scattering with a short mfp were to exist in a CNTFET, the on current would be severely degraded due to the one-dimensional channel geometry. At high drain bias, optical phonon scattering, which has a much shorter mfp (~10nm)(~10nm), is expected to dominate, even in a short-channel CNTFET. We find, however, that inelastic optical scattering has a small effect in CNTFETs under modest gate bias
- …