291 research outputs found
A Numerical Study of Scaling Issues for Schottky Barrier Carbon Nanotube Transistors
We performed a comprehensive scaling study of Schottky barrier carbon
nanotube transistors using self-consistent, atomistic scale simulations. We
restrict our attention to Schottky barrier carbon nanotube FETs whose metal
source/drain is attached to an intrinsic carbon nanotube channel. Ambipolar
conduction is found to be an important factor that must be carefully considered
in device design, especially when the gate oxide is thin. The channel length
scaling limit imposed by source-drain tunneling is found to be between 5nm and
10nm, depending on the off-current specification. Using a large diameter tube
increases the on-current, but it also increases the leakage current. Our study
of gate dielectric scaling shows that the charge on the nanotube can play an
important role above threshold.Comment: 26 pages, 8 figure
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
Signatures of disorder in the minimum conductivity of graphene
Graphene has been proposed as a promising material for future nanoelectronics
because of its unique electronic properties. Understanding the scaling behavior
of this new nanomaterial under common experimental conditions is of critical
importance for developing graphene-based nanoscale devices. We present a
comprehensive experimental and theoretical study on the influence of edge
disorder and bulk disorder on the minimum conductivity of graphene ribbons. For
the first time, we discovered a strong non-monotonic size scaling behavior
featuring a peak and saturation minimum conductivity. Through extensive
numerical simulations and analysis, we are able to attribute these features to
the amount of edge and bulk disorder in graphene devices. This study elucidates
the quantum transport mechanisms in realistic experimental graphene systems,
which can be used as a guideline for designing graphene-based nanoscale devices
with improved performance.Comment: Article: 14 pages, 4 figures. Supporting information: 8 pages, 3
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