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Research Overview
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Ninad Pimparkar
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Nanocomposites
Nano-bundle TFTs for
Macroelectronic Applications
Over the past fifteen
years, tremendous advances in thin-film transistor (TFT) technology for
large-area displays have complemented the equally phenomenal gains in
silicon CMOS technology for high performance computation and communication
ICs. However, performance limits of a-Si or organic TFT technologies make
them unsuitable for a range of potentially exciting and novel applications
in sensors, radars, and displays that could dramatically alter the
application-landscape of flexible electronic systems. For these applications,
researchers are exploring a new class of nano-composites based on bundles
of Si nanowires or Carbon Nanotubes. Although initial results promise
significant improvement in electro- thermal performance, but lack of
adequate transport models have stymied physical understanding and device
optimization. We are working on novel transport models for these composites
to simulate complete TFT characteristics so that experiments can be
interpreted, devices can be optimized, and ultimate limits of this new thin-film
material technology can be explored and established.
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Transport Physics in
Nanobundle Thin Film Transistors
Although Amorphous-silicon
(a-Si) based thin film transistors (TFTs) now dominate the market for
large-area flat-panel displays, and although low-cost organic-TFTs on
flexible, lightweight, plastic substrates or high-performance poly-silicon
TFT on glass-substrates are emerging as viable alternatives for many
non-display applications, still there are many potential macroelectronics
applications that demand better performance than a-Si or organics, while
simultaneously requiring large area fabrication on flexible substrates beyond
the reach of single crystal silicon (x-Si) or even poly-silicon TFTs.
Examples of such "high-performance" macroelectronics applications
include electronically steerable antennas for portable systems, conformal
radar for airborne applications, tunable frequency -selective surfaces,
biological and chemical sensors, adaptive surfaces for enhance surface
properties, etc. Since traditional macroelectronics materials may not be
suitable for these applications, therefore researchers are exploring a new
class of nano-composite TFTs based on bundles of silicon nanowires (Si-NWs)
or carbon nanotubes (CNTs). The potential advantages of NB TFTs include; i)
high performance, because single nanotubes have demonstrated very high
mobilities, ii) reduced fluctuations, because the effective grain size can be
controlled by the arbitrary length of the wire, iii) substrate neutrality,
because a temporary substrate can be used for growth and the NBs transferred
to a final substrate, and iv) reliability, because CNTs have no dangling
bonds and Si NW's can be passivated by techniques developed for x-Si. Indeed,
the initial experimental results of these NB-TFTs have been both exciting and
promising.
Unfortunately, however, the
lack of even rudimentary electronic transport models has made it very
difficult to interpret the initial measurements, optimize the transistors,
and explore their performance limits. The basic concern is, despite the
initial promise and excitement, whether NBT-TFTs can achieve - even in theory
-- the performance required by the macroelectronic applications described
above. Therefore, we propose (i) to work closely with experimentalists to
develop a comprehensive transport model for such nano-composite TFTs so that
the measurements from different laboratories can be interpreted within a
common framework and the performance of transistors can be optimized as a
function of nano- composite variables like bundle-density, tube-length,
device geometry, etc. and (ii) to encapsulate our insights from such physical
analysis within compact models to access overall system performance and embed
these models, in collaboration with industrial partners, in design platforms
for system integration. Over longer term, the simulation infrastructure
developed and insight gained as a result of this research will be broadly
applicable not only to NBT-TFTs for macroelectronics applications, but also
to evaluate the merits of any new TFT technologies based novel, engineered
nano-composites as well as innovative usage of traditional poly-Si or organic
thin-films.
BACKGROUND PAPERS
[1] C. R. Kagan and P. Andry, "Thin Film Transistors,"
Marcel Dekker, Inc. New York,
2003.
[2] C. Dimitrakopoulos and D. Mascaro, "Organic
TFT: A review of recent advances," IBM J. Res. & Dev. 45, 11-27,
2001.
[3] T. S. Brotherton et al., "High-speed,
short-channel poly-crystalline Silicon TFTs," APL, 84(2), pp. 293-295,
2004.
[4] B. Reuss,.
http://www.darpa.mil/mto/solicitations/BAA03-21/S/Brief.pdf. April, 2003.
[5] R. Wisnieff, "Printing Screens," Nature,
394, p. 225, 1998.
[6] E. Menard et al., "A printable form of
silicon for high peformance TFT on plastic," APL 84(26), 5398-5400,
2004.
[7] Duan, C. Niu, V. Sahl, J. Chen, J. W. Parce, S.
Empedocles, and J. Goldman, "High performance thin film transistors
using semiconductor nanowires and nanoribbons, " Nature, vol. 425, pp.
274-278, 2003.
[8] S. Kirkpatrick, "Percolation and
Conduction", Rev. of Mod. Phys. 45(4), 574-588, 1973.
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