Posted: May 2, 2008 |
Nano-designed transistors with disordered materials, but high performance |
(Nanowerk News) The Holy Grail for transistor designers has been the requirement to be
able to get high performance at reduced costs over very large substrate
areas. Transistors on cheap and flexible substrates like glass and
plastics are currently unable to deliver such performance and therefore
do not lend themselves to seamless monolithic integration of increased
electronic functions on human interface devices (displays and sensors).
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At present, high performance transistors are only available in
crystalline materials which are expensive and have to be attached
ex-situ onto larger area substrates, which adds to the expense and
complexity of system design. If both the electronics and display
substrates can be integrated onto one platform, it would usher a new
dawn in immersive and personal electronics. Individuals will thus be
able to communicate, send and receive information of value, and access
data about their current environment and health status with freedom, at
leisure, and in comfort. However, in general, the deposition of
semiconductor films used to make transistors on such substrates has to
be carried out at low temperatures to preserve substrate integrity. As a
result, the quality of the organic or inorganic semiconductor films is
severely constrained, and has a dramatic influence on the transistor
performance.
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In a recent report to be published in Science - 'Engineering
Perspectives', backed by a further paper to appear in IEEE Electron
Device Letters, engineers propose the use of clever transistor structure
designs to overcome some of the issues with obtaining suitably low power
and high speed operations in standard material systems. In the first
collaborative work with Hitachi Central Research Laboratory, Japan,
researchers at the Advanced Technology Institute of the University of
Surrey have experimentally and theoretically demonstrated that for
transistors of disordered silicon films, superior switching performance
(low leakage current, and steep sub-threshold slope) can be achieved by
making the conduction channel in the transistor very thin. A higher
ION/IOFF ratio, which exceeds 1011, can be achieved for devices with a
2.0-nm-thick channel.
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Another seminal work from the same research
laboratory at Surrey, is on the newly developed source-gated transistor
(SGT) concept by Professor John Shannon. Compared to a field-effect
transistor, the SGTs can operate with very short source-drain
separations even with a thick gate insulator layer to achieve high
speed, good stability and superior control of current uniformity,
providing a significant advantage in terms of the fabrication process.
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Dr Xiaojun Guo, one of the lead investigators, comments: "Engineering of
the transistor structure itself rather than the channel material can
lead to improved device performance. It will enable the design of
high-performance large area circuits and systems based on low-cost
reliable material processes".
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Professor Ravi Silva, Director of the Advanced Technology Institute
states: "This work will help extend the already well established CMOS
fabrication technologies for use in large area applications such as
displays and sensors, which are at the heart of consumer electronics.
The ATI is fortunate that we have been at the forefront of two potential
technologies that can lead to enhanced device performance in disordered
materials by clever nano-scale structural design of disordered
transistors. This type of work sponsored by the EPSRC forms the bedrock
for future electronic technologies".
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This research will be published in the journal Science, and a more
detailed version of the nano-designed transistor will appear in IEEE
Electron Device Letters.
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References:
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X. Guo and S.R.P. Silva, 'High-Performance Transistors by Design',
Science, vol 320, 02 May 2008
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X. Guo, T. Ishii, and S.R.P. Silva, 'Improving Switching Performance of
Thin-Film Transistors in Disordered Silicon', to appear in IEEE Electron
Device Letters, vol 29 Issue 6, 2008.
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