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Posted: February 19, 2008
NEC Fabs Flexible Nanotube Transistors
(Nanowerk News) Green printing technology has enabled researchers at NEC Corp. to fabricate high-speed carbon nanotube transistors on inexpensive flexible polymer substrates.
NEC claims its nanotube transistors are 100 times faster that competing flexible organic transistors, while its printing technique produces 90 percent fewer greenhouse gases than silicon transistors. NEC presented its research results last week at the International Nanotechnology Exhibition & Conference last week in Tokyo.
Organic transistors enable circuitry to be printed at room temperature on flexible polymer substrates, vastly lowering chip manufacturing costs compared to the high-temperature chemical vapor deposition processes. Low-temperature processing also produces less C02 gas, uses less water and produces fewer toxic byproducts than traditional silicon chip processing. However, the electron mobility of organic transistors is dismal compared to silicon.
NEC also claims to have combined the high-speed of carbon nanotube transistors with the low-temperature processing, resulting in transistors that are printable on organic substrates with electron mobilities 100 times greater than typical organic transistors.
"Typical organic semiconductor such as pentacene have an electron mobility around 1 cm2/Vs," said Shinichi Yorozu, senior manager of the Emerging Device Technology Group and NEC's Nano Electronics Research Laboratories (Tusukuba, Japan). "But our devices show an electron mobility of over 100 cm2/Vs--some were as high as 210 cm2/Vs."
The nanotubes were used as a channel for NEC'S experimental transistors. Unlike the difficult-to-fabricate transistors utilizing a single nanotube as the transistor channel, NEC created a nanotube-impregnated ink that can be spin-coated onto a polymer substrate. NEC claimed that the same nanotube ink could be applied with ink-jet printing processes as well.
By creating a slurry of randomly-oriented nanotubes, NEC's transistors achieved their relatively favorable electron mobility without having to first orient them. Instead of building transistors with nanoscale dimensions, the nanotube coating process produced circuitry on flexible substrates with micron-scale dimensions.
"We do not align our carbon nanotubes, and estimate that their densities are around 10 nanotubes per square micron," said Yorozu. "Our typical channel lengths were between 10 to 300 microns."
Next, NEC researchers will seek to achieve more consistency among devices, which may depend on sorting semiconducting from metallic nanotubes. Many researcher are working on methods of separating out semiconducting nanotubes useful for transistor channels, but no industrial-scale process has yet been achieved.
"Carbon nanotube purification is very important, since we want to utilize only the semiconducting nanotubes for transistor's channels, but today we have a mixture of metallic and semiconducting ones," said Yorozu.