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March 29, 2007

Progress in Nanotube Transistors

High-current transistors made from perfectly aligned carbon nanotubes show promise for use in flexible and high-speed nanoelectronics.


Tube transistors: Researchers at the University of Illinois at Urbana Champaign have developed a technique to grow thousands of carbon nanotubes (shown in blue and white in this colorized scanning electron micrograph). The researchers deposit electrodes (shown in gold) on two sides of the nanotube arrays to create transistors that have hundreds of nanotubes bridging the electrodes.
Credit: John Rogers, UIUC


In a Nature Nanotechnology paper, the researchers, led by John Rogers, a professor of materials science and engineering at UIUC, have demonstrated transistors made with about 2,000 nanotubes, which can carry currents of one ampere--thousands of times more than the current possible with single nanotubes. The researchers have also developed a technique for transferring the nanotube arrays onto any substrate, including silicon, plastic, and glass.

The nanotube transistors could be used in flexible displays and electronic paper. Because carbon nanotubes can carry current at much higher speeds than silicon, the devices could also be used in high-speed radio frequency (RF) communication systems and identification tags. In fact, the research team is working with Northrop Grumman to use the technology in RF communication devices, says Rogers

Until now, making transistors with multiple carbon nanotubes meant depositing electrodes on mesh-like layers of unaligned carbon nanotubes, Rogers says. But since the randomly arranged carbon nanotubes cross one another, at each crossing, flowing charges face a resistance, which reduces the device current. The perfectly aligned array solves this problem because there are "absolutely no tube-tube overlap junctions," Rogers says.

Making a well-ordered array in which parallel nanotubes are connected between the source and drain electrodes is a big achievement, says Richard Martel, a chemistry professor at the University of Montreal. The new work allows a true comparison between nanotube transistors and silicon transistors because an array of nanotubes gives a planar structure similar to silicon devices, he says. "They did exactly what needed to be done, and it's a significant step."

For now, the new transistors will be useful for larger electronics circuits such as those in flexible displays and RF chips, but to be used in high-performance electronics like computer chips, the devices need a much better structure and geometry, Javey says. For instance, the devices would need to be much smaller than they are now: the transistors are currently tens of micrometers long and wide.

To make smaller devices, the UIUC team is working on making the arrays denser. Right now, the distance between adjacent tubes is 100 nanometers, but theoretically, this separation could go down to only one nanometer without affecting electrical properties, Martel says.

Future work: find an effective way to make devices with only semiconducting nanotubes. Typically, a third of the nanotubes in any grown batch are metallic, which causes a small current to flow through a transistor even when it is turned off. The researchers use a common trick to get rid of metallic tubes: turn a transistor off and apply a high voltage that blows out the metallic tubes. But to make good-quality transistors on a larger scale, they would need to find a better way to get rid of the metallic tubes or selectively grow semiconducting tubes. That, according to Javey, is the "last big key" for making nanotube electronics.

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