The findings, reported in the April issue of Nano Letters, a peer-reviewed journal of the American Chemical Society, the world's largest scientific society, add to mounting enthusiasm about nanotechnology's revolutionary potential.
"Since the invention of nanotube transistors, there have been theoretical predictions that they can operate very fast," says Peter Burke, Ph.D., a professor of electrical engineering and computer science at the University of California, Irvine, and lead author of the paper. "Our work is the first to show that single-walled nanotube transistor devices can indeed function at very high speeds."
Burke and his colleagues built an electrical circuit with a carbon nanotube between two gold electrodes. When they varied the voltage, the circuit operated at a frequency of 2.6 gigahertz (GHz), which means electrical current could be switched on and off in about one billionth of a second. This is the first demonstration of a nanotube operating in the frequency range of microwaves — electromagnetic waves with faster frequencies than radio waves.
Although Burke's group demonstrated that nanotube transistors could work in the GHz range, he believes that much faster speeds are possible. "I estimate that the theoretical speed limit for these nanotube transistors should be terahertz [1 THz=1,000 GHz], which is about 1,000 times faster than modern computer speeds." His team is currently doing related research on the theoretical prediction of the cutoff frequency, or so-called speed limit, for these transistors.
Every transistor has a cutoff frequency, which is the maximum speed at which it can operate. For silicon, the cutoff is about 100 GHz, but current circuits typically operate at much slower speeds, according to Burke. For example, some of today's newest processor chips still operate below 5 GHz.
Nanotechnology is the science of the very small: a nanometer is one billionth of a meter, or about 1,000 times smaller than the width of a human hair. A nanotube is another form of carbon, like graphite or diamond, where the atoms are arranged like a rolled-up tube of chicken wire.
Electrons move without losing energy inside nanotubes, which makes them perfect candidates for connections in electrical devices. A semiconducting carbon nanotube can act as a transistor — the key component in all modern electronics — because it can be switched on and off.
High-speed nanotube transistors could be useful in a number of applications. "Theoretically, this can translate into very low noise microwave amplifiers that could increase the range in which cell phones operate," Burke says. A cell phone receives its radio signal at a very low strength, so a microwave amplifier is needed to boost the signal for further processing.
Nanotube transistors could also lead to very high quality microwave filters that can separate out many different phone conversations more efficiently than current filters, and at lower cost, according to Burke. "Right now, this one function requires a separate chip inside a cell phone," he says. If the filter could be integrated with the other processing parts, the entire radio system would be on one chip, saving power, space and cost.
This type of "integrated nanosystem" is a goal of Burke's research. "Ultimately, we would like more sophisticated circuits on a single chip," he says. "Our nanotube transistor is on a silicon substrate, but there are no active silicon devices." If all the transistors and electrical connections on a chip were made of nanotubes or nanowires, there would be no silicon parts to slow things down.
Burke expects to have a prototype transistor available within two years. "We still need to demonstrate operation at room temperature, which we are working on in my lab now. Also, we need to show that we can achieve amplification," he says. "But these are both achievable goals given one or two years of work."
The Army Research Office, the Office of Naval Research, and the Defense Advanced Research Projects Agency provided funding for this research.
— Jason Gorss
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