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World's first carbon-nanotube computer built

Posted: 27 Sep 2013     Print Version  Bookmark and Share

Keywords:Stanford  computer  carbon nanotubes  silicon chips 

A team of Stanford engineers, led by Indian-origin researcher Subhasish Mitra, has built a computer using carbon nanotubes, instead of silicon chips. Carbon nanotubes is a semiconductor material that has the potential to launch a new generation of electronic devices that run faster, while using less energy.

"This unprecedented feat culminates years of efforts by scientists around the world to harness this promising material," claim the researchers.

"People have been talking about a new era of carbon nanotube electronics moving beyond silicon," said Subhasish Mitra, an electrical engineer and computer scientist and Chambers Faculty Scholar of Engineering, who also led the research. "But there have been few demonstrations of complete digital systems using this exciting technology. Here is the proof."

The new development is expected to galvanise efforts to find successors to silicon chips, which could soon encounter physical limits that might prevent them from delivering smaller, faster, cheaper electronic devices.

Carbon nanotubes (CNTs) are long chains of carbon atoms that are extremely efficient at conducting and controlling electricity. They are so thin—thousands of CNTs could fit side by side in a human hair—that it takes very little energy to switch them off, according to Wong, a co-author of the paper.

CNTs have long been considered as a potential successor to the silicon transistor. But until now it hasn't been clear that CNTs could fulfil those expectations. It was roughly 15 years ago that carbon nanotubes were first fashioned into transistors, the on-off switches at the heart of digital electronic systems. But a bedeviling array of imperfections in these carbon nanotubes has long frustrated efforts to build complex circuits using CNTs.

Over time, researchers have devised tricks to grow 99.5 per cent of CNTs in straight lines. But with billions of nanotubes on a chip, even a tiny degree of misaligned tubes could cause errors, so that problem remained.

Depending on how the CNTs grow, a fraction of these carbon nanotubes can end up behaving like metallic wires that always conduct electricity, instead of acting like semiconductors that can be switched off.

The Stanford paper describes a two-pronged approach that the authors call an "imperfection-immune design."

To eliminate the wire-like or metallic nanotubes, the Stanford team switched off all the good CNTs. Then they pumped the semiconductor circuit full of electricity. All of that electricity concentrated in the metallic nanotubes, which grew so hot that they burned up and literally vaporized into tiny puffs of carbon dioxide. This sophisticated technique eliminated the metallic CNTs in the circuit.

Bypassing the misaligned nanotubes required even greater subtlety.

The researchers created an algorithm that maps out a circuit layout that is guaranteed to work no matter whether or where CNTs might be askew.

"This 'imperfections-immune design' [technique] makes this discovery truly exemplary," said Sankar Basu, a programme director at the National Science Foundation.

The team used this imperfection-immune design to assemble a basic computer with 178 transistors, a limit imposed by the fact that they used the university's chip-making facilities rather than an industrial fabrication process.

Their CNT computer performed tasks such as counting and number sorting. It runs a basic operating system that allows it to swap between these processes.





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