This wafer contains tiny computers using carbon nanotubes (Credit: Norbert von der Groeben)

World's first carbon nanotube computer

The world’s first carbon nanotube computer has been built by Stanford University engineers.

Scientists have tried for years to harness the promising material – a semiconductor that has the potential to launch a new generation of electronic devices that run faster, while using less energy, than those made from silicon chips.

And while it has been roughly 15 years since carbon nanotubes were first fashioned into transistors, the on-off switches at the heart of digital electronic systems, imperfections have frustrated efforts to build complex circuits using the material.

But using new processes that overcome these imperfections the Stanford team has assembled a basic computer able to perform tasks such as counting and number sorting as well as running a basic operating system that allows it to swap between these processes.

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

The team hope the research, published in the journal Nature, will galvanize efforts to find successors to silicon chips, which could soon encounter physical limits that might prevent them from delivering smaller, faster, cheaper electronic devices.

Progress in electronics has meant shrinking the size of each transistor to pack more transistors on a chip, but as transistors become tinier they waste more power and generate more heat.

"Carbon nanotubes (CNTs) have long been considered as a potential successor to the silicon transistor," said Professor Jan Rabaey, a world expert on electronic circuits and systems at UC Berkeley.

"There is no question that this will get the attention of researchers in the semiconductor community and entice them to explore how this technology can lead to smaller, more energy-efficient processors in the next decade."

Unlike silicon based electronics, CNTs – long chains of carbon atoms that are extremely efficient at conducting and controlling electricity – are so thin that it takes very little energy to switch them off, according to professor HS Philip Wong, co-author of the paper.

"Think of it as stepping on a garden hose," Wong said. "The thinner the hose, the easier it is to shut off the flow. CNTs could take us at least an order of magnitude in performance beyond where you can project silicon could take us."

However CNTs do not necessarily grow in neat parallel lines, as chipmakers would like, and 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.

Since mass production is the eventual goal, researchers had to find ways to deal with misaligned and metallic CNTs without having to hunt for them like needles in a haystack prompting them to design a two-pronged approach that they call an "imperfection-immune design."

"We needed a way to design circuits without having to look for imperfections or even know where they were," Mitra said.

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

To bypass the misaligned nanotubes the researchers created a powerful 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 program director at the National Science Foundation.

The Stanford 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.

In a demonstration of its potential, the researchers also showed that the CNT computer could run MIPS, a commercial instruction set developed in the early 1980s by then Stanford engineering professor and now university President John Hennessy.

Though it could take years to mature, the Stanford approach demonstrates the possibility of industrial-scale production of carbon nanotube semiconductors, according to Naresh Shanbhag, a professor at the University of Illinois at Urbana-Champaign and director of SONIC, a consortium for next-generation chip design research.

"The Wong/Mitra paper demonstrates the promise of CNTs in designing complex computing systems," Shanbhag said, adding that this "will motivate researchers elsewhere" toward greater efforts in chip design beyond silicon.


How the Stanford team overcame the problems of misaligned and metallic carbon nanotubes

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