News / Science & Technology

Working Computer Built with Carbon Nanotubes

Max Shulaker a graduate student at Stanford University shows off the new material used to make new computers, August 19, 2013, at the Stanford School of Engineering. ( Photo by Norbert von der Groeben )
Max Shulaker a graduate student at Stanford University shows off the new material used to make new computers, August 19, 2013, at the Stanford School of Engineering. ( Photo by Norbert von der Groeben )

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Researchers at Stanford University may have found the successor to silicon-based microchips.

Using carbon nanotubes (CNTs), a “quirky” material, the team of engineers announced they have built a basic computer. They say the breakthrough has the potential to launch a new generation of electronic devices that run faster, while using less energy than those made from silicon chips.

The Stanford team says their computer has 178 transistors and performs tasks such as counting and number sorting. It runs a basic operating system that allows it to swap between these processes.

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.

"People have been talking about a new era of carbon nanotube electronics moving beyond silicon," said Subhasish Mitra, an electrical engineer and computer scientist at Stanford in a statement. "But there have been few demonstrations of complete digital systems using this exciting technology. Here is the proof."

Carbon nanotubes have been used as transistors, the on-off switches at the heart of nearly all digital electronic systems, but they have been prone to imperfections, which has made building complex circuits difficult.

Over the years, pressure has been placed upon semiconductors to be smaller and faster. That means shrinking the size of each transistor to pack more of them on a chip. But as transistors become tinier, they waste more power and generate more heat – all in a smaller and smaller space, as evidenced by the warmth emanating from the bottom of a laptop.

Many researchers believe that this power-wasting phenomenon could spell the end of Moore's Law, named for Intel Corp. co-founder Gordon Moore, who predicted in 1965 that the density of transistors would double roughly every two years, leading to smaller, faster and cheaper electronics.

"Energy dissipation of silicon-based systems has been a major concern," said Anantha Chandrakasan, head of electrical engineering and computer science at MIT and a world leader in chip research. He called the Stanford work "a major benchmark" in moving CNTs toward practical use.

In theory, this combination of efficient conductivity and low-power switching make carbon nanotubes excellent candidates to serve as electronic transistors, but inherent imperfections have stood in the way of putting this promising material to practical use.

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

Over time, researchers have devised tricks to grow 99.5 percent 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.

Another problem is that 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/or metallic CNTs without having to hunt for them like needles in a haystack.

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

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 technique eliminated the metallic CNTs in the circuit.

Bypassing the misaligned nanotubes required another method.

The Stanford 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.

Despite the breakthrough, researchers say industrial-scale production of CNTs is still years away.

The achievement is reported today in an article in the journal Nature.

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