Stanford researchers develop first computer using only carbon nanotube transistors

Carbon nanotube transistors are seen as a successor to silicon transistors, as silicon is expected to meet its physical limits

Researchers at Stanford University have demonstrated the first functional computer built using only carbon nanotube transistors, according to an article published Wednesday on the cover of scientific journal Nature.

Scientists have been experimenting with transistors based on carbon nanotubes or CNTs as successors to silicon transistors, as silicon is expected to meet its physical limits in delivering the increasingly smaller transistors required for higher performance in smaller and cheaper computing devices that are less power-consuming. Digital circuits based on the long chains of carbon atoms are expected to be more energy-efficient than silicon transistors.

The rudimentary CNT computer, developed by the researchers at Stanford, is said to run a simple operating system that is capable of multitasking, according to a synopsis of the article.

Made of 178 transistors, each containing between 10 and 200 carbon nanotubes, the computer can do four tasks summarized as instruction fetch, data fetch, arithmetic operation and write-back, and run two different programs concurrently.

As a demonstration, the researchers performed counting and integer-sorting simultaneously, according to the synopsis, besides implementing 20 different instructions from the MIPS instruction set "to demonstrate the generality of our CNT computer," according to the article by Max Shulaker and other doctoral students in electrical engineering. The research was led by Stanford professors Subhasish Mitra and H.S. Philip Wong.

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

Carbon nanotubes still have imperfections. They do not, for example, always grow in parallel lines, which has led researchers to devise techniques to grow 99.5 percent of CNTs in straight lines, according to the press release. But at billions of nanotubes on a chip, even a small misalignment of the tubes can cause errors. A fraction of the CNTs also behave like metallic wires that always conduct electricity, instead of acting like semiconductors that can be switched off.

The researchers describe a two-pronged approach called an "imperfection-immune design". They passed electricity through the circuits, after switching off the good CNTs, to burn up the metallic nanotubes, and also developed an algorithm to work around the misaligned nanotubes in a circuit.

The basic computer was limited to 178 transistors, which was the result of the researchers using the university's chip-making facilities rather than an industrial fabrication process, according to the press release.

Other researchers are also working on CNTs as they worry about silicon hitting its physical limits. IBM said last October its scientists had developed a way to place over 10,000 transistors made from the nano-sized tubes of carbon on a single chip, up from a few hundred carbon nanotube devices at a time previously possible. This density was, however, far below the density of commercial silicon-based chips, but the company said the breakthrough opened up the path for commercial fabrication of "dramatically smaller, faster and more powerful computer chips."

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John Ribeiro

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