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Silicon Valley may very soon be a misnomer.

Instead of a new microprocessor, the transistors (tiny electronic switches performing computations) are made with carbon nanotubes, rather than silicon.

By devising techniques to overcome the nanoscale defects that often threaten individual nanotube transistors (SN: 7/19/17), researchers have produced the first computer chip that uses thousands of these switches to run programs.

The prototype, reported in the Aug. 29 Nature, is not yet as fast or as small as popular silicon devices. But carbon nanotube computer chips may finally give rise to a new generation of faster, more energy-efficient electronics.

This is “a very important milestone in the development of this technology,” says Qing Cao, a materials scientist at the University of Illinois at Urbana-Champaign not involved in the work.

The core of every transistor is a semiconductor component, traditionally made of silicon, which can work either like an electrical conductor or an insulator. A transistor’s “on” and “off” states, where current is moving through the semiconductor or not, encode the 1s and 0s of computer data (SN: 4/2/13). By building leaner, more conventional silicon transistors, “we used to get exponential gains in computing every single year,” says Max Shulaker, an electrical engineer at MIT. But “now performance increases have started to level off,” he says. Silicon transistors can’t get much tinier and more efficient than they already are.

Because carbon nanotubes are nearly atomically thin and ferry electricity so well, they make better semiconductors than silicon. In essence, carbon nanotube processors could run three times quicker while using about one-third of the energy of their silicon predecessors, Shulaker says. But until now, carbon nanotubes have shown too finicky to create complex computing systems.

One problem is that, when a network of carbon nanotubes is installed onto a computer chip wafer, the tubes manage to bunch together in lumps that limit the transistor from working. It’s “like trying to build a brick patio, with a giant boulder in the middle of it,” Shulaker says. His team resolved that problem by overlaying nanotubes on a chip, then using shakes to gently shake unwanted bundles off the cover of nanotubes.




Shrinking the nanotube transistors would help electricity zip through them with less resistance, allowing the devices to switch on and off more quickly, Arnold says. And aligning the nanotubes in parallel, rather than using a randomly oriented mesh, could also increase the electric current through the transistors to boost processing speed.
Shrinking the nanotube transistors would help electricity zip through them with less resistance, allowing the devices to switch on and off more quickly, Arnold says. And aligning the nanotubes in parallel, rather than using a randomly oriented mesh, could also increase the electric current through the transistors to boost processing speed.

Another difficulty the team faced is that each batch of semiconducting carbon nanotubes includes about 0.01 percent metallic nanotubes. Since metallic nanotubes can’t suitably flip between conductive and insulating, these tubes can confuse a transistor’s readout.

In search of a work-around, Shulaker and colleagues analyzed how badly metallic nanotubes affected different transistor configurations, which perform different kinds of operations on bits of data (SN: 10/9/15). The researchers found that defective nanotubes affected the function of some transistor configurations more than others — alike to the way a missing letter can make some words illegible, but leave others essentially readable. So Shulaker and colleagues delicately designed the circuitry of their microprocessor to withdraw transistor configurations that were most confused by metallic nanotube glitches.

“One of the biggest things that impressed me about this paper was the cleverness of that circuit design,” says Michael Arnold, a materials scientist at the University of Wisconsin–Madison not involved in the work.

With more than 14,000 carbon nanotube transistors, the resulting microprocessor performed a simple program to write the message, “Hello, world!” — the first program that numerous newbie computer programmers learn to write.




The newly minted carbon nanotube microprocessor isn’t yet ready to unseat silicon chips as the mainstay of modern electronics. Each one is about a micrometer beyond, compared with current silicon transistors that are tens of nanometers across. And every carbon nanotube transistor in this prototype can flip on and off about a million times each second, whereas silicon transistors can flicker billions of times per second. That puts these nanotube transistors on par with silicon components produced in the 1980s.

Narrowing the nanotube transistors would improve electricity zip through them with less resistance, enabling the devices to switch on and off more quickly, Arnold says. And adjusting the nanotubes in parallel, rather than using a randomly oriented mesh, could also improve the electric current through the transistors to boost processing speed.




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