The Nanotube Computer Debuts

The world’s first computer built with carbon nanotubes has been unveiled by researchers at Stanford University (Nature 2013, DOI: 10.1038/nature12502). The machine—albeit very limited in function—is a major step toward energy-efficient alternatives to the silicon-based electronic components that dominate the computing landscape today.

Computer technology has long followed Moore’s law, the prediction that every two years electronics firms will be able to double the number of transistors—the semiconducting building blocks of circuits—packed onto their chips. This increase has enabled computers and other electronics, such as smartphones, to become progressively smaller and faster.

But that progression isn’t sustainable, experts say. As silicon-based transistors have become smaller, they’ve also become less efficient conductors, wasting energy and emitting heat.

Enter the carbon nanotube. Scientists have long considered replacing silicon-based circuit elements with nanotubes because the tiny tubes transport electrons more rapidly and require less energy.

After the first carbon nanotube transistor was created in 1998, researchers made progress by building other circuit elements such as logic gates. But a computer with an all-nanotube central processor remained out of reach.

To make even a simple computer’s circuits out of nanotubes requires tens of thousands of the ultrathin tubes—and they all need to be in particular spots and aligned with one another, explains Subhasish Mitra, an engineer and computer scientist at Stanford. In addition, he points out that nanotube synthesis typically yields a mix of semiconducting and metallic tubes, the latter of which are undesirable for electronics.

Mitra, Stanford engineer H.-S. Philip Wong, and colleagues addressed these challenges by developing a technique, called imperfection-immune design methodology, to selectively obliterate metallic nanotubes from chips and to pattern circuits so that misaligned tubes don’t cause computational errors. They used the approach to fabricate a 178-transistor computer chip capable of running both a counting and a number-sorting program.

Cees Dekker, whose group at Delft University of Technology, in the Netherlands, made the first nanotube transistor 15 years ago, says the new computer is the most complex carbon-based electronic system yet realized. But, he adds, it remains to be seen whether these carbon-nanotube computers really will outcompete silicon-based computers.

“So much money has been invested in silicon that Silicon Valley isn’t going to become Carbon Valley overnight,” says Max M. Shulaker, a graduate student and lead author of the Nature report. Plus, the Stanford team’s computer still has plenty of room for improvement.

According to Shulaker, the team is limited in how many transistors it can put onto a chip because of the academic fabrication facility it uses. Increasing the processing speed of the computer and giving it more functionality, he says, will require help from industry, a step the team is hoping to take in the future.