March/April 2008

TR10: Graphene Transistors

A new form of carbon being pioneered by Walter de Heer of Georgia Tech could lead to speedy, compact computer processors.

By Kevin Bullis

Credit: Maxwell Guberman, Georgia Tech

Multimedia •  Walter de Heer on graphene transistors.

Who: Walter de Heer, Georgia Tech Definition: Transistors based on graphene, a carbon material one atom thick, could have extraordinary electronic properties. Impact: Initial applications will be in ultrahigh-speed communications chips, with computer processors to follow. Context: A number of academic researchers and several electronics companies are studying graphene-based electronics.

The remarkable increases in computer speed over the last few decades could be approaching an end, in part because silicon is reaching its physical limits. But this past December, in a small Washington, DC, conference room packed to overflowing with an audience drawn largely from the semiconductor industry, Georgia Tech physic­s professor Walter de Heer described his latest work on a surprising alternative to silicon that could be far faster. The material: graphene, a seemingly unimpressive substance found in ordinary pencil lead.

Theoretical models had previously predicted that graphene, a form of carbon consisting of layers one atom thick, could be made into transistors more than a hundred times as fast as today's silicon transistors. In his talk, de Heer reported making arrays of hundreds of graphene transistors on a single chip. Though the transistors still fall far short of the material's ultimate promise, the arrays, which were fabricated in collaboration with MIT's Lincoln Laboratory, offer strong evidence that graphene could be practical for future generations of electronics.

Today's silicon-based computer processors can perform only a certain number of operations per second without overheating. But electrons move through graphene with almost no resistance, generating little heat. What's more, graphene is itself a good thermal conductor, allowing heat to dissipate quickly. Because of these and other factors, graphene-based electronics could operate at much higher speeds. "There's an ultimate limit to the speed of silicon--you can only go so far, and you cannot increase its speed any more," de Heer says. Right now silicon is stuck in the gigahertz range. But with graphene, de Heer says, "I believe we can do a terahertz--a factor of a thousand over a gigahertz. And if we can go beyond, it will be very interesting."

Besides making computers faster, graphene electronics could be useful for communications and imaging technolo­gies that require ultrafast transistors. Indeed, graphene is likely to find its first use in high-frequency applications such as terahertz-wave imaging, which can be used to detect hidden weapons. And speed isn't graphene's only advantage. Silicon can't be carved into pieces smaller than about 10 nanometers without losing its attractive electronic properties. But the basic physics of graphene remain the same--and in some ways its electronic properties actually improve--in pieces smaller than a single nanometer.

Interest in graphene was sparked by research into carbon nanotubes as potential successors to silicon. Carbon nanotubes, which are essentially sheets of graphene rolled up into cylinders, also have excellent electronic properties that could lead to ultrahigh-­performance electronics. But nanotubes have to be carefully sorted and positioned in order to produce complex circuits, and good ways to do this haven't been developed. ­Graphene is far easier to work with.