Tuesday, March 06, 2007

New Graphene Transistors Show Promise

Electronics made of a single sheet of carbon could be created much smaller than those made with silicon.

By Kate Greene

Single-electron logic: A single-electron transistor carved entirely in a graphene sheet. The central element is a so-called quantum dot, which allows electrons to flow one by one. The dot is connected to wider regions that have contact pads used to turn the transistor on and off. Credit: University of Manchester

Researchers at the University of Manchester have announced a single-electron transistor made out of graphene, a single sheet of graphite only one atom thick. The researchers' device, which is the first single-electron transistor to operate at room temperature, offers evidence that graphene is a promising alternative to silicon, which, in the coming decades, is expected to reach its physical limits as a material for making ever-smaller transistors.

Engineers in the semiconductor industry expect that by 2025, the features on chips will need to shrink to about 15 nanometers, from today's 45-nanometer size, but few people expect that silicon is the material that can lead the charge. "I don't know how they're going to do this," says Andre Geim, professor of physics at the University of Manchester and lead researcher on the work. "Essentially," he says, "the semiconductor road map sees a cul-de-sac in 2025." Geim believes that graphene, however, "can extend beyond this cul-de-sac."

The single sheet of graphite is a relatively new material that was first reported by Geim's group in 2004. It is essentially an unrolled carbon nanotube--a tube made of a single sheet of graphite with a diameter of about a nanometer--which is also being explored as a material for transistors. A growing number of scientists are researching the material because it has an exceptionally useful electrical property: electrons can travel through the material quickly and without bouncing off atoms and scattering. This property, called ballistic transport, could make graphene useful for a range of applications, from making faster, smaller electrical devices to testing theories from a branch of physics called quantum electrodynamics.

When members of Geim's group first announced their fabrication of graphene, they also demonstrated how it could be used to make a transistor. However, the early graphene transistors were impractical because electrical current would leak from them, making them inefficient.

"In recent years," says Geim, "there's been a drive to try to find a solution around this problem." Some scientists have looked at using ribbons of graphene and placing them side by side to try to create artificial electrical properties that confine the current. But in this configuration, says Geim, the ribbons of carbon simply mimic the behavior of carbon nanotubes, and they don't take full advantage of the properties of graphene.

The researchers' new approach uses a transistor design known for years, called a single electron transistor, which funnels one electron through the transistor at a time. Single-electron devices based on this design usually consist of electrical contacts, called a source and a drain, that supply and collect the current, and a nanometer-size island of conducting material, called a quantum dot, in between the contacts.