Thursday, January 26, 2012
The smallest carbon-nanotube transistor ever made, a nine-nanometer device, performs better than any other transistor has at this size.
For over a decade, researchers have promised that carbon nanotubes, with their superior electrical properties, would make for better transistors at ever-tinier sizes, but that claim hadn't been tested in the lab at these extremes. Researchers at IBM who made the nanotube transistors say this is the first experimental evidence that any material is a viable potential replacement for silicon at a size smaller than 10 nanometers.
"The results really highlight the value of nanotubes in the most sophisticated type of transistors," says John Rogers, professor of materials science at the University of Illinois at Urbana-Champaign. "They suggest, very clearly, that nanotubes have the potential for doing something truly competitive with, or complementary to, silicon."
The shrinkage of silicon transistors over the past several decades has reduced the cost of electronics and led to more processing power with less energy consumption. But the downsizing of silicon electronics might hit a roadblock at around 10 nanometers, says Aaron Franklin, a researcher at the IBM Watson Research Center in Yorktown Heights, New York. "We are now reaching physical limits," he says. As transistors are made smaller, it gets more difficult to control how electrons move through the silicon channel to turn the transistor on and off. Faced with this unruly, power-draining behavior, Intel announced last year that it would switch to a new, three-dimensional transistor design for its 22-nanometer generation of chips. Other companies are working on so-called ultrathin body transistors. No matter how it's shaped, though, silicon is silicon, and dealing with it at extremely small sizes presents problems even in these new transistor designs.
Many materials have been hyped as a potential replacement for silicon, including carbon nanotubes. That material and others have shown promise in larger transistors, but until now, no one had demonstrated a carbon-nanotube transistor smaller than 10 nanometers. "If nanotubes can't go much further than silicon, then working on them is a waste of time," says Franklin. "We've made nanotube transistors at aggressively scaled dimensions, and shown they are tremendously better than the best silicon devices."
To test how the size of a nanotube transistor affected its performance, Franklin's group made multiple transistors of different sizes along a single nanotube. This enabled them to control for any variations that might occur from nanotube to nanotube. First, they had to lay down a very thin layer of insulating material for the nanotube to sit on. And they developed a two-step process for adding electrical gates to the nanotube without damaging it. These techniques are by no means ready for manufacturing, but they enabled the IBM group to make the first nanotube devices smaller than 10 nanometers to test in the lab. The work is described online in the journal Nano Letters.
The IBM group demonstrated that its nine-nanometer nanotube transistor had much lower power consumption than other transistors the same size. And it can carry more current than comparable silicon devices, which means a better signal.
Several major engineering problems remain, says Franklin. First, researchers have to come up with better methods for making pure batches of semiconducting nanotubes—metallic tubes in the mix will short out integrated circuits. Second, they must come up with a way to place large numbers of nanotubes on a surface with perfect alignment.