Faster Graphene Transistors
A pair of research groups, working independently, report making graphene-based transistors that work at the highest frequencies reported to date. The new transistors are a promising first step toward ultrahigh radio-frequency (RF) transistors, which could be useful for wireless communications, remote sensing, radar systems, and weapons imaging systems.
The reports come from researchers at the IBM T. J. Watson Research Center in Yorktown Heights, NY, and at the HRL Laboratories in Malibu, CA. The IBM transistors work at frequencies up to 26 gigahertz. Both the IBM and HRL work was funded by the U.S. military’s Defense Advanced Research Projects Agency (DARPA). Kostya Novoselov, a physicist and graphene researcher at the University of Manchester, in the U.K., says that the results are “a really big step forward to demonstrating that high-frequency graphene transistors should work.”
Graphene, a flat sheet of carbon atoms, is a promising material for RF transistors. Typical RF transistors are made from silicon or more expensive semiconductors like indium phosphide. In graphene, for the same voltage, electrons zip around 10 times faster than in indium phosphide, or 100 times faster than in silicon.
Graphene transistors will also consume less power and could turn out to be cheaper than those made from silicon or indium phosphide. Yu-Ming Lin, who led the work at IBM, says that silicon technology is extremely mature, but graphene could “achieve device performance that may never be obtained with conventional semiconductors.”
The eventual goal of the DARPA program is to demonstrate an amplifier circuit for signals with frequency greater than 90 gigahertz. The circuit should be made using transistors on an eight-inch-wide wafer employing processes compatible with the current fabrication methods for silicon circuits.
Andre Geim, a professor of physics at the University of Manchester, who discovered graphene and fabricated some of the first graphene transistors, says that even “90 gigahertz is really nothing for graphene. The frequency could be 10 times higher–around the terahertz range.” Such extremely high-frequency transistors would be useful for terahertz imaging, which could detect hidden weapons.
The HRL researchers make their transistors on two-inch-wide graphene pieces. They grow these two-inch pieces by heating a silicon carbide wafer to extremely high temperatures–over 1,200 °C. The silicon evaporates and leaves behind carbon atoms, which arrange themselves in a single layer and form graphene. The researchers fabricate the transistors by depositing insulating aluminum oxide and metal electrodes on top of the graphene.
Lin and his colleagues at IBM use graphene made by peeling single graphene sheets off graphite, which is a stack of graphene sheets. This gives small but very high-quality flakes. As a result, the researchers can shrink transistor features, resulting in devices that work at higher frequencies.
Many challenges remain in order for the researchers to meet DARPA goals. One is a practical way to cover larger areas with graphene. Jeong-Sun Moon, who led the HRL efforts, says that even though the silicon carbide method results in subpar-quality graphene, it has potential for evolving into an eight-inch wafer-scale process. “The method would be crucial to getting graphene to become a real technology,” he says. Others are also using various tricks to make wafer-size graphene for circuits.
“The achieved frequencies are very far from what is possible,” Geim says. Nevertheless, he believes that it shouldn’t be too long before graphene transistors exceed expectations and come close to their terahertz capacity. “I personally believe this is a done deal,” he says. “The fundamentals are all there. Now it’s down to engineers to polish the processes involved.”
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