The molecular structure of graphene
Thanks to a change in recipe, IBM has created a graphene-based processor that can execute 100 billion cycles per second (100GHz), almost four times the speed of previous experimental graphene chips.
With this research, IBM has also shown that graphene-based transistors can be produced by the wafer, which could pave the way for commercial-scale production of graphene chips, said Yu–Ming Lin, the IBM researcher who led the work.
If commercialized, such graphene processors could be the basis of superior signal processing componentry, improving the fidelity of audio and video recording, radar processing and medical imaging.
IBM conducted the work on behalf of the U.S. Defense Department's DARPA (Defense Advanced Research Projects Agency), under a program to develop high-performance RF (radio frequency) transistors. A write-up of the results has been published in the Feb. 5 issue of Science.
Graphene, a single-atom-thick honeycomb lattice of carbon atoms, can transport electrons more quickly than other semiconductors, a quality called electron mobility. "That makes graphene a promising material for high-speed or high-frequency electronic components," Lin said.
This prototype processor was created on a 2-inch wafer, though in principle it could be done on even larger wafers, which should bring the production costs down, Lin said. Graphene is produced by heating a silicon carbide wafer, allowing the silicon to evaporate.
Until now, the downside of graphene has been that it is very sensitive to the environment. During the fabrication process, an oxide layer is deposited over the graphene to form the gate insulator. Typically, this deposition degrades the graphene's electron mobility, due to defects in the oxide that scatter electrons in the graphene. The IBM researchers minimized the damage by separating the graphene from the oxide with a very thin polymer layer.
This new approach has been instrumental in allowing the researchers to almost quadruple the frequency of graphene chips. Last year, research teams from IBM and the Massachusetts Institute of Technology both demonstrated graphene processors capable of frequencies around 26GHz. By comparison, silicon-based transistors of the same gate length (240 nanometers) have only been able to scale up to a clock rate of 40Ghz or so.
It also sets the stage for commercial production. The research shows that "high-quality graphene can be produced on a wafer scale, and graphene transistors can be fabricated with those processes used in the semiconductor industries," Lin said.
Lin cautioned against thinking of graphene as a substitute for the silicon-based microprocessors used in today's computers, at least at anytime in the near future. One major roadblock is that graphene does not work easily with discrete electronic signals, he explained. Because graphene is a zero bandgap semiconductor, meaning there is no energy difference between its conductive and nonconductive states, transistors made of the semiconductor cannot be turned on and off. In contrast, silicon has a bandgap of one electron volt, making it good for processing discrete digital signals, Lin said.
Instead, graphene is better suited for making analog transistors, such as signal processors and amplifiers. Today, such circuitry is largely made from GaAs (gallium arsenide), though GaAs offers nowhere near the same electron mobility, Lin said.