In a just-published paper in the magazine Science, IBM researchers demonstrated a radio-frequency graphene transistor with the highest cut-off frequency achieved so far for any graphene device—100 billion cycles/second (100 GHz).
This accomplishment is a key milestone for the Carbon Electronics for RF Applications (CERA) program funded by DARPA, in an effort to develop next-generation communication devices.
The high frequency record was achieved using wafer-scale, epitaxially grown graphene using processing technology compatible to that used in advanced silicon device fabrication.
"A key advantage of graphene lies in the very high speeds in which electrons propagate, which is essential for achieving high-speed, high-performance next generation transistors," said T.C. Chen, vice president, Science and Technology, IBM Research. "The breakthrough we are announcing demonstrates clearly that graphene can be utilized to produce high performance devices and integrated circuits."
Graphene is a single atom-thick layer of carbon atoms bonded in a hexagonal honeycomb-like arrangement. This two-dimensional form of carbon has unique electrical, optical, mechanical and thermal properties and its technological applications are being explored intensely.
“We had previously used exfoliation from graphite crystal. That approach was good for the small scale, but what we have demonstrated here is a 2-inch wafer process technology that distributes the graphene uniformly across the wafer. This opens up a whole new venue. We can leverage what we know from silicon-based processes and manufacture state-of-the-art transistors on a wafer scale,” says Yu-Ming Lin, a lead author of the study.
Uniform and high-quality graphene wafers were synthesized by thermal decomposition of a silicon carbide (SiC) substrate at more than 1,000°C. The graphene transistor itself utilized a metal top-gate architecture and a novel gate insulator stack involving a polymer and a high dielectric constant oxide. The gate length was modest, 240 nanometers, leaving plenty of space for further optimization of its performance by scaling down the gate length.
“Because of the atom-thin layer of graphene, we are able to make ultra-small devices,” says Lin. “If we can make them smaller, we can make them faster.”
The intrinsic electronic properties of graphene contribute much higher electron mobility levels than those found in silicon CMOS or MOSFET architectures. For the 240-nm gate length, for example, a typical silicon-based transistor would operate at 40GHz maximum. And the potential is there for much higher frequencies if the process is adapted to lithographic standards that are now down below 50 nm.
However, Lin was unable to say how soon such prototypes would find their way into productions.
“To achieve a better-quality graphene, we are working to remove impurities and defects,” he says.
The frequency performance of the graphene device exceeds the cut-off frequency of state-of-the-art silicon transistors of the same gate length. Similar performance was obtained from devices based on graphene obtained from natural graphite, proving that high performance can be obtained from graphene of different origins. Previously, the team had demonstrated graphene transistors with a cut-off frequency of 26 GigaHertz using graphene flakes extracted from natural graphite.
Study abstract
CERA at DARPA