Researchers at IBM say they have achieved a new top mark for graphene-based transistors, setting a new record of 100GHz cutoff frequency (100 billion cycles/sec), the highest achieved so far for any graphene device.
Graphene, the single-atom-thick layer of hexagonally-arranged carbon atoms, possesses unique and intriguing electrical, optical, mechanical, and thermal properties. "A key advantage of graphene lies in the very high speeds in which electrons propagate," which scientists hope to leverage into vastly faster transistors, stated T.C. Chen, VP of science & technology at IBM Research. (In Dec. 2008 IBM set the previous mark of 26GHz cutoff frequency.)
The latest work, part of the DARPA-backed Carbon Electronics for RF Applications (CERA) program as part of efforts to develop next-generation of communication devices, was achieved using wafer-scale epitaxially grown graphene and process technology "compatible" with that used in advanced silicon device fabrication. Uniform, high-quality graphene wafers were synthesized by thermal decomposition of a silicon carbide (SiC) substrate. The graphene transistor itself has a metal top-gate architecture,
and a "novel" gate insulator stack (polyhydroxystyrene polymer spin-coated to 10nm thickness, and oxide deposited by ALD); the high dielectric constant layer has capacitance of about 195nF/cm2. Gate length was a "modest" 240nm, IBM says, with plenty of room for scaling and optimization (the goal is to get to ~50nm); maximum frequency could be increased by lowering gate resistance with a thicker metal stack or multifinger gate layout, IBM says. Results were published in the Feb. 5 issue of Science.
IBM also notes that the frequency performance of this graphene transistor exceeds the cutoff frequency of silicon transistors of the same gate length (~40GHz). Also, devices based on graphene obtained from natural graphite showed similar performance, indicating that performance can be obtained from graphene with different origins. (The Dec. 2008 work used graphene flakes extracted from natural graphite.) — J.M.