Graphene could be a useful material for high-performance transistors because it carries electrons faster than silicon. Since graphene transistors can’t be turned off, they’re more useful for RF applications than logic circuits. Now researchers in California have boosted the performance, while simplifying the production, of graphene RF transistors.

A traditional transistor has a silicon or metal semiconductor channel sandwiched between source and drain electrodes. Applying voltage between a gate electrode on top of the channel and the source electrode allows current to flow through the channel. Adding a small RF signal to the gate electrode while the transistor is carrying current amplifies that signal as it comes out of the drain.

Since graphene carries electrons faster than silicon, filling channels with the carbon sheet could speed signals running through the transistor. But high-performance graphene transistors can’t be made using standard fabrication techniques. Building a gate electrode atop graphene damages the carbon sheet and reduces its electron-carrying ability.

In a 2010 paper in Nature, Xiangfeng Duan, of the University of California in Los Angeles, and his colleagues side-stepped this difficulty by transferring a prebuilt nanowire gate electrode to a pristine strip of graphene. Additionally, the size of the nanowire helped position the source and drain electrodes close to the channel. This precise positioning helped the researchers build a transistor that could handle the highest incoming current frequency for a graphene transistor–until now.

The scientists have boosted the cutoff frequency of graphene transistors again, while simplifying production to be more compatible with industrial methods. In this new paper, Duan transfers stacked gate electrodes to graphene strips using special sticky tape.

To build the gates, the scientists first place a thin layer of gold on a silicon wafer. Then they cover the surface with aluminum oxide and add tiny strips of titanium and gold using standard fabrication methods like etching and lithography.

The scientists cover the stacks with the special tape and peel them off the silicon wafer. They transfer the gate stacks to strips of graphene on glass and remove the tape. Finally, the scientists build source and drain electrodes with lithography.

The performance of these transistors, partially described by the cutoff frequency, depends on the quality of the underlying graphene. When the graphene was made by vapor deposition, 46-nm channel length transistors had a cutoff frequency of 212 GHz at 0.6 V. The cutoff frequency increases to 427 GHz with 1.1V applied across 67-nm transistor of higher-quality peeled graphene–the highest frequency for such a transistor.

The other number that describes the performance of these transistors is the power gain. A 220-nm transistor of vapor-deposited graphene has a maximum oscillation frequency (where the power gain equals one) of 29 GHz. That’s about 10 times lower than indium phosphide or gallium arsenide transistors of comparable size, says Frank Schweirz, of the Ilmenau University of Technology in Germany. This value is generally lower for graphene transistors because the carbon sheet behaves more like a metal than a semiconductor.

Schweirz thinks the fabrication method is very clever. Graphene transistors cannot yet compete with current transistors due to their lower power gain, he adds, but they might be able to in the future if scientists can find a way to alter some of the properties of graphene.

UPDATE: This article was corrected to say that 29 GHz is the maximum oscillation frequency, not the power gain.

Proc. Natl. Acad. Sci., 2012. DOI: 10.1073/pnas.1205696109 (About DOIs).