Because of graphene’s physical properties, electronics made from the material may be cheaper and speedier than current technology. In a step toward practical graphene electronics for telecommunications, engineers at the IBM have made graphene transistors that break speed records for devices made from the material (Nano Lett., DOI: 10.1021/nl300904k).
Transistors are electronic devices, usually made from semiconductor materials, that can turn on, turn off, and amplify electrical signals in circuits. The earliest graphene electronics were high-quality graphene transistors made one at a time from single, pristine flakes of graphene. But the complex circuits in today’s electronics require integration of a large number of transistors, making these one-off transistors impractical, says Phaedon Avouris of IBM Watson Research Center in Yorktown Heights, N.Y. For the past couple years, researchers including Avouris have focused on making large numbers of transistors on wafers, similar to how the semiconductor industry produces silicon-based devices.
Making large arrays of graphene transistors that maintain the same quality as those made one at a time, though, is something engineers are still figuring out how to do. Recent attempts have led to defects in the graphene that cut the device performance.
The IBM group has developed new methods for making high-quality, wafer-scale graphene electronics, and the resulting transistors have set two speed records. At more than 300 GHz, their wafer-scale transistors beat the previous record for a measurement called cutoff frequency, which describes how fast electrons can move through the graphene channel at the heart of the transistor. They also beat the record for how fast the transistors can switch without degrading the outgoing signal. That operating speed is 44 GHz, about twice the previous record for graphene transistors made on wafers.
To show they could integrate the transistors into a working circuit, the researchers also used the devices to build an amplifier circuit, a basic component of analog electronics that increases the power of a signal. The circuit could amplify an incoming signal by 3 decibels while operating at a speed of 5 MHz. While the performance of the graphene devices isn’t impressive compared to conventional materials, the ability of a graphene circuit to amplify a signal at all is an important step, says Avouris. The IBM group made the first ever graphene integrated circuit just one year ago, he points out.
The IBM group actually made two types of 140 nm-long graphene transistors, each using graphene grown by different methods: chemical vapor deposition (CVD) and epitaxial graphene. On top of the each device’s graphene layer, the team applied an insulating layer, which helps transistors in the off state remain off. For the CVD graphene device, they used two methods to put down a layer of oxidized aluminum, while the epitaxial graphene transistor got a layer of silicon-nitride.
This focus on optimizing all the device materials is what enabled the improvements in device performance, says Deji Akinwande, an electrical engineer at the University of Texas, Austin. He says similar work is what researchers need to push graphene from experiments in the lab toward actual applications.
However, graphene electronics still have a ways to go, says Avouris. The IBM group is now focusing on how to improve device performance, while also shrinking the transistors.