Researchers from the University of California – Riverside and Rice University have made the first single-transistor amplifier from graphene. The device is better than conventional amplifiers thanks to the "ambipolar" nature of graphene, which means that it could find use in wireless and audio applications. It might also be used to design simpler analogue circuits for communications.

A single-transistor amplifier, which consists of one transistor and one resistor, is one of the most basic building blocks in analogue circuits. There are three types of single-transistor amplifiers: common-source, common-drain and common-gate. Each of these has different characteristics that depend on the small-signal voltage gain in the device (ΔVout/ΔVin). The common-source amplifier provides negative gain while the other two provide positive gain.

Different applications call for different types of amplifier, but the ideal device should be one that can be configured into more than one type after fabrication – something that is impossible to do with conventional silicon-based metal-oxide semiconductor field-effect transistor (MOSFET) technology.

Graphene amplifiers
Amplifiers made from graphene could come into their own here, say Alexander Balandin and colleagues. Graphene – a 2D sheet of crystalline carbon just one atom thick – could be ideal for future nanoelectronics devices thanks to its unique properties, which include the fact that it is an excellent conductor of electricity and heat. Transistors made from graphene also have a very high cut off frequency above 100 GHz and show low levels of noise.

Graphene is "ambipolar" too. This means that electrical current in the material can be carried by both electrons and holes, and the type of carrier utilized can be switched by simply applying a gate bias. This is somewhat different to conventional semiconductors, explains Balandin, where the type of carrier is pre-determined by the doping in the device.

"The fact that the type of carrier can be switched by the gate is reflected in the well-known 'V-shaped' current-voltage characteristic of graphene," he said. "We are capitalizing on this to achieve greater functionality from graphene transistors and use it in the amplifier design."

The UCR – Rice University team made a "triple-mode" device, which means that the amplifier can operate in one of three modes (common-source, common-drain or frequency multiplication) depending on where exactly the transistor is biased in the V-shaped ambipolar curve. "The three points one can choose are some place on the left branch of the V, some place on right branch or on the minimum conduction point where the branches meet," said co-team leader Kartik Mohanram of Rice. "Each operating point has different characteristics and we took advantage of the ability to switch between these bias points during operation when designing our amplifier."

Such triple-mode devices could lead to simpler circuits that show lower parasitics, have a larger bandwidth and consume less power. And being able to switch between the three modes this way will be important for "phase shift keying" and "frequency shift keying", processes that are widely used in wireless and audio applications, including Bluetooth, RFID and ZigBee.

The team now plans to employ more advanced top-gate transistors, which will allow for higher gain because of the much smaller gate thickness. Balandin and Mohanram's group has already built such transistors with low flicker noise, something that is crucial for graphene transistors in any analogue and communication application. "Now we have to put them to work in amplifiers," added Balandin.

"Our result is a major step forward in graphene technology because it marks the transition from making individual graphene devices to making fully fledged graphene circuits and chips. Clearly, there are several challenges ahead and the community as a whole is actively working on developing solutions."

The work was reported in ACS Nano. It can also be seen on arXiv.