Graphene, a two-dimensional sheet of carbon atoms, has become a major focus of device research because of its high conductivity and compatibility with planar fabrication processes. Graphene-based transistors demonstrated to date have used a single gate contact to control the flow of current. Now, Naoki Harada and colleagues at Fujitsu Laboratories in Japan1 have demonstrated a graphene transistor with two gates, allowing a greater range of functions to be achieved.

Much of the improvements in electronic device performances over the past 50 years have resulted from reducing the size of silicon-based transistors. This trend, however, is expected to soon come to an end as further size reduction efforts run into fundamental physical limits. The use of novel materials such as graphene represents a promising alternative to further miniaturization by allowing performance gains to be achieved from higher conductivities.

In addition to its high conductivity, graphene is also unique because, unlike silicon, it conducts both positive and negative charge (‘holes’ and electrons, respectively) with equal efficiency. Harada and his colleagues exploit this trait — called ambipolarity — in their two-gate transistor geometry. While one gate controls the magnitude of current flowing through the transistor, the other gate controls the polarity of that current.

Fig. 1: A digital modulator for communications circuits based on graphene modulates a carrier wave connected to the top gate of a transistor with digital data fed into the bottom gate.

This allowed the Fujitsu team to build a novel type of electronic inverter — a device that inverts an input signal applied to one gate depending on the voltage applied to the other gate. The researchers used the inverter to construct a digital data modulator of the kind that is used in many modern digital communications systems (Fig. 1). A digital data stream delivered to one gate modulates, or changes, a carrier signal fed into the second gate. This modulated signal can then be broadcast over a channel, and decoded by a second circuit.

The Fujitsu transistor represents the first graphene-based digital communications circuit, and its function is made possible by the team’s unique two-gate transistor design. The research demonstrates the potential for the use of novel materials in electronics. “Our goal is to realize convenient and low-power-consumption electronic devices by using nano-carbon materials,” says Harada. “We hope that more and more researchers in the field will be inspired by our results and become interested in graphene.”