Mohanram and Balandin’s device is the first that can do this level of signal processing in a single transistor. Usually such signaling requires multiple transistors. Their transistor is a proof-of-concept device, but Mohanram says it demonstrates what might be possible with graphene.
Other groups have demonstrated multimode transistors using graphene, carbon nanotubes, and organic molecules. The researchers say that the new graphene triple-mode circuit can be controlled better than those devices.
Control is critical when designing transistors that are ambipolar, says Subhasish Mitra, professor of electrical engineering and computer science at Stanford University. “People used to consider ambipolarity a bad thing” because it’s typically difficult to control how an ambipolar transistor will behave, which makes it difficult to use them at all, he says.
Mitra notes that the benefits shown at the single-transistor level must now be demonstrated in systems. The electrical gates needed to control the behavior of arrays of ambipolar transistors might end up making circuits much harder to design and fabricate. “Now that they have shown that they can do this, we need to see what benefit it brings at a system level,” he says.
Balandin and Mohanram are now working on graphene circuits to test the benefits of ambipolarity at a higher level. They’re also changing the design of the transistors themselves to make them more efficient.
No one has yet published any articles on the creation of integrated circuits made of graphene transistors, but Balandin says researchers are now on the verge of putting it all together. As materials scientists and device fabricators work on overcoming the challenges of working with graphene, says Mohanram, circuit designers should keep pace with them and think creatively about ambipolarity and other possibilities opened up by graphene and other nanomaterials. “New designs and new ways of thinking can lag behind the development of new materials,” he says.