Graphene's potential was recognized earlier this month when those who first studied it in the labwon the 2010 Nobel Prize in Physics. But researchers are just beginning to figure out how to take advantage of the novel carbon material in electronic devices.
Researchers have already made blisteringly fast graphene transistors. Now they've used graphene to make a transistor that can be switched between three different modes of operation, which in conventional circuits must be performed by three separate transistors. These configurable transistors could lead to more compact chips for sending and receiving wireless signals.
Triple transistor: Single graphene transistors like this one can be made to operate in three modes and perform functions that usually require multiple transistors in a circuit.
Chips that use fewer transistors while maintaining all the same functions could be less expensive, use less energy, and free up room inside portable electronics like smart phones, where space is tight. The new graphene transistor is an analog device, of the type that's used for wireless communications in Bluetooth headsets and radio-frequency identification (RFID) tags.
Graphene's perfect structure at the atomic level provides smooth sailing for electrons, and the material conducts electrons better than any other materials do at room temperature. So far, it's been used to make transistors that switch at about 100 gigahertz, or 100 billion times per second, 10 times faster than the best silicon transistors; it's predicted the material could be made into transistors that are even 1,000 times faster than this. And because graphene is smooth and flat, it should be compatible with the chip-making equipment at semiconductor fabs.
But graphene offers other properties besides just being a great conductor of electrons, says Kartik Mohanram, professor of electrical and computer engineering at Rice University. It's also possible to change the behavior of a graphene transistor on the fly, something that can't be done with conventional silicon transistors. The transistors that make up conventional silicon logic circuits can only behave in one of two ways, called "n" for negative or "p" for positive--they either control the flow of electrons or the flow of "holes," or positive charges. Whether a conventional transistor is p-type or n-type is determined during fabrication. But graphene is ambipolar: it can conduct both positive and negative charges.
Mohanram has designed a transistor that can be changed, and has made and tested it with Alexander Balandin, professor of materials science and engineering at the University of California, Riverside. By changing the voltage applied to a sheet of graphene using three electrical gates, they could switch the graphene between three different modes: n-type, p-type, and a mode where it conducted positive and negative charge equally. This triple-mode transistor acts as an amplifier and can be used to encode a data stream by changing the frequency and the phase of a signal. Changes in phase and frequency are used to encode data in telecommunications devices such as Bluetooth headsets and RFID tags.
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.
By Katherine Bourzac
From Technology Review