Graphene has already brought us the world’s smallest transistor – twice – and now the one atom thick form of carbon that recently won its discoverers the Nobel Prize has been used to create a triple-mode, single-transistor amplifier. The new transistor has the potential to replace many traditional transistors in a typical integrated circuit and its developers say the device could become a key component in future electronic circuits.
Aside from being very strong, nearly transparent and being a very good conductor of electricity, graphene is also ambipolar, meaning it is able to switch between using positive and negative carriers on the fly depending on the input signal. In comparison, traditional silicon transistors usually only use one or the other type of carrier, which is determined during fabrication. It is graphene’s ambipolar property that has allowed the researchers from Rice University and the University of California, Riverside, to develop the new three-terminal single-transistor amplifier.
The triple-mode transistor can be changed during operation to any of three modes at any time using carriers that are positive, negative or both. This provides opportunities that are not possible with traditional single-transistor architectures, said Kartik Mohanram, an assistant professor of electrical and computer engineering at Rice.
Mohanram likened the new transistor's abilities to that of a water tap. "Turn it on and the water flows," he said. “Turn it off and the water stops. That's what a traditional transistor does. It's a unipolar device – it only opens and closes in one direction. But if you close a tap too much, it opens again and water flows. That's what ambipolarity is – current can flow when you open the transistor in either direction about a point of minimum conduction.”
This means a graphene-based transistor can be “n-type” (negative) or “p-type” (positive), depending on whether the carrier originates from the source or drain terminals, which are effectively interchangeable. When the input from each carrier is equal, a third function appears with the transistor becoming a frequency multiplier. By combining the three modes, the Rice-Riverside team demonstrated such common signaling schemes as phase and frequency shift keying for wireless and audio applications.
"Our work, and that of others, that focuses on the applications of ambipolarity complements efforts to make a better transistor with graphene," Mohanram said. "It promises more functionality." The research demonstrated that a single graphene transistor could potentially replace many in a typical integrated circuit, he said. Graphene's superior material properties and relative compatibility with silicon-based manufacturing should allow for integration of such circuits in the future, he added.
However, technical roadblocks still need to be overcome before that happens. Fabrication steps such as dielectric deposition and making contacts actually disturb graphene’s lattice structure, scratching it and introducing defects, which immediately limits its signal gain and degrades its performance. This means the team has to exercise a lot of care when making the transistors.
But Mohanram is confident these problems can be overcome, saying, “the technology will mature, since so many research groups are working hard to address these challenges."
A paper detailing the triple-mode transistor appears in the online journal ACS Nano.