Graphene works as a frequency multiplier
Mar 25, 2009 3 comments
Electrical engineers at Massachusetts Institute for Technology (MIT) have created a carbon-based component which could significantly increase the speed of computers and communication devices, they say. Tomas Palacios and his team have used the “wonder material” graphene — sheets of carbon just one atom thick — to create a frequency multiplier that can double the amplitude of an electrical signal.
"With hindsight, it is hard to believe that no one previously thought of this application," Andre Geim, University of Manchester
“In electronics, we’re always trying to increase the frequency.” says Palacios, “It’s very difficult to generate high frequencies above 4 or 5 gigahertz,” he says.
This new graphene technology could lead to practical systems in the 500 to 1,000 gigahertz range such as digital and analogue communication devices, radio-astronomy and THz sensing.
Graphene comes of age
Amongst its portfolio of useful properties graphene is a special semiconductor whose electrical state can be adjusted by simply applying a voltage across a region of a graphene sheet — rather than by introducing chemical impurities as in silicon. As a result, some researchers have tried to create transistors that are smaller and faster than traditional silicon-based devices. Indeed, in some cases, transistors have been made for intended use in wireless communication.
Despite these advances, Palacios believes there is a critical limitation in developing graphene transistors — graphene’s very small bandgap will always restrict performance. So, bypassing this obstacle, the researchers focussed on developing graphene frequency multipliers (FM) — a more advanced electronic component which produces an output signal whose frequency is an exact integral multiple of the input signal.
“With hindsight, it is hard to believe that no one previously thought of this application,” said Andre Geim, one of the researchers credited with the discovery of graphene back in 2004.
Using mechanical exfoliation — basically, “sticky tape” — the researchers removed flakes from a larger sample of graphene and deposited them onto 300 nm of silicon dioxide with an underlying silicon wafer which acted as the gate for the incoming and outgoing electric currents.
To double the outgoing frequency, Palacios and his team used graphene’s “ambipolarity”. That is, electrons and holes will conduct in alternative half cycles to produce an output signal, whose fundamental frequency is twice that of the input. They applied a certain voltage across the gate in order to make the amplitudes of the electron and hole currents roughly symmetrical. Using this setup they converted a 10 kHz input signal to a 20 kHz output.
“In the last few months, the understanding of the transport properties of graphene has improved considerably — this definitely helped us to create our new device,” Palacios told physicsworld.com.
“From what they have demonstrated, the work is more concrete than other fancier proposals,” said Yu-Ming Lin, a nanotechnology researcher at the IBM T.J. Watson Research Center in New York.
Lin was a bit less optimistic about scaling this effect up to boost the input and output signals by include multiple layers of graphene.
“One of the challenges will be to control of number of graphene layers in the final device… the domain size and the uniformity need to be further addressed and explored in my opinion.”
Palacios told physicsworld.com that his group intend to now recreate their device using graphene grown by chemical deposition on large wafers — a key step towards commercialization.
The findings will be published in the May Edition of Electron Device Letters
About the author
James Dacey is a reporter for physicsworld.com