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U-bend breakthrough for superfast graphene transistors

With its potential for superfast processing speeds, graphene has long been heralded as a key component in the supercomputers of the future. But the problem with making transistors out of the stuff is finding a way to turn them off.

Now however, a new type of design suggests that simply creating "U" bends in it could do the trick.

Graphene is the thinnest material known, made up of sheets of carbon arranged in a honeycomb structure just a single atom thick. This structure allows electrons to pass through it faster than most other materials, making it an ideal candidate from which to make electronic devices like transistors, says Zakaria Moktadir at the Nano Research Group at the University of Southampton, UK.

Binary bind

In order to make computers faster, circuits need to be turned on and off with an extremely high switching speed, something at which graphene excels.

Indeed, just last year IBM scientists demonstrated a graphene transistor with a switching rate of 100 gigahertz – that is, capable of switching between a "1" and "0" state 100 billion times a second, more than twice that of even the fastest silicon transistors.

Ideally these binary states would correspond to a current flowing ("1") and zero current ("0"). However graphene's structure means that a current flows through the device in both states, even when the transistor is supposed to be switched off.

"The intrinsic physical properties of graphene make this current flow difficult to turn off," says Moktadir.

Power problem

The difference between these two states is called the current on/off ratio, and graphene's low on/off ratio has long been a major barrier to using it in transistors for logic gates and ultimately computer chips.

A low on/off ratio means that the processor would have to use more power, and so would be less efficient.

The problem can be overcome by using either a strong electric field to switch the transistor or by making the transistors out of extremely narrow ribbons of graphene, just a few nanometres wide. But large electric fields require large voltages, and it's difficult to make nanoribbons narrower than 5 nanometres with any degree of accuracy.

But Moktadir reckons he has discovered a third way to get graphene switching more efficiently. By making a normally flat transistor similar to a U-shape, but with corners instead of a curve at the bottom, he has found that he can switch it off entirely, increasing the current on/off ratio thousandfold. "We are still trying to explain why it's happening, but we attribute it to the corners," he says.

Clever cuts

Working with graphene just a few hundred nanometres wide, sharp corners are known to produce unusual behaviour, says Moktadir. But making such corners is challenging because conventional electron-beam lithography – using electrons to create a circuit on a nanometre scale - leaves corners somewhat rounded.

Moktadir and colleagues got round this by using a gallium focused ion beam to create U-shaped ribbons of graphene 300 nanometres wide and a few micrometres long. Reporting their findings in Electronics Letters, Moktadir says this new approach overcomes the issue.

"If they have made some progress then that's a big deal," says C. David Wright at the Centre for Graphene Science at the University of Exeter, UK.

Journal reference:



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