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Speedy chip secret ingredient: Carbon

Posted: 27 Mar 2008 ?? ?Print Version ?Bookmark and Share

Keywords:carbon transistor? electron mobility? fast chip?

Researchers at the University of Maryland claim that carbon transistors could outperform the fastest chip materials, including indium antimonide.

The College Park team recently characterized graphene monolayers, sheets of pure carbon just one atom thick. They discovered that graphene appears to be unphased by temperature, unlike most semiconductors. Usually, speedcalled electron mobilityis proportional to temperature (the colder the better since fewer lattice vibrations, called phonons, can scatter flowing electrons).

Instead, pure graphene transistors appear able to achieve their maximum possible speed at room temperature, according to the researchers, if chip makers choose the right substrate.

Electro mobility
"We measured the electron mobility of graphene monolayers between 50K (- 370F) and 500K (450F) and found [electron mobility] to be about 15,000cm?/Vs regardless of temperature, which is unusual," said team leader Michael Fuhrer of the university's Center for Nanophysics and Advanced Materials, and the Maryland NanoCenter.

In silicon, electron mobility is about 1,400cm?/Vs, and the highest known electron mobility in any material is about 77,000cm?/Vs in indium antimonide. By contrast, the lattice vibrations in graphene were measured by the Maryland researchers to be so weak that secondary effects like impurities and substrate choice had a bigger impact than phonons.

"What we now think is that phonon scattering of electrons in graphene is very weak, and that leads us to believe that we are being limited by impurities. If we can remove those impurities, we think we can achieve electron mobilities of 200,000cm2/Vs at room tempteraturewhich is more than 100 times better than silicon," said Fuhrer.

By comparison, electron mobility in carbon nanotubes has been measured at about 100,000cm?/Vs, or half that of graphene monolayers. But to achieve the highest mobility possible for pure carbon transistors fabricated from graphene monolayers, the researchers said they will need a substrate other than ordinary silicon dioxide, which was used in their current tests. Candidates include silicon carbide and diamond.

Also being considered is eliminating the substrate altogether and using air gaps beneath graphene transistor channels.

Since "the lattice vibrations were so weak in the graphene itself, a secondary, little-studied effect seemed to dominate," said Fuhrer. "The phonons in the substrate, which for us was silicon dioxide, appeared to scatter electrons in the graphene, which we believe will limit its electron mobilities to about 40,000cm?/Vs."

Next, the researchers will try widely available silicon carbide, which is prefabricated into wafers. The team also plans to test graphene deposited atop diamond substrates. They will also try using air gaps, which Fuhrer said will be much more difficult to fabricate in commercial devices.

- R. Colin Johnson
EE Times

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