September 6, 2013

New Hope for Graphene-based Logic Circuits

Tiffany Trader

For more than a half century, computer processors have increased in power and shrunk in size at a phenomenal rate, but the exponential advances described by Moore’s law are winding down. Electronics based on silicon complementary metal–oxide–semiconductor (CMOS) technology are coming up against the physical limitations of nanoscale. Currently, there is no technology to take the place of CMOS, but a number of candidates are on the table, including graphene, a one-atom thick layer of graphite. Research suggests this incredibly strong and lightweight material could provide the foundation for a new generation of nanometer scale devices.

Scanning electron microscopy image of graphene device used in the study. The scale bar is one nanometer.

As an excellent conductor of heat and electricity, graphene is a promising electronics substrate, yet other characteristics of this material have stymied its progress as a silicon alternative. To address these limitations, researchers at the University of California Riverside have taken a completely new approach.

Semiconductor materials have an energy band gap, which separates electrons from holes and allows a transistor to be completely switched off. This on/off switch enables Boolean logic, the foundation of modern computing.

Graphene does not have an energy band gap, so a transistor implemented with graphene will be very fast but will experience high leakage currents and prohibitive power dissipation. So far, efforts to induce a band-gap in graphene have been unsuccessful, leaving scientists to question the feasibility of graphene-based computational circuits.

But Boolean logic is not the only way to process information. The UC Riverside team showed that it was possible to construct viable non-Boolean computational architectures with the gap-less graphene. Their solution relies on specific current-voltage characteristics of graphene, a manifestation of negative differential resistance. The researchers demonstrate that this intrinsic property of graphene appears not only in microscopic-size graphene devices but also at the nanometer-scale – a finding that could set the stage for the next generation of extremely small and low power circuits.

“Most researchers have tried to change graphene to make it more like conventional semiconductors for applications in logic circuits,” Alexander Balandin, a professor of Electrical Engineering, said. “This usually results in degradation of graphene properties. For example, attempts to induce an energy band gap commonly result in decreasing electron mobility while still not leading to sufficiently large band gap.”

“We decided to take alternative approach,” Balandin continued. “Instead of trying to change graphene, we changed the way the information is processed in the circuits.”