Graphene, a 2D sheet of carbon just one atom thick, is ideal for making nanoelectronic devices because it a very good electrical conductor as well as being the thinnest material known. However, the fact that it lacks a bandgap limits its use in digital electronics based on current switching. Indeed, as a result, the on/off current switching ratio in graphene devices made so far is typically around 5, which is too low for practical applications.

Recently, researchers have also been studying bilayer graphene, which can develop a bandgap when a perpendicular electric field is applied. Until now, though, efforts to open up a large bandgap at room temperature have not been all that successful and higher on/off ratios were only possible at very low temperatures. Phaedon Avouris and colleagues at IBM's TJ Watson Research Center in New York have now demonstrated a bilayer field-effect transistor (FET) that has an electric-field induced bandgap of 120 meV and an off/ratio of as high as 100 at room temperature.

Novel gate dielectric stack
The bilayer transistor is based on a pure AB-stacked graphene bilayer incorporated in a double-gated FET structure, explains Avouris. "Key to the high performance of the device is a novel gate dielectric stack composed of a thin polymer layer in contact with the graphene, followed by a high dielectric constant HfO2 film deposited by atomic layer deposition," he said.

It is the polymer seed layer employed in the gate stack that we have to thank for producing the high on/off ratio. This is because it reduces scattering of charge carriers in graphene introduced by charged impurities in the oxide dielectric, which is no longer in direct contact with the graphene. "At the same time, the gate stack and the double-gate structure allow us to generate the high fields needed to open the bandgap," Avouris told nanotechweb.org.

The team now plans to further develop its device, which is far from optimized. For example, if the insulating layers are scaled down and their quality improved, much higher fields could be achieved, adds Avouris. "We can also improve the purity of graphene itself and so attain significantly higher on/off ratios. At the same time, we will keep exploring applications."

The work was published in Nano Letters.