High frequency, high gain graphene transistors

A graphene transistor that outperforms previous state-of-the-art has been developed by an international team of scientists gathered around the Graphene Flagship project. By utilizing a thin top gate insulator material, researchers from Italy, Sweden, the USA and Spain optimized transistor figures of merit such as maximum oscillation frequency, cutoff frequency, forward transmission coefficient, and open-circuit voltage gain, realizing devices that show prospect of using graphene in a wide range of electronic applications.

The use of graphene field effect transistors (GFETs) in digital electronics has been problematic due to the lack of a bandgap in graphene. The lack of bandgap results in the inability to switch the transistors off, effectively rendering the “0” state in digital logic inaccessible. For most analog applications, however, a bandgap is not necessary. The only undesired side-effect of using GFETs in analog circuits is a poor saturation of the drain current, which prevents high-gain operation. Researchers have now succeeded in improving saturation by optimizing the dielectric material (AlOx) that is used to electrically insulate the top gate of the GFET. An improved quality of gate dielectric resulted in strong control over carriers in the graphene channel, yielding overall performance benefits.

“Such GFETs could find applications in analog circuits, in which high gain and operating speed could be traded off against power consumption”, says Roman Sordan, leader of the group that performed the research. “Some examples include amplifiers, oscillators, and very simple mixed-signal circuits (e.g., for baseband processing)”, he adds.

Graphene transistors

Image: Graphene transistors (copyright Nature Publishing Group, Creative Commons)

The paper was published in the Nature Publishing Group journal Scientific Reports. As part of the project, the authors have identified critical technological parameters that could be optimized to further improve the maximum oscillation frequency, showing that the proposed improvements are of a technological nature, rather than a fundamental obstacle. The research suggests that a further improvement of CVD graphene quality, smoother substrates, reduced contact and gate resistances should yield devices with even higher real use potential.

This research is a result of the Graphene Flagship, a billion-euro ten-year project of the European Commission.