Researchers at the University of Texas at Austin in the US say that they have made state-of-the-art flexible graphene field-effect transistors with record current densities and the highest power and conversion gain ever. The transistors also show near-symmetric electron and hole transport, are the most mechanically robust flexible graphene devices fabricated to date, and can be immersed in a liquid without any ill effects.

Graphene is a single, flat sheet of carbon arranged in a honeycombed lattice. It has many unique electronic and mechanical properties, such as extremely high carrier mobility – which means that it is an ideal material for use in ultrafast transistors. The material can also absorb light over a range of wavelengths in the electromagnetic spectrum from the visible to mid-infrared and is highly transparent to light. The fact that it is mechanically flexible while being incredibly strong is good news too.

The researchers, led by Deji Akinwande and Rodney Ruoff, made their graphene field-effect transistors (GFETs) directly atop patterned dielectrics on plastic sheets using conventional microelectronic lithography. The devices have a unique structure, explains Akinwande, in which multi-finger metal gate electrodes are embedded in the plastic sheet. They are also made using graphene that has been grown by chemical vapour deposition (CVD), which can now produce as good graphene flakes as can be obtained by exfoliation (the famous "sticky-tape" method).

Record properties

The innovative production technique means that graphene can easily be integrated and fabricated on plastic sheets that have been pre-patterned with metal gates. This produces transistors in which charge carriers can move extremely fast and in which electrons and holes move in the same way. The devices are also extremely compliant and can accommodate mechanical strains of up to 9% and can be bent and unbent over for more 20 continuous cycles – a record number for flexible GFETs.

"Overall, our transistors feature record circuit performance, the largest mechanical bending and the highest extrinsic cut-off frequencies (of about 2.23 GHz) to date for any graphene flexible nanoelectronic device," says Akinwande. "What is more, the devices are liquid-resistant thanks to the fact that the surface of the graphene is passivated with silicon nitride and the plastic substrate is self-passivated. In short, we found that they could be accidentally dropped into everyday liquids, such as milk, tea or coffee, and can even survive being run over by a moving vehicle – all without suffering damage to their outstanding properties."

Smart applications

The extremely flexible, high-performance devices could be ideal for smart, conformal, advanced electronics that could offer performance capabilities beyond today’s silicon-based technology while also being cheaper, lighter, more environmentally friendly and with arbitrary form factors, claims Akiwande. "Potential applications include flexible smartphones, displays, fabric and even smart walls," he adds.

The team, which is presenting its work this week at the International Electron Devices Meeting in San Francisco, is now busy trying to make flexible wireless radios and mobile systems using the new GFETs at gigahertz frequencies. "From a basic research point of view, we are also looking into heat management in these devices on flexible plastic substrates, which is a major issue for transistors operating at high speeds and current densities," adds Akinwande.