The researchers – led by Professor Jack Ma and research scientist Jung-Hun Seo – fabricated a transistor that operates at 38GHz, but note their simulations show the device could operate at 110GHz.
Using low temperature processes, Prof Ma and his team patterned the circuitry on their flexible transistor – single-crystalline silicon placed on a polyethylene terephthalate (PET) substrate – using nanoimprint lithography.
Through the use of selective doping, impurities were introduced in precise locations to enhance material properties – in this case, electrical conductivity. In order to overcome the short channel effect, the researchers blanketed the single crystalline silicon with a dopant, rather than doping it selectively.
A photoresist layer was then added and electron beam lithography applied to the photoresist to create a reusable mould. This was applied to an ultrathin, flexible silicon membrane to create a photoresist pattern, after which a dry etching process cut precise, scale trenches in the silicon.
With 3D current flow, the transistor is said to consume less energy and to operate more efficiently. And because the approach enables narrower trenches than conventional fabrication processes, it could lead to greater transistor density.
Ultimately, says Prof Ma, because the mould can be reused, the method could scale for use in roll to roll processing. “Nanoimprint lithography addresses future applications for flexible electronics,” says Prof Ma. “We don't want to make them the way the semiconductor industry does now. Our step, which is most critical for roll-to-roll printing, is ready.”
Other collaborators in the project were University of Michigan, University of Texas at Arlington and University of California, Berkeley.
