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Engineers Develop World’s Fastest Silicon-based Flexible Transistors

20 April 2016

Engineers from the University of Wisconsin-Madison, along with fellow engineers from across the country, used a very tiny knife to easily and inexpensively create high-performance transistors with wireless capabilities on huge rolls of flexible plastic.

Zhenqiang (Jack) Ma, the Lynn H. Matthias Professor in Engineering and Vilas Distinguished Achievement Professor in electrical and computer engineering, research scientist Jung-Hun Seo, and the rest of the team were responsible for the transistor that operates at 38 gigahertz. However their simulations even indicate that it could potentially operate at 110 gigahertz—in other words, at lightning-fast processor speeds.

UW–Madison engineers has fabricated the world’s fastest silicon-based flexible transistors, shown here on a plastic substrate. (Image Credit: Jung-Hun Seo)UW–Madison engineers has fabricated the world’s fastest silicon-based flexible transistors, shown here on a plastic substrate. (Image Credit: Jung-Hun Seo)

The transistor can relay data or transfer power wirelessly, so it could find potential applications in wearables and sensors.

How they did it

The researchers used a nanoscale fabrication that was a bit different than traditional lithographic approaches, which use light and chemicals to pattern flexible transistors. Instead, they used low-temperature processes to pattern the circuitry on their flexible transistor, comprised of single-crystalline silicon placed on a polyethylene terephthalate (commonly known as PET) substrate, in a simpler and less expensive process called nanoimprint lithography.

And, instead of using a typical method called selective doping, where impurities are introduced into materials in precise locations to enhance their properties, like electrical conductivity, they covered their single-crystalline silicon with a dopant, rather than selectively doping it.

They then added a light-sensitive material and employed electron-beam lithography, in which a focused beam of electrons creates shapes as narrow as 10 nanometers wide, in order to create a reusable mold. The mold was used to create a flexible silicon membrane. The entire process was completed when they used a nanoscale knife to cut precise, nanometer-scale trenches in the silicon following the patterns in the mold, then added wide gates, which function as switches, on top of the trenches.

The 3-D pattern allows the transistor to consume less energy and operate more efficiently.

Because the researchers sliced very narrow trenches during the fabrication process, semiconductor manufacturers can squeeze even more transistors onto an electronic device.

According to Ma, because the mold can be reused, the method could easily scale for use in a technology called roll-to-roll processing, which would allow semiconductor manufacturers to repeat their pattern and mass-produce many devices on a roll of flexible plastic.

"Nanoimprint lithography addresses future applications for flexible electronics," said 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."

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