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US researchers offer a new definition for the word 'woodchip'

01 July 2015

A biodegradable silicon transistor based on material derived from wood, opens the door for green, flexible, low-cost portable electronics.

A fresh CNF based microwave silicon transistor chip. After three weeks of putting it in a woodpile, the chip was partially degraded with the help of fungi (images: Jung-Hun Seo, Shaoqin Gong and Zhenqiang Ma/University of Wisconsin-Madison)

Researchers from the University of Wisconsin-Madison have come up with a new solution to alleviate the environmental burden of discarded electronics. They have demonstrated the feasibility of making microwave biodegradable thin-film transistors from a transparent, flexible biodegradable substrate made from wood-derived cellulose nanofibrillated fibre (CNF). The work is published in the journal, Applied Physics Letters.

"We found that cellulose nanofibrillated fibre based transistors exhibit superior performance as that of conventional silicon-based transistors," says team leader, Professor Zhenqiang Ma. "And the bio-based transistors are so safe that you can put them in the forest, and fungus will quickly degrade them. They become as safe as fertilizer."

The majority of portable electronics are based on non-renewable, non-biodegradable materials such as silicon wafers, which are highly purified, expensive and rigid substrates. Cellulose nanofibrillated fibre films, however, have the potential to replace silicon wafers as electronic substrates in environmental friendly, low-cost, portable gadgets or devices of the future.

Cellulose nanofibrillated fibre is a sustainable, strong, transparent nanomaterial made from wood. Compared with other polymers like plastics, the wood nanomaterial is biocompatible and has relatively low thermal expansion coefficient. These properties make cellulose nanofibril an outstanding candidate for making portable green electronics.

An array of microwave silicon transistors sitting on a wood-derived CNF substrate (image: Jung-Hun Seo, Shaoqin Gong and Zhenqiang Ma/University of Wisconsin-Madison)

To create high-performance devices, Ma's team employed silicon nanomembranes as the active material in the transistor. These are pieces of ultra-thin films peeled from the bulk crystal that are bonded to the cellulose nanofibrill substrate to create a flexible, biodegradable and transparent silicon transistor.

But to make portable electronics, the biodegradable transistor needs to be able to operate at microwave frequencies, which is the working range of most wireless devices. The researchers thus conducted a series of experiments such as measuring the current-voltage characteristics to study the device's functional performance, which finally showed the biodegradable transistor has superior microwave-frequency operation capabilities comparable to existing semiconductor transistors.

Ma and colleagues now plan to develop a more complicated circuit system based on their biodegradable transistors.

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