New transistor could revolutionise thin-film electronics

A new transistor could facilitate the development of flexible electronic devices in areas such as biomedical imaging and renewable energy.

Efforts by researchers and the consumer electronics industry to improve the performance of the transistors have been slowed by the challenges of developing new materials or slowly improving existing ones for use in traditional thin-film transistor architecture, known technically as the metal oxide semiconductor field effect transistor (MOSFET).

But the University of Alberta team did a run-around on the problem. Instead of developing new materials, the researchers improved performance by designing a new transistor architecture that takes advantage of a bipolar action. In other words, instead of using one type of charge carrier, as most thin-film transistors do, it uses electrons and the absence of electrons (referred to as ‘holes’) to contribute to electrical output. Their first breakthrough was forming an ‘inversion’ hole layer in a ‘wide-bandgap’ semiconductor, which has been a great challenge in the solid-state electronics field.

Once this was achieved, the researchers were able to construct a unique combination of semiconductor and insulating layers that allowed us to inject ‘holes’ at the MOS interface, said Gem Shoute, a PhD student in the Department of Electrical and Computer Engineering who is lead author on the article. Adding holes at the interface increased the chances of an electron ‘tunnelling’ across a dielectric barrier. Through this phenomenon, a type of quantum tunnelling, the researchers were able to create a transistor that behaves like a bipolar transistor. 

“This kind of device is normally limited by the non-crystalline nature of the material that they are made of,” said Materials Engineering Professor Ken Cadien, a co-author on the paper. The dimension of the device itself can be scaled with ease in order to improve performance and keep up with the need of miniaturisation. An advantage that modern TFTs lack. The transistor has power-handling capabilities at least 10 times greater than commercially produced thin-film transistors.

Electrical Engineering Professor Doug Barlage, who is Shoute’s PhD supervisor and one of the paper’s lead authors, said his group could produce a high-power thin-film transistor — it was just a matter of finding out how.

“Our goal was to make a thin-film transistor with the highest power handling and switching speed possible. Not many people want to look into that, but the raw properties of the film indicated dramatic performance increase was within reach,” he said.

“The high-quality sub-30 nanometre (a human hair is 50,000 nanometres wide) layers of materials produced by Professor Cadien’s group enabled us to successfully try these difficult concepts.”

In the end, the team took advantage of the very phenomena other researchers considered roadblocks.

“Usually tunnelling current is considered a bad thing in MOSFETs and it contributes to unnecessary loss of power, which manifests as heat,” explained Shoute. “What we’ve done is build a transistor that considers tunnelling current a benefit.”

The team has filed a provisional patent on the transistor. Shoute said the next step is to put the transistor to work “in a fully flexible medium and apply these devices to areas like biomedical imaging, or renewable energy”.