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Tokyo group makes transparent transistors manufacturable

Posted: 17 Jan 2005 ?? ?Print Version ?Bookmark and Share

Keywords:oregon state university? transparent transistor? tokyo institute of technology? zno?

About a year ago, the world's first transparent transistor was demonstrated at Oregon State University. Since then, several other laboratories have created similar zinc-oxide-based transparent transistors. However, critics charged that organic and polysilicon transistors would be easier to mass-produce because they do not require the expensive substrates or pulsed-laser deposition.

Now, a group at the Tokyo Institute of Technology claims to have solved these problems at room temperature by adding gallium and indium to ZnO, opening the gates to mass-production of transparent circuitry using industry-friendly sputtering, and at temperatures low enough for direct deposition on cheap, flexible polymers.

"This is a breakthrough for flexible electronics," said Hideo Hosono, a professor in the Yokohama-based institute's Materials and Structures Laboratory.

Scientists researching flexible electronics are struggling to get enough performance out of organic and polysilicon transistors. However, Hosono says his material makes their work moot because it achieves the same goal with conventional processing techniques. "Our physical deposition process for the demonstration was pulsed-laser deposition, but sputtering will also work," he said. "Our process works at room temperature--no heating is needed to crystallize the amorphous thin films, which work as excellent transistors."

"This announcement from the Tokyo Institute of Technology is something pretty fabulous," said John Wager, the OSU EE professor who reported the world's first transparent transistor in 2003. OSU has partnered with Hewlett-Packard Co. to create a commercial version. "In our lab, we are doing similar things and have already demonstrated that sputtering on amorphous oxides at room temperature works fine," Wager said.

Performance gap

The new InGaZnO material outperforms by up to fiftyfold amorphous-silicon substrates, such as those now in the planning stage for next-generation displays and solar cells. "Right now, polysilicon outperforms our materials, but we still have a long way to go. I think there will be significant new performance gains for amorphous oxides over the next few years," said Wager. "And if we can drop the transparency requirement for some applications, then we can probably get much higher performance."

Hosono's transparent amorphous oxide semiconductor used indium gallium zinc oxide for the active channel in a transparent TFT. The group deposited the InGaZnO on a polyethylene terephthalate (PET) flexible substrate at room temperature. It exhibited Hall-Effect mobilities an order of magnitude larger than for normal hydrogenenated amorphous silicon. The transparent TFTs functioned stably during repetitive bending of the flexible substrate.

The top-gate flexible transparent TFTs used the InGaZnO film as an n-channel active layer atop the 200?m-thick PET film. The source, drain, gate contacts and a gate insulator were defined using standard photolithography and lift-off techniques. Then, a 140nm-thick yttrium-oxide layer was laid over the gate as insulator. Connections to the source, drain and gate then were formed with indium-tin-oxide transparent electrodes.

"This is totally different from organic semiconductors," said Hosono. "We use almost all of the normal lithographic processes, and we can use sputtering for deposition and indium-tin-oxide electrodes."

The semiconductor material was optically transparent over the entire visible and nearIR regionsor in the wavelength of 390nm to 3,200nm.

Existing display technologies could benefit from transparent TFTs by letting more light through to brighten them, but the long-term benefit will be for flexible electronics of all types&mdashsolar cells, computers and displays.

- R. Colin Johnson

EE Times

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