US-Tech Online. -> July 07 - Transparent Transistors Herald New Displays, E-Paper

July 07 - Transparent Transistors Herald New Displays, E-Paper

Working at a micro-manipulation probe station in research using nanotechnology to create transparent transistors and circuits are Purdue University doctoral student Sanghyun Ju, sitting, and David B. Janes, a professor, both in the School of Electrical and Computer Engineering.

By Emil Venere, Purdue University
West Lafayette, IN — Researchers at Purdue University have used nanotechnology to create transparent transistors and circuits, a step that promises a broad range of applications, from e-paper and flexible color screens for consumer electronics to "smart cards" and "heads-up" displays in auto windshields. The transistors are made of single "nanowires," or tiny cylindrical structures that were assembled on glass or thin films of flexible plastic.

"The nanowires themselves are transparent, the contacts we put on them are transparent and the glass or plastic substrate is transparent," said David Janes, a researcher at Purdue University's Birck Nanotechnology Center and a professor in the School of Electrical and Computer Engineering.

Other researchers had previously created nanowire transistors, but the metal electrodes in the transistors were non-transparent, which made the overall structure opaque, said Tobin J. Marks, the Vladimir N. Ipatieff Professor of Chemistry and a professor in the Department of Materials Science and Engineering at Northwestern University.

Transparent Electronics
"Our study demonstrates that nanowire electronics can be fully transparent, as well as flexible, while maintaining high performance levels," Marks said."This opens the door to entirely new technologies for high-performance transparent flexible displays." Findings were detailed in a research paper in the journal Nature Nanotechnology, June 2007.

The advancement has three broad areas of potential applications:

Transparent displays for uses such as heads-up displays on windshields and information displays on eyeglasses and visors. The displays enable drivers to see information without looking down at the dashboard and could project information on visors for workers without obstructing their view. Potential applications also include sports goggles for spectators to follow a particular player while having relevant statistics displayed and real-time interactive information for soldiers and surgeons.
Flexible displays for future "e-paper," promising to allow full-motion video. E-paper is a technology designed to mimic regular ink on paper. Unlike conventional flat-panel displays, which use a backlight to illuminate pixels, e-paper reflects light like ordinary paper and is capable of holding text and images indefinitely without drawing electricity while allowing the image to be changed later. Potential uses of e-paper include low-cost, energy efficient ways of displaying information and video as a replacement for paper in magazines, newspapers, books, electronic signs and billboards.
Transparent and flexible electronics for RFID, electronic bar codes and smart credit cards, which resemble ordinary credit cards but contain an embedded microprocessor. This microprocessor replaces the usual magnetic strip on a credit or debit card, increasing the security of data stored on the card and enabling computers to "talk" to the microprocessor. Such a technology could be used to display balances on cards and could be used for the free flow of people through transportation systems, avoiding the need of ticketing machines or validation gates. The cards could contain encryption software, secure data for use in pay phones and banking, and to contain health-care data for patients and allow tamper-proof identification information for workers.

The nanowires were made of zinc oxide or indium oxide.

Low-Temperature Processing
Unlike conventional CMOS computer chips, the thin-film transistors could be produced less expensively in low temperatures, making them suitable for incorporation into plastic films, which would melt during high-temperature processing.

Liquid crystal displays now used in applications such as color cell phone screens are made with thin-film electronics. This thin-film technology makes it possible to lay down electronic devices in large sheets containing individual pixels. Current thin-film electronics use technologies known as amorphous silicon and poly-silicon. "These approaches work fine if you have a flat, rigid display that's going to be opaque," Janes said."They require fairly high-temperature processing, so they are not good on plastic, although industry is working really hard to get them on plastic and make them lightweight, flexible and transparent."

An alternative, emerging technology uses so-called "organic" or "plastic" transistors to replace the conventional silicon that has been a mainstay of microelectronics for decades. While this technology enables transistors to be embedded in or printed on flexible plastic, it has lower performance, although major advances are being made, Marks said. The new research represents the best of both worlds.

"You can get high performance because the nanowires themselves give you some unique performance advantages, and you could still think of dispersing them down over large areas for displays, smart credit cards and other applications," Janes said.

The nanowires are transparent because they are made of materials that do not absorb light in the visible range of the spectrum. In conventional electronics, transistors are connected to the rest of the circuitry by tiny lines of metal that act as wires. But in the new approach, the nanowires themselves are the transistors. "This is a different kind of wire," Janes said."It is basically taking the place of the silicon in silicon electronics."

One reason for the higher performance realized in the new technology is that the devices have a better "on-off ratio" than previous thin-film technologies, Janes said. Having a good on-off ratio helps conserve power, making the new thin-film transistors practical for portable battery-powered devices.

"In a transistor, you are trying to turn it off and on, like a switch," Janes said."But unlike a wall switch in your house, a transistor never really turns completely off. There is always a little bit of leakage through it, sort of like crimping a garden hose."

The nanowire transistors help to reduce this leakage while also offering the possibility of precisely controlling the pixels in displays.

Research has been funded by NASA through the Institute for Nanoelectronics and Computing, based at Purdue's Discovery Park, and at Northwestern University.

Nanotechnology is critical for the advancement because electricity flows differently on the scale of nanometers, or billionths of a meter, than it does in larger wires. The nanowires used in the research measure as small as 20 nanometers in diameter. A single nanometer is roughly the size of 20 hydrogen atoms strung together. Future research is expected to include work to integrate the thin-film transistors into large circuits and to develop ways to interconnect numerous transistors.

For more information, contact: Purdue News Service.

765-494-2096 E-mail: purduenews@purdue.edu

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