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Tuesday, July 3, 2007

See-Through Transistors

Transparent transistors made from nanowires could mean bright and clear OLED displays.

Organic light-emitting diode (OLED) displays are currently found on mobile phones and digital cameras. But in the future, manufacturers expect bigger, bendable, and completely transparent versions. They envision bright maps on visors and windshields, television screens built into eyeglasses, and roll-up, see-through computer screens. And although the OLEDs themselves can be transparent, to make a clear display, the transistors that control each display's OLED, or pixel, need to be transparent as well.

Researchers at Purdue University and Northwestern University have now made flexible, see-through transistors using zinc-oxide and indium-oxide nanowires. By contrast, the amorphous or polycrystalline silicon transistors used in existing displays are not transparent. The new transistors also perform better than their silicon counterparts and are easier to fabricate on flexible plastic.

The transistors could lead to brighter see-through OLED displays, says Purdue electrical- and computer-engineering professor David Janes, who led the work published in last week's Nature Nanotechnology. When conventional nontransparent transistor circuitry is placed around the OLED, it takes up space on the display that could otherwise be emitting light. But, says Janes, "you could put transparent transistors underneath or on top of the pixel," increasing the light-emitting area.

To make the transistors, Janes and his colleagues first deposit an indium-zinc-oxide gate electrode on glass or plastic. Then they put a nanowire solution on the surface. After finding a nanowire that is aligned appropriately, they deposit source and drain electrodes made from indium tin oxide on either side of the nanowire. Both indium zinc oxide and indium tin oxide are transparent materials.

The nanowire transistors have high electron mobility, which determines how fast the transistor can work and how much current it can carry. In fact, the mobility is a few hundred times better than it is for transistors made from amorphous silicon, which is widely used in the electronics for displays. Because of that, the transistors could be made smaller and faster, Janes says. More-compact transistors, he says, would mean an even larger pixel area. What's more, the nanowire transistors are much easier to make on plastic than silicon transistors are because they don't need high-temperature processing.

Research groups have recently made transparent transistors using thin films of zinc oxide or indium oxide, or using carbon nanotubes. (See "Cheap, Transparent, and Flexible Displays.") Both technologies face unique issues. While the carbon-nanotube transistors have much better electron mobilities and are stronger than the new nanowire transistors, they aren't totally transparent because they need tiny metal contacts to connect the nanotubes to the electrodes. Thin-film transistors, on the other hand, are easier to fabricate on various surfaces but have much lower mobilities.


For the new transistors, "the performance in terms of the mobility, flexibility, and transparency is very impressive," says John Wager, an electrical-engineering and computer-science professor working in the area of transparent electronics at Oregon State University. Now the biggest remaining question, he says, is, "Can all of this be translated into real-world manufacturability?"

Right now, there is no method to control where nanowires get deposited on a surface or how they line up. "In experimental demonstrations, you throw down a couple thousand nanotubes and hope one aligns in the direction you want," Wager says. But randomly depositing nanowires on a surface will not work if one is to manufacture transistors for large displays.

Indeed, says Janes, "you have to have some way of putting the desired number of nanowires in the location you want." At this point, all three technologies to make transparent transistors--nanowires, thin films, and carbon nanotubes--have a fair shot at replacing silicon transistor technology for future transparent, flexible displays, Janes says.

According to John Rogers, a professor of materials science and engineering at the University of Illinois at Urbana Champaign, the ultimate commercial success of one of the three technologies will depend on how they measure up on many different factors: transparency, electrical performance, flexibility, and the ease and cost of manufacturing them. "It will be a good horse race to see which approach wins," Rogers says.

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