Printable semiconductors move a step closer

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Xerox and TDA have both unveiled methods for making transistors out of plastic instead of silicon

If recent research projects bear fruit, it won't be too many years before magazines play videos and semiconductors roll out of inkjet printers.

Workers at Xerox and TDA Research independently unveiled methods last week for making transistors out of plastic rather than silicon, in ways that could be commercially viable.

Such a shift in materials could drastically reduce the cost of computer displays because chipmakers would not have to build multibillion dollar factories to make semiconductors to power these devices.

Just as important, it could greatly expand the range of objects that connect to the Internet, because electronic connections would be handled by a thin film or mouldable material, rather than rigid chips. A thin screen could be bound into a magazine, for instance, and connect wirelessly to a Web site, or plastic soda bottles could transmit signals to inventory devices.

"Within two to three years, you might be able to see something on the market, like a low-performance display" made with these types of materials, said Beng Ong, a Xerox fellow who headed a team at the Xerox Research Centre of Canada that developed a series of electronic polymers.

Similarly, TDA last week made the debut of Oligotron, a conductive polymer that could be used to make inexpensive solar cells or environmental sensors. The National Science Foundation funded the project.

Conductive polymers have been made in the past; however, they've often proved impractical. Generally, these materials are not soluble in liquid, which makes them difficult to shape or spray. And they typically corrode easily in air or water.

In its project, Xerox developed a molecule that can act as a semiconductor (i.e., transport electricity on command, like silicon) and also remain relatively stable in earthly environments. Manufacturers would want to control dust, but the material would not require the sort of clean-room atmosphere chips are now produced in.

"It allows you to print in ambient conditions," Ong said. "In order to print for consumer electronics, you have to be able to print in the air."

Once sprayed, the molecule then orients itself with regard to its neighbours. "The molecule has to be able to self-assemble into a proper structure to transport charges," he said.

Xerox has also developed two other designer molecules: one that acts like a conductor, which transmits electricity, similar to how a metal works; and one that behaves like a dielectric, or insulator.

Together, all three of these could conceivably enable Xerox to make printable chips, as all three are integral parts of semiconductors. Semiconductors switch off and on to create the basic digital signals, conductors connect semiconductors, and insulators prevent cross-signalling.

The Palo Alto Research Centre (PARC), a subsidiary of Xerox, has already created a functioning electronic array that serves as a backplane for a display, said Ong. The backplane can be turned off and on, proving that the semiconductor material works.

Xerox, Motorola and Dow Chemical are now conducting further experiments on integrating the material into various devices and on controlling the application of the conductive material through different printing techniques and specialised masks. Ong's group will conduct experiments on spraying the conductor and insulator materials.

Oligotron, meanwhile, is a variant of a material called Pedot (polyethylene dioxythiophene). Although Pedot conducts, water corrupts it, making it unattractive, according to TDA.

To get around this problem, TDA attached two additional molecules to the core Pedot molecule and developed a solvent bath that allows manufacturers to manipulate the material. In the solvent, manufacturers could shape a soda bottle out of Oligotron. When it cured, the bottle could then be used to transmit signals.

Both experiments represent examples of nanotechnology because their distinct properties only emerge when the material is manipulated at the molecular level.

Other companies are tinkering with creating similar materials with carbon nanotubes or OLEDs (organic light emitting diodes).

Neither material will rival silicon in the general market for silicon. For one thing, plastic transistors are huge. Xerox's circuits can measure 20 to 50 microns wide, while manufacturers like Intel are churning out chips with 90-nanometre features. A micron, a millionth of a metre, is 1,000 times larger than a nanometre, a billionth of a metre.

Still, that's small enough to create a video or PDA display. Typically, displays might only have 110 pixels per square inch, said Ong.

The Xerox materials are also good at conducting electricity, like a certain well-known metal. "Our conductor," said Ong, "is almost as good as gold."

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