Mar. 30, 1998 DALLAS -- Imagine a flexible computer display screen that does not break when it kaplunks on the ground. Or how about a credit card-sized smart card, chock full of vital statistics and buying power, that is virtually indestructible and does not require regular replacement?
Plastic transistors may lead to these breakthroughs, and the process for making them could be as simple as printing a newspaper.
A team of researchers at Bell Labs, which is the research and development arm of Lucent Technologies, produced the first fully "printed" transistor last year, and they presented their latest research findings here at today’s national meeting of the American Chemical Society.
Besides developing plastic transistors that are smaller and also more intricate in their design, the researchers have developed a new process that involves spraying liquids instead of printing. Because the spraying technique requires less material than printing, manufacturing costs would decrease even further, compared to making conventional silicon transistors.
Although plastic transistors never would replace the silicon variety, which are found in virtually everything from computers to talking toys, they would be very useful in certain high-volume applications.
Besides roll-up computer screens and smart cards, other potential uses include luggage tags that help airport personnel locate lost suitcases, or tags on groceries that verify whether they were transported under the right conditions to the supermarket.
Plastic transistors can be disposable because of the inexpensive manufacturing process. In today’s factories, for instance, machines deposit and etch all sorts of materials on silicon to make multi-layered transistors, which essentially are microscopic switches that form the guts of today’s electronics. This process requires high temperatures, stringently controlled environments, and precision optics, all of which are very costly.
"We’re interested in the most low-tech approaches," says chemist Howard Katz. After several years of development, Katz and his colleagues have created plastic transistors in normal laboratory conditions. The major challenges have included developing materials with the necessary electrical characteristics, without sacrificing their flexibility and long-term reliability.
Using thin sheets of plastic -- similar to overhead transparencies -- as the base, the researchers print a transistor’s multiple layers one layer at a time. They use a squeegee to apply a liquid plastic mixture over a stainless-steel mesh.
After the solvent evaporates, the plastic remains. "The process is very similar to silk screening T-shirts," says chemist Zhenan Bao. The challenge is creating a liquid mixture with the correct viscosity. The mixture, for instance, must pass through the mesh, but it must be viscous enough so it will not spread too much on the surface.
Since the researchers introduced this liquid-based approach last year, they have been working to reduce the transistor’s size, especially concentrating on the distance between the transistor’s two top electrodes. As the distance decreases, the transistor’s speed increases, and it also requires less power to operate.
So far, the smallest distance achieved by the Bell Labs’ scientists for the printed plastic transistor is 75 microns (A human hair is roughly 100 microns), compared to 0.25 micron in the fastest silicon transistors.
"We’re at the stage in plastic transistor technology where silicon was several decades ago," says chemical engineer V. Reddy Raju. The other three researchers on the Bell Labs’ team are physicists Ananth Dodabalapur and Andrew Lovinger and chemist John Rogers.
"Right now," continues Raju, "we’re using inexpensive processes that produce larger features. Fortunately, there are some applications where size is not an issue. With a smart card, for example, people are happy with the size, but the plastic transistor would make it more flexible and durable than putting a silicon transistor on a card."
Another potential application is the flexible computer screen because today’s laptop display has two major disadvantages: cost and lack of toughness. With a plastic display, the transistors would be implanted directly on a plastic sheet, adjacent to light emitting diodes that would light up the screen.
The most crucial building blocks for the plastic transistors are huge molecules that resemble spaghetti in terms of their molecular shapes. Ordering these molecules to achieve adequate transistor activity has been a considerable challenge. However, the spaghetti-like molecules are useful because they form continuous, flexible films.
It may soon be possible to use much smaller molecules, which would be easier to purify and align than the longer molecules. As a result, the electrical properties would be easier to control, creating more precise switching within transistors.
"We found it’s possible to make transistors by spraying dilute solutions of small molecules directly on a flat sheet," Katz says. "The droplets evaporate and leave behind the film that becomes the most crucial layer of the transistor. So far, the results have been similar to those achieved with the printing process."
Lucent Technologies, headquartered in Murray Hill, N.J., designs, builds and delivers a wide range of public and private networks, communications systems and software, data networking systems, business telephone systems and microelectronic components. For more information on Lucent Technologies, visit the company's web site at http://www.lucent.com.
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