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Printing Plastic Transistors

Mike May

Advances in electronics—especially integrated circuits—often arise from new manufacturing techniques. Making conventional integrated circuits requires steps that include high-energy or high-vacuum processes, not to mention pristine conditions. A relatively new approach to electronics relies on much simpler production techniques, which resemble the printing processes that made this magazine. This process generates transistors—the switching devices that dominate electronics—made of plastics rather than conventional semiconductors.

CircuitsClick to Enlarge Image

You might wonder how plastic can carry the electric currents that make a circuit work. In general, people think of plastic as an insulator, a material that doesn't conduct electricity very well. In fact, all materials conduct some electricity, and many applications do not require a material that conducts as well as copper wire, for instance. Although plastic lacks some of the benefits of the best conductors, it provides other advantages, such as ease of manufacturing.

Nearly a decade ago, one of the first plastic transistors came from Francis Garnier at the Centre National de la Recherche Scientifique's Molecular Materials Laboratory. To build that device, Garnier and his colleagues first discovered a group of organic—essentially plastic—semiconductors called conjugated oligomers. Most conventional transistors rely on the semiconductor silicon, which is made from sand as a raw material. Although he helped develop the new organic semiconductors, Garnier did not make his transistor entirely from plastic but used metal for the electrodes. A few years later, the group replaced the metal electrodes with a polymer-based conducting ink, making their transistor all-plastic.

Since the first transistor in 1947, most new designs could be described by one word—smaller. Just 0.25 micrometer separates the two upper electrodes of today's fastest silicon transistors. But plastic transistors are much bigger. The smallest plastic transistor made at Bell Labs, the research and development arm of Lucent Technologies, has 1 micrometer separating the two electrodes.

The benefit of these transistors comes from the simplicity and economy of the steps needed to make them. Howard Katz, a chemist at Bell Labs, says, "What we are really focusing on is that all the steps involve simple printing." The team at Bell Labs, which also includes Zhenan Bao and Ananth Dodabalapur, starts with a piece of plastic, like a transparency for an overhead projector. Then layers of various plastics can be added by applying the liquid plastic over a mesh that forces the material to create a pattern. "The process is very similar to silk-screening T-shirts," according to Bao. In this way, a transistor can be built—layer upon layer—without any high-energy or high-vacuum steps. These printing-like steps lead to less expensive transistors. "We're imagining being able to print a circuit based on these transistors for a few cents," says Katz. "And this can be compared to manufacturing and mounting a silicon chip for a dollar or two. So we're talking about an order of magnitude of cost difference."

Printed plastic transistors might be used in various applications. Dodabalapur and his colleagues announced a device that includes a light-emitting diode and a controlling transistor, all made of organic components, except for the metallic electrodes. This so-called "smart pixel" could be the basis of a display, say on a pager. In addition, plastic transistors can be made transparent, so that they might be used in display systems incorporated in an automobile's windshield. The plastic allows these circuits to be bent along the curvature of a windshield or around a package. Investigators at Philips Research in The Netherlands have developed a disposable identification tag that can be incorporated in the wrapping of a soft package. The Philips device also leads the way in linking large numbers of all-plastic transistors into circuits. This 326-transistor device can be bent in half and still work properly.

Modern electronics—a world dominated primarily by teeny-weeny, yet brittle devices—rarely makes room for a comparatively gigantic contraption. Nevertheless, the low cost of printable production, light weight and flexibility might make technology wrap itself around all-plastic transistors.

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