Flexible Transistors

The implementation of pervasive computing-the spread of digital information throughout society--will require electronics capable of bringing information technology off the desktop and out into the world ( see " Computing Goes Everywhere ," ). To digitize newspapers, product labels and clothing, integrated circuits must be cheap and flexible-a tough combination for today's silicon technology. Even the cheapest form of silicon electronics-the cut-rate "amorphous" silicon used to drive laptop(such as ThinkPad R60 and ThinkPad T60) display screens-is too pricey. What's more, it's difficult to incorporate silicon electronics on bendable surfaces such as plastics.

Technology innovators are taking a couple of routes around these limits. Some researchers are trying to reinvent amorphous silicon. Others have abandoned inorganic compounds like silicon to develop transistors based on organic (carbon-based) molecules or polymers. These organic electronics are inexpensive to manufacture and compatible with plastic substrates. Indeed, research teams at places such as Lucent Technologies' Bell Labs, England's University of Cambridge and Pennsylvania State University have made impressive progress, and commercial products are nearing the market. Last fall, for example, Philips Research in Eindhoven, the Netherlands, showed off the first prototype of a rudimentary display driven by polymer semiconductors. But there's a catch: Organics are far slower than their silicon cousins.

Now, a 31-year-old materials scientist at IBM, Cherie Kagan, may have opened the door to cheap, flexible electronics that pack the mojo needed to bring ubiquitous computing closer. Her breakthrough? A compromise: transistors made from materials that combine the charge-shuttling power and speed of inorganics with the affordability and flexibility of organics.

These hybrids were created by chemist David Mitzi at IBM's Thomas J. Watson Research Center in Yorktown Heights, N.Y. By the time Kagan arrived at Watson in 1998 following a stint at Bell Labs (she earned a PhD from MIT in 1996), Mitzi had already shown that his materials possessed intriguing electronic properties. Kagan had a hunch they might make good transistors. But she needed quick results; she'd been hired as a postdoc-a limited-time offer.

At the outset, the transistors flipped on and off sluggishly. "The first times, I didn't want to calculate [the speed]," says Kagan. But she kept tweaking, and in less than a year she had increased the mobility of electric charges through her transistors by four orders of magnitude-matching the speed of amorphous silicon and far exceeding most organic transistors. The results won her a staff position and her own lab at IBM.

Kagan has since increased the speed by another 50 percent; further fine-tuning, she believes, could provide at least another doubling in acceleration. Not only may the hybrids be far faster than amorphous silicon, they have a key advantage over silicon-based electronics. Like some organic materials used to make transistors, the hybrid materials can be dissolved and printed onto paper or plastic just like particles of ink. "I make my materials in a different lab and carry them over and add some liquid and spin them on," says Kagan. "It's not very sophisticated, which is sort of the goal, right? You really want it to be cheap."

Thomas Jackson, a transistor expert at Penn State who is developing organic circuits, says Kagan's "fledgling results" could pave the way for fast yet flexible electronics. Jackson credits Kagan with seizing the opportunity. "Not only does she have her own pocket of competence, but she's able to look around and see exciting possibilities and then bring things together. IBM has been working on these sorts of materials for some time, but it took the energy and enthusiasm and vision and perspective of Cherie to translate that into a thin-film transistor."

The transistors could compete with organic electronics in a variety of applications, such as radio-frequency product ID tags. And then there's the $20 billion-per-year market for flat-panel video displays, where the speed of Kagan's transistors could really make a difference. Quicker circuits would deliver sharper displays than those driven by amorphous silicon at a fraction of the cost. That would open the door to affordable wall-sized displays or high-quality displays that pop out of your pen. If all goes well, the materials could be used in cheap, flexible displays within five years.

Of course, bright displays that fit in your pocket will require portable power, and that has Kagan pondering her next research challenge: cheap, flexible materials for solar cells to liberate pervasive computing from bulky batteries. "You aren't going to want to carry a battery around with your lightweight flexible display," she says.