Printing Plastic Transistors

Murray Hill, N.J. (March, 1998) -- "Think plastics," was the career advice given to the young hero of the movie, "The Graduate." Prophetic advice, as researchers today are looking at carbon-based compounds for new and simpler ways to make integrated circuits, often called "organic" or "plastic" transistors.

Bell Labs researchers have been in the lead in this effort, having successfully investigated P-type compounds that have electron mobilities as high as 0.1, which is the mobility that can be achieved by amorphous silicon semiconductors, and work is also proceeding apace on the N-type side of the ledger.

Hopefully, research into organic transistors may lead to new uses of these promising devices, such as new generations of smart cards, toys, appliances, and many other things that might not be physically or commercially viable using today's silicon-based technology. The work continues at Bell Labs.

They won't supplant silicon devices...but they could offer some interesting new options.

Carbon-based semiconductors

Bell Laboratories researchers are studying carbon-based compounds - popularly called plastic or organic transistors - to make integrated circuits that may someday complement silicon-based devices in special applications.

Dick Slusher, head of Bell Labs Optical Physics Research, one of the departments in which the organic transistors are being created, admits, "This is very long range stuff that we're talking about," Slusher says. "It's not that we have targeted a market of $50 billion to enter within the next five years. This is research which might open a market in the five-to-twenty-year time frame."

The purpose of this technology is not to replace conventional semiconductor chips used in computers, but rather to compete with, and even go beyond, amorphous silicon technology currently found in displays.

A Variety of Uses

What could organic transistors be used for? Lightweight and flexible plastic chips could usher in new generations of smart cards, toys, appliances, and many other things that might not be physically or commercially viable using today's silicon-based technology.

Some speculate about low-cost transistor-impregnated attire, which could be used for everything from ticket-less checkouts at retail stores to monitoring personnel at high-security installations.

The possibilities are endless. In addition to being highly flexible and lightweight, plastic transistors hold the promise of tremendously reducing production costs.

Printing Transistors

To create traditional amorphous silicon devices, circuit patterns are defined on glass sheets using expensive lithographic, vacuum deposition, and etching techniques. A typically device consists of four layers. The first is a gate metal. Then comes a gate insulator, an active semiconductor, and finally source and drain electrodes.

This plastic substrate contains some 2,400 organic transistors, each about 1/4 millimeter long and as wide as a human hair.

"In creating plastic transistors, we use a layering structure quite similar to that used with amorphous silicon, but the methods by which the layers are formed are much less expensive," notes Ananth Dodabalapur, one of the researchers.

"We'd like to be able to eventually print the active material in the same way a high-speed press prints a modern-day newspaper," said Slusher

Squeegee Technique

While this is the long-term goal, present research is aimed at developing methods of replacing the expensive vacuum chambers which are required to eliminate unwanted impurities in building up chips, layer by layer. Hopefully, the layers of material could be applied either by spinning liquid semiconductor material onto a plastic sheet -- thereby creating a thin film -- or by spreading the liquid semiconductor using a high-tech squeegee, known as a doctor blade.

The squeegee technique is used to print the circuitry in a process similar to silk-screening, in which the doctor blade is used to press organic ink through small openings in a screen, and onto the plastic surface.

Materials Complement Silicon Devices

While this process is less expensive than etching and vacuum deposition, it admittedly cannot produce transistors as small as those made the conventional way.

"By printing organic films, one can create lines down to about 50 microns in size, while the standard silicon process goes to 0.5 microns," says Slusher. "So instead of megabytes of memory we're talking about kilobytes. And in speed, instead of megahertz we're talking kilohertz."

Nonetheless, Slusher does not see this as a problem, since the much lower cost and unique materials used may suggest completely new uses. "We're not trying to re-invent what silicon does," Slusher says. "Nor are we competing directly with silicon. We're trying to create new markets."

Ananth Dodabalapur places a plastic sheet containing organic transistors in a cryo-probe station used to measure various characteristics of the material.

Plastic as a Conductor?

While one doesn't normally think of plastic as an electrical conductor, the study of the electronic conduction properties of organic materials has been going on at Bell Labs and elsewhere for many years.

"Certain plastics have amazing electronic properties," explains Dodabalapur. "These materials have an electronic structure which makes it very easy for carriers (electrons) to move through them." However, he notes, "Not all plastics do this; only a very small subset."

As with silicon semiconductors, plastic devices fall into two primary categories- P-type, which conduct positive carriers, and N-type that carry negative charges.

Three years ago, another Bell Labs researcher, Howard Katz, began investigating P-type organic semiconductors in hopes of developing high performance plastic transistors.

"We started working on a compound that had been reported to be a good P-type organic semiconductor, know as alpha-6T," said Katz. "We devised new ways to produce it and, more importantly, new ways to purify the compound, thereby greatly increasing its performance."

Search for Better Materials

But even with these improvements, alpha-6T didn't quite measure up, so the search for better semiconducting plastics continue.

One method of measuring transistor performance is known as field-effect mobility, which is a measure of how fast a charge will move in a material at a certain voltage. Stated in centimeters squared per volt per second (cm2/Vs), field effect mobility is basically a measure of the speed at which a transistor will turn on and off. With the alpha-6T semiconductor, "We achieved a mobility of about 0.02," said Katz. "That's much too low, about a factor of 10 from where we wanted to be, so we started working on other P-type materials. For example, we synthesized a compound that has a slightly different structure than alpha-6T...and showed it can have a mobility of 0.04, or double that of alpha-6T, but still much, much too low."

The Research Team Expands

As improvements began to come rapidly, so too did the pace of work on this technology, and additional scientists were added to the team. Zhenan Bao joined the staff, as did Joyce Laquindanum, a postdoctoral fellow working with Katz.

This team has successfully investigated P-type compounds that have mobilities as high as 0.7, which is the mobility that can be achieved by amorphous silicon semiconductors.

Work is also proceeding apace on the N-type side of the ledger. " We recently discovered that certain compounds based on naphthalene have a fairly high mobility for negative charges, exceeding 0.001," said Katz. "Admittedly, that's almost 100 times lower than amorphous silicon, but considering that the compounds are fairly simple, and fairly stable, that represents a major advance over where other N-type materials had been."

Others Companies Involved

These Bell Lab scientists are not alone, however, in their quest to come up with commercially viable products. "There's been a lot of progress in the last two to three years, and it is anticipated that more will be made in the coming years. As a result, the action has really heated up," says Dodabalapur.

Other significant players in the field include IBM, Mitsubishi, and Philips. And at Pennsylvania State University, Professor Tom Jackson has reported that pentacene can have a mobility of 1.

Recent Bell Labs Contributions

One recent advance at Bell Labs is the first demonstration of a completely organic complementary circuit -- the basic building block of logic circuits -- using both N- and P-type materials in the same circuit.

Additionally, Bao, Dodabalapur and Katz, working in conjunction with Reddy Raju and post doctoral fellow Yi Feng, have fabricated an organic transistor in which every component was printed rather than deposited in a vacuum.

This constitutes an advancement "over not only our own previous work, but over what anybody else in the world has done," said Dodabalapur. And, in fact, strains of Bach have already wafted across their lab from an organic amplifier built using plastic transistors by Dodabalapur and Allen Mills.

"These and other advancements have brought the performance of plastic semiconductors," as Slusher notes, "very close to what amorphous silicon is able to do, and that's very encouraging. The main thing that characterizes this field is growth in understanding, performance and fabricability by a factor of two to three each year."

"It's a very rapid learning curve that people are on," he said, "and we don't know where we'll end up."