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