SHARE

When did you start working on organic transistors, and what prompted the choice of subject?

I started working on organic electronics, and on organic transistors in particular, for my thesis work with Prof. Tom Jackson at Penn State in the late 1990s. Tom is the corresponding author for all these early studies from 1999 and 2000. He started working on organic electronics around 1993-1994, and I was his fourth or fifth doctoral student.

Are these highly-cited papers all a direct continuation of your work with Tom Jackson?

Yes. Take the 2002 Journal of Applied Physics paper that's our most cited (Klauk H, et al., "High-mobility polymer gate dielectric pentacene thin film transistors," J. Appl. Phys. 92[9]: 5259-63, 1 November 2002). We were trying to make thin-film transistors using organic semi-conductors.

There's really nothing sensational about the work published in that article. We changed one or two of the materials from what we had originally developed at Penn State, which made the transistors a little better. They were a factor of two better than the ones we had made a couple years earlier.

So why do you think that paper has been so highly cited?

That's a good question. So let me try and explain. The main motivation for organic electronics is that these organic materials, unlike inorganic semiconductors, can be processed at relatively low temperatures.

To make silicon or gallium arsenide devices, inorganic devices, you need fairly high temperatures. These inorganic semiconductors are tough materials, and to process them you need temperatures of 600 degrees, 800 degrees, maybe 1,000 degrees Celsius.

Organic semiconductors, on the other hand, are soft, and you can process them essentially at room temperature. This is the main reason why organic transistors are of interest. It allows you to make these transistors at room temperature, and you can start thinking about making them on a variety of unconventional substrates, like plastic, wood, paper or cloth—my t-shirt, for example, or the windshield of your car. If you need 800 or 1,000 degrees to process the materials, that's going to certainly destroy my t-shirt or the windshield.

The transistor consists of more than just the semiconductor. It's also contacts and what's called the gate dielectric, which is a thin insulating film. Those were typically still made using inorganic materials.

What my colleagues and I did in 2002 was say, "Let's not just make the semiconductor organic; let's make the insulator organic, too."

"The main motivation for organic electronics is that these organic materials, unlike inorganic semiconductors, can be processed at relatively low temperatures."

Of course, organic gate dielectrics had been used before, but had always resulted in much lower transistor performance, because these organic dielectrics usually don't allow the organic semiconductors to perform at full potential. But when we combined the organic semiconductor with our newly designed organic insulator, the result was a transistor that was not just as good as one with an inorganic dielectric, but actually better by a factor of two.

That's probably the main reason why this paper is cited so often. It's a fairly good transistor that utilizes organic materials not just for the semiconductor, but also for the gate insulator.

Why did you publish that in the Journal of Applied Physics instead of, say, Advanced Materials?

We could have sent it to Advanced Materials. I can't think of any particular reason why I sent it to an applied physics journal, other than that maybe at the time Advanced Materials might not have been publishing as many articles on organic transistors. Now they are, but back then they didn't as much. So the Journal of Applied Physics seemed like the logical choice.

What you have to understand is that everything we publish is multidisciplinary, by definition, since organic electronics is a multidisciplinary field. It combines physics, chemistry, materials science, and electrical engineering.

Even mechanical engineering is an important part, because when we try to make flexible circuits—to make them on plastic or paper, say, as we discussed—that has a mechanical engineering component, as you can imagine. So it's really interdisciplinary. Every paper we publish includes most of these disciplines.

What about the 2004 paper in Nature on low-voltage organic transistors (Halik M, et al., "Low-voltage organic transistors with an amorphous molecular gate dielectric," 431[7011]: 963-6, 21 October 2004)? What was that reporting and why is it so highly cited?

That was very interesting; that was the first time that organic transistors were able to operate with low voltages and showed good performance.

A silicon transistor, to cite one example, usually operates with a voltage of 1 or 2 volts. The microprocessor in your computer, for example, uses 1.2 or 1.5 volts. And the reason is that the dielectric is quite thin in silicon transistors. It's only a few nanometers thick, and the voltage required to operate the transistor is essentially proportional to the thickness of the dielectric. A dielectric 4 nanometers thick allows the transistor to operate with 1 or 2 volts.

Until that Nature paper, most organic transistors required much higher voltages—20, 50, even 100 volts—because the gate insulator was much thicker, usually 100 or 200 nanometers, even a micron. You have to apply a high voltage to get appreciable charge accumulation in the semiconductor if the gate dielectric is that thick.

When I started working on this in Tom Jackson's laboratory, back in 1996, we had a meeting and we looked at the transistor characteristics, and I said, "But you need 100 volts to operate these." Tom said, "Okay, but we just got started. We'll get lower voltages eventually."

The problem is that when you make the insulator thinner, you start to introduce defects. There was some very nice pioneering work on thin gate dielectrics for organic transistors by a group in France, but their process was a little bit difficult to control, and the transistor performance was somewhat disappointing.

So the key was to find an insulator that can be thin but doesn't have a lot of defects and is easy to handle. There we got lucky.

We worked with three excellent chemists from the University of Stuttgart who had developed a very thin organic insulator, only about 3 or 4 nanometers thick. They thought it didn't have a lot of defects, and so it should be possible to use it to make organic transistors that could be operated at a low voltage.

As it turned out, it worked. The results were remarkable. We got very good transistor characteristics and the transistors were very easy to make. I would have thought it would be a lot more difficult, but it turned out to be easier to make actually than our other transistors, and it gave us very good transistor performance at a low voltage.

Our colleagues at MIT confirmed that the dielectric, despite its small thickness, indeed had a very small density of defects.

Page 2 >

EMAIL   |   BACK TO TOP

• 2010
• June 2010 - Author Commentaries
• Hagen Klauk on the Quest for Organic Transistors