“...we have been working to demonstrate the characteristics that might make OTFTs compelling from a commercial standpoint.”

For my early education I attended Calvin College in Grand Rapids, Michigan, for two years, then completed BS, MS, and Ph.D. degrees in Electrical Engineering at the University of Michigan in Ann Arbor. I then spent 12 years as a Research Staff Member at IBM Research in Yorktown Heights, New York, before joining the faculty in the Electrical Engineering Department at Penn State University in 1992. I have also found continuing education quite important; for example, I never had a course in organic chemistry, but my research interest in organic semiconductors has led me to learn at least some basics.

For several years at IBM Research I worked on III-V (GaAs and similar) device technology. Our task there was to try to answer the question "Should IBM build computers from III-V materials?" Our answer was no, they should make them from silicon. This got me thinking about what cannot be done easily or at all with silicon and I joined a flat-panel display technology group working on hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs).

Flat-panel displays are odd beasts and require oxymoronic materials like transparent conductors and opaque insulators, and at one point I was working on an organic version of an opaque insulator. Partly I was working to decrease the already low mobility in a strongly absorbing, small-molecule, conjugated organic material. So my research path went from III-V heterojunction materials with a carrier mobility of about 10⁴ cm²/V× s, to a-Si:H with a carrier mobility of about 1 cm²/V× s, to organic materials with a mobility of about 10^-9 cm²/V× s.

About that time there was an interesting paper from Garnier, Horowitz, Peng, and Fichou that reported a carrier mobility of about 0.1 cm²/V× s in a -sexithiophene. So, after losing about thirteen orders of magnitude in mobility, when I moved to Penn State I decided to look in the other direction. After some early work with a -sexithiophene, we decided to look for other high-mobility materials and picked pentacene as one to look at because of some notions about how trapping in that material might impact carrier transport. So far I have recovered about nine or ten orders of magnitude in mobility.

Starting about 1996, we began to publish high mobility organic TFT (OTFT) results for pentacene. The mobilities we reported were much higher than had been seen earlier and initially some other groups had difficulty repeating our results. At that time a not uncommon occurrence was a call from someone who had not obtained high mobility, sometimes asking for advice, but frequently explaining, with more or less politeness, on the basis of their results or from theoretical considerations, that we must be wrong.

Dr. Thomas Jackson's most-cited paper with 309 cites to date: Nelson SF, et al., "Temperature-independent transport in high-mobility pentacene transistors," Appl. Phys. Lett. 72(15): 1854-6, 13 April 1998. Source: Essential Science Indicators.

The variable temperature measurements in the 1998 APL paper were an attempt to gain a better understanding of the fundamental nature of transport in pentacene. Two of my then students, Yen-Yi Lin (now at Texas Instruments) and Dave Gundlach (now at NIST) fabricated the devices and did simple screening at 77 K and Shelby Nelson (then a professor of physics at Colby College, now at Kodak) did many very nice variable temperature measurements. We found that a variety of mobility versus temperature characteristics could be observed including a dependence that looks very much like uncorrelated hopping (Holstein-like), or like correlated hopping (Emin-like), or a surprising nearly temperature independent mobility.

Our point was that coincidence does not imply causality and that though we had difficulty saying precisely what the nature of the transport was in high mobility TFTs, we could say what it was not (not simple hopping or correlated hopping). The paper also makes the useful point that elucidating transport from TFT measurements is non-trivial.

Two main directions. First, I am an engineer and so my group has worked to provide engineering demonstrations that OTFTs can be useful for practical applications. For example, we have demonstrated OTFT circuits, sensors, and simple flexible substrate displays as existence theorems that such applications are possible. Second, we have been working to demonstrate the characteristics that might make OTFTs compelling from a commercial standpoint. This relates in part to cost and we have been working on solution processible organic semiconductors as a vehicle for much lower cost manufacturing than is possible with inorganic semiconductors or vapor-deposited organic semiconductors.

To the best of my knowledge there are no current products, though there have been some piloting demonstrations. I think one of two things is required for commercialization. The first is a compelling application that simply cannot be built with inorganic devices. For example, one might imagine a smart bandage that would monitor for infection or other problems or sense the need for medication and provide it. In this case having a soft material that can be processed at low temperature may be the key. Second is materials and processes that allow existing applications, for example displays, to be fabricated at substantially reduced cost. Both areas are being worked on worldwide and OTFTs are close to having all the characteristics required for commercial use.

We continue to work on high-mobility solution processible organic semiconductors and have demonstrated devices with mobility > 2 cm²/V× s. Recently, working with John Anthony (University of Kentucky), we have demonstrated devices where a differential organic semiconductor microstructure is tied to the underlying device structure. We have also demonstrated patterning of organic semiconductors without the use of photolithography.

Because an underlying interest is in demonstrating things that silicon cannot do we have also been working on ZnO TFTs and, working with Kodak, have recently demonstrated ZnO TFT circuits with propagation delay < 100 ns/stage. My group also has projects with nano-biomotors (kinesin-microtubule), a-Si:H sensors and circuits, and UHF ultrasound transducer arrays.

Thomas Jackson, Ph.D.
The Pennsylvania State University
University Park, PA, USA