Fast switching and printable transistor invented

All six articles in his doctoral thesis were recently published in journal Advanced Materials.

The thesis claims that with the help of polymers, plastics, which are already manufactured on a large scale, it is possible to manufacture transistors that are fast and can run on small printed batteries, where the drive voltage is around 1 volt.

They are particularly suitable for printed electronics.

The transistor is made up of two polymers, one of which acts as a semiconductor and the other as an electrolyte; a substance containing mobile charged ions that controls the current flowing through the transistor.

Polymers consist of linked chains of molecules. Thanks to the fact that one type of charged particle in the electrolyte, be it positive or negative ions, binds to the polymer chain in the semiconducting polymer. The active layer, in which the electric field is concentrated in the electrolyte, becomes very thin (1 nanometre) irrespective of the thickness of the electrolyte layer.

Whether it is a negative or positive ion that binds depends on whether it is a transistor that is hole-conducting (p-channel) or if it is electron-conducting (n-channel).

The thin active layer permits the use of very low driving voltages. By combining p- and n-channel transistors, Lars Herlogsson has constructed complementary circuits, CMOS circuits, which reduces the power consumption.

"This is robust CMOS technology which allows for very low drive voltages, and besides that, it is well suited to printed electronics," he says.

To achieve these low drive voltages using conventional technology would require nanometre thin layers. Printing such thin layers is impossible because the printing surface on paper or plastic film is typically rough. However, printing a 100-nanometre thick layer, as in this case, is possible using conventional printing techniques.

The idea of creating a thin active layer also impressed electronics Professor Christer Svensson, now emeritus of the examining committee.

"A scientifically very neat job, an intelligent idea that he clearly showed works in reality. There may be applications for this type of electronics such as in large TV screens where silicon is unable to compete," Svensson says.

The focus of Lars Herlogssons thesis has been to produce a material system for polymer-based organic transistors that can be printed at a reasonable price. The result is a transistor that within traditional electronics is called a field-effect transistor. Four of the thesis articles are related to just that, but the other two articles are related to the following:

• one addresses woven electronics where the organic electrolyte transistors are embedded in the intersections between textile microfibers.
• The other shows how to produce an organic field-effect transistor with a drop of water as the electrolyte.

All of the six articles in the dissertation have been published in the scientific journal Advanced Materials.

Now, after spending years on research, Lars Herlogsson has taken a step closer to production. September 1, he began working at the company Thin Film Electronics in Linkoping to develop inexpensive printed memories.

"As scientists, our task is to push the boundaries and show what is practical and possible. Industry can produce the organic electronics better than we can and there are many talented plastic electronics companies, says Magnus Berggren, Professor of organic electronics at Linkoping University.

Thesis: Electrolyte-Gated Organic Thin-Film Transistors, Lars Herlogsson, Department of Science and Technology, Linköping University, Campus Norrköping, 2011