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Full control of threshold voltage

Loïg Kergoat

Loïg Kergoat, researcher in organic electronics at Linköping University, has shown how it is possible to control one of the most important parameters in printable plastic transistors in a recently published article in the highly ranked journal PNAS.

In his doctoral thesis from barely a year ago, Lars Herlogsson of the Organic Electronics Group in the Department of Science and Technology succeeded in showing that with the help of polymers, plastics that are already under large-scale production, it is possible to manufacture fully functional transistors that are fast, and additionally, can be powered by small, printed batteries where the operating voltage is around 1 volt. This makes plastic transistors, or organic transistors as they are also called, quite suitable for printed electronics.

Now, yet another breakthrough from the same research group has been released, led by Magnus Berggren, Professor of Organic Electronics.

Kergoat, a post-doc in the group, discovered a method of controlling with great precision one of the most critical parameters of those organic transistors.

Typically, a transistor has three electrodes. By putting a charge across one of the electrodes, known as the gate electrode, the current flow (or number of electrical charges) passing through both the other electrodes is controlled. A transistor must also be able to quickly switch between on and off, one and zero, and the state – known as the threshold voltage – must be well-defined if it is to be usable in a logic circuit. Up to a certain voltage, the transistor is off; over the threshold voltage, it’s on.

The research group work on organic transistors – organic field-effect transistors – and are constructed from two polymers where one functions as a semiconductor and the other as a solid electrolyte. As a voltage is applied on the gate electrode, ions are polarised in the electrolyte, thus regulating the electronic charge transport between the two other electrodes.

Kergoat has now shown that by changing the material on the gate electrode, the threshold voltage can also be gradually shifted. He’s tested gate electrodes in gold, nickel, copper, titanium, aluminium, and calcium. The other electrodes have consisted only of gold.

“It’s important to be able to control the transistor with weak voltages, preferably as close to zero as possible,” Kergoat says.

By changing from a gold to a calcium electrode, for example, he has reduced the threshold voltage by as much as 0.9V. The material that, incidentally, turned out to provide the threshold voltage closest to zero was titanium. This exact control of the threshold voltage in transistors now makes it possible to optimise the properties and performance of the circuits.

Magnus BerggrenResearch was conducted in collaboration with a research group at the Université Paris Diderot, Paris 7, where Berggren was a guest professor from 2009 to 2011. The results have recently been published in the highly ranked journal PNAS, the Proceedings of the National Academy of Sciences of the United States.

Kergoat has now been appointed to a Marie Curie position, financed by the EU and located in Berggren’s research group:
“The next step is to combine several transistors into a complementary circuit, or CMOS, which is the cornerstone of the energy-saving logic circuits of the future,” he says.

Circuits that are, therefore, well-suited for printed electronics, and which can be used to process and govern electronic signals in a number of various integrated printed electronics systems.

Article: “Tuning the Threshold Voltage in Electrolyte-Gated Organic Field-Effect Transistors”, published in PNAS, the Proceedings of the National Academy of Sciences of the United States.


  • Loïg Kergoat, Department of Science and Technology (ITN), Linköping University
  • Lars Herlogsson, Thin Film Electronics AB, Linköping University
  • Benoit Piro and Minh-Chau Pham, Université Paris Diderot, Paris 7
  • Gilles Horowitz, Ecole polytechnique i Palaiseau
  • Xavier Crispin and Magnus Berggren, ITN, Linköping University


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Monica Westman Svenselius Tue May 15 08:30:00 CEST 2012

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