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Philips' ambipolar plastic transistor

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Scientists at Philips Research have made organic field-effect transistors that conduct both positive holes and negative electrons within a single sheet of material. The company believes that the discovery enables the design of robust digital circuits with low power dissipation and high product yield. Philips scientist Dr Eduard Meijer (pictured) recently received the Else Kooi award for his PhD research that led to the discovery.
Scientists at Philips Research have made organic field-effect transistors that conduct both positive holes and negative electrons within a single sheet of material. The company believes that the discovery enables the design of robust digital circuits with low power dissipation and high product yield. Philips scientist Dr Eduard Meijer (pictured) recently received the Else Kooi award for his PhD research that led to the discovery.

The researchers used two different approaches to achieve ambipolar current flow in plastic. One approach blended p-type and n-type materials, in combination with source and drain electrodes of gold. This results in ambipolar conduction in a single layer of the blended material. Materials with low energy barriers for electron and hole injection were mixed to solve the usual charge injection problems with organic devices.

The second approach achieves ambipolar transistor operation in a single organic semiconductor. Low band gap organic semiconductors were chosen to reduce the energy barrier at the source and drain electrodes for both electrons and holes.

Both approaches produced both n-type and p-type transistor operation in a single organic semiconductor layer with a single type of source and drain electrodes. The Philips scientists also made the first working ambipolar inverter circuits with good noise margins and high gain values.

Until now, organic semiconductors have only shown the flow of one type of charge. This is caused by the occurrence of a high energy barrier for either electron or hole injection from the metal source and drain electrodes caused by the relatively large band gap of organic semiconductors. The absence of ambipolar charge transport has complicated the fabrication of CMOS-like circuits using organic semiconductors. Apart from the newly developed techniques, such organic production processes involve separate steps to make the n- and p-type transistors. Large-scale low-cost production of plastic manufactured by simple techniques such as spin coating or large-area printing makes it highly desirable to realise ambipolar transistor operation in a single layer of deposited material.

Complementary metal-oxide-semiconductor (CMOS) ICs on silicon are based on electronic circuits that use both n-type transistor channels with electrons conducting the electric current and p-type channels where holes make up the majority of charge transport. CMOS is the dominant technology in today's silicon-based industry.

The Else Kooi award is granted annually to reward excellent work in the area of applied semiconductor research and IC design. Candidates are students and young researchers employed by universities or public research institutes in the Netherlands.

Eduard Meijer carried out his PhD work at Philips Research within the framework of the "Lab Zonder Muren" programme run by the Dutch Foundation for Fundamental Research on Matter (FOM). He received his doctorate from the Delft University of Technology. The work was carried out in co-operation with the University of Groningen and the Bergische Universitaet Wuppertal.

The Else Kooi Award was established in 1995 by the DIMES microelectronics research centre at TU Delft, MESA+ at Twente University and Philips Research in Eindhoven. Dr Else Kooi was a former director of Philips Research.

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