Electrons and holes

Scientists at Philips have taken an important step towards the development of plastic electronics by showing that it is possible to fabricate field-effect transistors that conduct both holes and electrons within a single sheet of material.

The discovery will enable digital circuits with a low power dissipation and a high yield to be designed and fabricated. Today, Philips scientist Eduard Meijer received the Else Kooi award for his PhD research that led to the discovery.

Transistors in electronic circuits comprise both an n-type semiconductor, in which electrons conduct the electric current, and a p-type semiconductor, in which holes make up the majority of charge transport. These form the basis of complementary metal-oxide-semiconductor (CMOS) ICs, which is the dominating technology in today’s semiconductor industry.

Unfortunately, until now, organic semiconductors only showed the flow of one type of charge. This is not an intrinsic property of these materials per se, but is caused by the occurrence of a high energy barrier for either electron or hole injection from the metal source and drain electrodes of the transistor, which is caused by the relatively large bandgap of organic semiconductors.

As a consequence of the absence of ambipolar charge transport, the fabrication of CMOS-like circuits using organic semiconductors was only possible through complicated processes that involved separate steps to make the n-type and p-type transistors.

However, to fulfil the promise of plastic electronics, namely that of large-scale production of low-cost electronic devices, easily manufactured by spin coating or large-area printing, it is desirable to realise ambipolar transistor operation in a single layer of deposited material.

The scientists at Philips have now realised this breakthrough in two different ways. In one approach, they used a blend of p-type and n-type materials. The charge injection barrier problem was solved by mixing a material with a low energy barrier for electron injection, with a material with a low barrier for hole injection in combination with source and drain electrodes made of gold.

In the second approach, ambipolar transistor operation was achieved by using a single organic semiconductor with a low bandgap to reduce the energy barrier at the source and drain electrodes for both electrons and holes.

The Philips scientists also built the first working ambipolar inverter circuits that showed good noise margins and high gain values.