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Posted on June 27, 2007 by  & 

Ambipolar Semiconductors - Useful or Just Fascinating?

In 2003, the University of Groningen in the Netherlands pointed out that there is ample evidence that organic field-effect transistors have reached a stage where they can be industrialized, analogous to standard metal oxide semiconductor (MOS) transistors.
Monocrystalline silicon technology is largely based on complementary MOS (CMOS) structures that use both n–type and p–type transistor channels. This complementary technology has enabled the construction of digital circuits, which operate with a high robustness, low power dissipation and a good noise margin.
For the design of efficient organic integrated circuits, there is an urgent need for complementary technology, where both n–type and p–type transistor operation is realized in a single layer, while maintaining the attractiveness of easy solution processing. The University of Groningen demonstrated, by using solution–processed field–effect transistors, that hole transport and electron transport are both generic properties of organic semiconductors. This ambipolar transport is observed in polymers based on interpenetrating networks as well as in narrow bandgap organic semiconductors. The University combined the organic ambipolar transistors into functional CMOS–like inverters.
In 2004, the University announced a complementary–like inverter comprised of two identical ambipolar field–effect transistors based on the solution processable methanofullerene [6,6]–phenyl–C61–butyric acid methyl ester (PCBM). The transistors were capable of operating in both the p–and n–channel regimes depending upon the bias conditions. However, in the p–channel regime transistor operation was severely contact limited due to the presence of a large injection barrier for holes at the Au/PCBM interface. Despite this barrier, the inverter operated in both the first and third quadrant of the voltage output versus voltage input plot exhibiting a maximum gain in the order of 20. Since the inverter represents the basic building block of most logic circuits the researchers anticipated that other complementary–like circuits could be realized by this approach. However, in 2007, there is still nothing being sold.
Unfortunately, ambipolar semiconductors tend to have high leakage current, which is the opposite of the benefit of a CMOS circuit. Ambipolar conduction is more widespread than was initially realised. For instance, it has also been seen in PEDOT conductive polymer and carbon nanotubes but it still seems to be a long way from the marketplace. There is also interest in ambipolar conduction in light emitting organic transistors, so perhaps that will find a use.
At the International Conference on Organic Electronics in Eindhoven the Netherlands in June 2007 a lecture on "Ambipolar light emitting transistors of tetracene single crystals" was delivered by staff of the Institute of Materials Research at Tohoku University in Japan. Many other research centres continue to study potentially printed ambipolar semiconductors but, so far, they bring problems as well as opportunities.
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