Organic Field-Effect Transistors

Transistors are considered a fundamental “building block” of modern electronic devices, either amplifying signals or operating as on-off switches.

There are many different types of transistors. Most organic transistors are organic field-effect transistors (OFETs). OFETs have several unique properties not shared by silicon transistors, most notably their flexibility.

Because OFETs can be manufactured at or near room temperature, they enable the manufacture of integrated circuits on plastic or other flexible substrates that would otherwise not withstand the high-temperature conditions of silicon-based device manufacture.

OFETs are also highly sensitive to specific biological and chemical agents, making them excellent candidates for biomedical sensors and other devices that interface with biological systems. With the synthesis of new organic materials, chemists have improved charge-carrier mobilities for small-molecule OFETs from <1cm2/Vs in 2000 to 8-11cm2/Vs today. Initially, the improved mobilities were obtained under very clean conditions in ultra-high vacuum chambers. However, recent results suggest that high-performance OFETS can be fabricated using simple and relatively inexpensive techniques, such as solution processing. By 2020, with the synthesis of even more advanced materials, mobilities could increase to 100cm2/Vs. As with small -molecule OFETs, polymer OFETs have also increased in performance, with typical mobilities increasing from about 0.01cm2/Vs in 2000 to greater than 1.0-3.0cm2/Vs in 2010.

Several of the leading display companies have expressed their intention to introduce flexible OLED displays in the near future, which will be lighter and more robust than glass-based displays and will allow novel display applications with new form factors. In principle, OFET technology could be an ideal backplane for this application because of the close materials compatibility between OLEDs and OFETs and their excellent mechanical properties, which might ultimately even allow foldable displays that would require tight bending on flexible substrates to the very small radius of curvature on the order of 100 μm. For this, the mechanical properties of OFETs are potentially superior to silicon or oxide based TFTs. The integration of OFETs with OLEDs was shown early, and several groups have realized prototype OFET-driven OLED displays. However, a commonly held view is that the performance, uniformity and operational stability achievable with OFETs is insufficient to realize a backplane with the same display performance and lifetime as a display based on an oxide or poly-Si TFTs.

Part of the challenge for the new ERA Chair will be to examine this view with respect to recent significant improvements in OFET materials and device performance. Another challenge the ERA Chair should tackle is high threshold voltage. For the practical application of OFET’s and to reduce energy consumption, better materials need to be found. Low power consuming electronic devices like RFIDs and electrophoretic displays are primary target applications for OFETs, which require low threshold voltage (Vt) for these devices. Low turn-on voltage of the OFETs has been achieved by using high-k organic/inorganic dielectrics or by application of very thin gate dielectric layers.

However, application of ultra-thin gate dielectric allows severe leakage currents through the device, which eventually impedes the device performance. The next viable means of obtaining high capacitance density and thus low voltage operation in OFET devices will involve using high-k organic, inorganic, and hybrid gate dielectrics. However, the poor semiconductor-dielectric interface and high electrical leakage decrease the transistor performance.