High Performance Organic Field-Effect Transistors and Integrated Inverters
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High Performance Organic Field-Effect Transistors and Integrated Inverters A. Ullmann, J. Ficker, W. Fix, H. Rost, W. Clemens, SIEMENS AG, Erlangen, Germany I. McCulloch, M. Giles, Merck NB-SC, UK, Southampton, UK Abstract Integrated plastic circuits (IPCs) will become an integral component of future low cost electronics. For low cost processes IPCs have to be made of all-polymer Transistors. We present our recent results on fabrication of Organic Field-Effect Transistors (OFETs) and integrated inverters. Top-gate transistors were fabricated using polymer semiconductors and insulators. The source-drain structures were defined by standard lithography of Au on a flexible plastic film, and on top of these electrodes, poly(3alkylthiophene) (P3AT) as semiconductor, and poly(4-hydroxystyrene) (PHS) as insulator were homogeneously deposited by spin-coating. The gate electrodes consist of metal contacts. With this simple set-up, the transistors exhibit excellent electric performance with a high source-drain current at source - drain and gate voltages below 30V. The characteristics show very good saturation behaviour for low biases and are comparable to results published for precursor pentacene. With this setup we obtain a mobility of 0.2cm2/Vs for P3AT. Furthermore, we discuss organic integrated inverters exhibiting logic capability. All devices show shelf-lives of several months without encapsulation. Introduction During the past few years the performance of Organic Field-Effect Transistors has continuously increased [1], [2], [3]. OFETs constitute a possible alternative to conventional silicon-based transistors for future applications in devices such as antitheft stickers, ID-tags and active matrix displays. An important factor for industrial mass-production is the possibility of low-cost fabrication. This includes low cost processing techniques based on solution processing, such as spin-coating and standard lithography or printing techniques. However, one of the most critical steps in producing these devices is the deposition of the semiconductor under ambient conditions. Although the effect on the semiconductor is still not clearly understood when processed under non-inert atmosphere, we present a way of fabricating high performance polymer transistors without encapsulation or any special processing conditions. Another important condition for future applications is that OFETs show small variation in their performance between different circuits. In addition, a fast switching behaviour is necessary, where the mobility ยต is the most important parameter and a high On-Off ratio is needed. Field-Effect Transistors are often called Organic-FETs, even if only the semiconducting layer consists of organic material. Only a small number of groups are working on OFETs which have more than one organic layer, e.g. Philips and Lucent [1], [4]. For low-cost mass-production of Integrated Plastic Circuits the need for an all plastic OFET is very important. Two different set-ups were used to do this: a top-gate set-up and a bottom-gate set
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