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High performance transistors are indispensable building blocks for future flexible electronic circuits. In order to overcome performance limitations such as low switching speed and high driving voltage, it is necessary to shorten the channel length and use high mobility semiconductors. Vertical organic field-effect transistors (VOFET) may overcome these limitations since their channel length can be reduced to a few nanometer without being restricted by photolitographic resolution limits. Previously reported VOFETs consist of small molecules from vacuum deposition. However, these semiconductors do not show as high mobilities as achieved with new semiconductors deposited from solution and are less suited for low-cost applications because of cost considerations.
Here, we present our investigations on the electrical performance of solution-processed VOFETs with the polycrystalline organic semiconductor 6,13-Bis(triisopropylsilylethynyl)pentacene (TIPS-Pentacene) which shows mobilities up to 2 cm²/Vs. One major problem hindering the VOFET performance is contact resistance due to non-ohmic source/drain contacts as well as a non-linear channel resistance.
Thus, we discuss the VOFET performance with regard to both kinds of resistances and reveal that the gate voltage induced modulation of contact resistance governs the IV curve for these VOFETs. The findings are supported by device simulations helping to create an improved understanding of the electric field and charge carrier distribution. We also demonstrate our results on channel resistance and compare experimental results with theoretical simulations proving that substantial improvements regarding contact resistance need to be made in order to obtain higher cutoff frequencies in organic transistors.
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