Reduced Contact Resistances in Organic Transistors with Secondary Gates on Source and Drain Electrodes
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Reduced Contact Resistances in Organic Transistors with Secondary Gates on Source and Drain Electrodes K. Nakayama1, T. Uemura1, M. Uno1,2, and J. Takeya1 1 Graduate School of Science, Osaka University, 1-1 Machikaneyama, Toyonaka 560-0043, Japan 2 TRI-Osaka, Ayumino, Izumi 594-1157, Japan ABSTRACT Secondary-gate electrodes are introduced in organic thin-film transistors to reduce carrier-injection barriers into air-stable organic semiconductors. The additional gate electrodes buried in the gate insulators under the source and drain electrodes form “carrier-rich regions” in the vicinity of source and drain electrodes with the application of sufficiently high local electric fields. Fabricating the structure with dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene, known for its excellent air-stability, it turned out that the contact resistance is drastically reduced especially when operated at low gate voltage in the main channel. The result demonstrates carrier injection with a minimized potential barrier realizing that from the same semiconductor material in the absence of peculiar interfacial trap levels at metal-to-semiconductor junctions. INTRODUCTION To realize the maximum device performance of organic field-effect transistors (OFETs), a significant challenge is to achieve efficient carrier injection from the electrodes. Such contact performance is the most seriously concerned for short-channel devices with the channel length typically less than sub-micrometers, though they are highly attractive because of their highfrequency response and capability of high-density integration. The problem of the contact barriers appear to be serious also for air-stable p-type organic semiconductors because stability to the oxidization is naively linked to difficulty to extract electrons causing slightly larger energy-level mismatch between the potentials of their highest occupied molecular orbitals (HOMOs) and the Fermi level of gold in most materials. Indeed, recently synthesized highperformance materials more air-stable than pentacene are reported to have HOMOs lower than that of pentacene by a few hundred mV [1,2]. Though contact resistance is reduced for such materials when relatively high gate voltage is applied and high-density carriers are accumulated in the channel, it is desired to develop methods of actively improving the contact problem in order to maximize the attractiveness of the high-mobility air-stable organic transistors with the capability of their low-power operation. As compared to common silicon metal-oxide-semiconductor field-effect transistors (SiMOSFET) where heavily doped carrier-rich region is incorporated next to the channel, all the reported organic field-effect transistors suffer from the carrier injection from different materials such as metals. Previously, we reported a device structure with “split gates” on the source and drain electrodes buried in the gate-insulating layers, so that the carrier density in the organic semiconductors in the vicinity of the source and drain electrodes can
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