Ultra-low Voltage, Self-aligned OTFTs for Frequency Applications
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Ultra-low Voltage, Self-aligned OTFTs for Frequency Applications Stefano Lai1, Piero Cosseddu1,2, Gian Carlo Gazzadi2, Giovanni Martines1, Annalisa Bonfiglio1,2, and Massimo Barbaro1 1 Department of Electrical and Electronic Engineering, University of Cagliari, Piazza d’Armi, 09123, Cagliari (Italy); 2 CNR – Institute of Nanoscience, S3 Centre, Via Campi 213A, 41100, Modena, Italy, ABSTRACT A novel structure for Organic Thin-Film Transistor (OTFT) is here presented. The devices are fabricated using a one-mask, photolithographic self-alignment technique which can be performed with standard photoresists and without further chemical treatments. This technique, combined with a novel technology for the realization of low voltage OTFTs, allows a dramatic reduction of the parasitic capacitances thus leading to a remarkable cut-off frequency. In this paper, the main electrical parameters of low voltage, self-aligned devices are reported, and a complete frequency characterization of the devices is given. These characteristics make the reported approach suitable for the development of basic circuitries for frequency applications. INTRODUCTION After the pioneering works about conductivity in polymers1, a lot of confidence has been placed in organic materials as an alternative to inorganic semiconductors for the development of electronic devices. However, in the sequent decades several drawbacks have emerged; in particular, low mobilities in organic semiconductor films avoided the employment of Organic Thin Film Transistors (OTFTs) in electronic applications which require high switching properties and high operating frequencies, as digital and analog circuitries. Only in the last years impressive improvements in materials and technologies have brought to the development of OTFTs with high performances, in particular as regards power consumption2, switching properties3 and most of all operating frequencies. As the cutoff frequency of an OTFT can be written as gm fT = (1) 2π (CG + C P ) were gm is the transconductance, CG is the channel capacitance and CP the parasitic capacitances, high operating frequencies can be obtained by employing high-mobility organic semiconductors and by optimizing the geometry of the transistor, i.e. channel length and overlap between source/drain and gate contacts. Even if several examples of high-mobility materials have been proposed4, layout optimization represents the most interesting way for producing OTFTs which can operate at high frequency independently from the mobility of the organic semiconductor. In principle, the combination of short channel and low parasitic capacitances represents the best way to improve the operating frequencies. Palfinger et al.5 used nanoimprint lithography (NIL) to fabricate narrow gate OTFTs, on top of which photolithographic self-alignment is performed. In self-aligning, the gate is used to define perfectly aligned source and drain contacts; in this way, in principle, there is no overlap between source/drain and gate. A maximum cutoff frequency of 40 kHz was measur
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