The Direct-Write Polymer Field-Effect Transistor
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The Direct-Write Polymer Field-Effect Transistor M. I. Sluch, R. M. Taylor, and K. H. Church Sciperio, Inc., 5202-2 North Richmond Hill Road, Stillwater, OK 74075, U.S.A. ABSTRACT Field-effect transistors (FET’s) have been directly written onto glass and Si substrates. The FET’s use the regioregular conjugated polymer poly(hexylthiophene) as the active semiconductor, with a formed polyurethane insulator layer and drain, source, and gate electrodes written in Ag. The FET’s were fabricated solely by direct writing techniques. The low processing temperatures, the elimination of photolithography, and the flexibility of the direct writing process make this technology especially attractive for low-frequency applications. INTRODUCTION Conjugated polymers are currently the subject of considerable interest because of their applicability to electronic devices, including field-effect transistors (FET’s), light-emitting diodes, and integrated optoelectronic devices [1–5]. Mostly conjugated polymer semiconductors, derivatives of polythiophene [1–7], and small aromatic molecules, derivatives of pentacene [8–18], have been used in the fabrication of organic FET’s. Thin-film transistor structures are used to build up polymer transistors. FET’s require three electrical connections, source, drain, and gate. The source and drain contacts are Ohmic contacts directly connected to the semiconductor channel. The gate is connected to the insulator. No depletion region isolates the source and drain from the semiconductor. Low off current is only guaranteed by the low conductivity (σ) of the polymer semiconductor. Polymer FET’s operate in accumulation mode in contrast to the inversion mode of inorganic metal–oxide–semiconductor FET’s. The performance of organic transistors has improved over the past few years. Several research teams [1–5, 7–18] have reported that the channel mobility (µc) of derivatives of polythiophene FET’s ranges from 0.02–0.05 cm2·V−1·s−1 and that of pentacene ranges from 0.6– 0.8 cm2·V−1·s−1, values similar to the mobility (µ) of amorphous silicon. However, from the points of view of stability, applied voltage (V), and frequency (f) response, the best performances of organic FET’s still fall short of those of conventional Si-based FET’s. The remaining advantage of polymer FET’s is their low production cost. Thus, it becomes extremely important to remove such expensive processes as photolithography, high processing temperatures, and vacuum deposition techniques from polymer FET fabrication. Inkjet printing The source, drain, and gate electrodes of polymer FET’s are conventionally formed by inkjet deposition of a solution of a conductive conjugated polymer, poly(ethylenedioxythiophene) (PEDOT), onto a glass substrate patterned in advance [2]. The insulator layer, typically a poly(vinylphenol) film of thickness d = 400–500 nm, and the semiconductor layer, typically a film of polythiophene or poly(9,9-dioctylfluorene-co-bithiophene) derivatives of d = 40–300 nm, are spin-coated onto the substrate. The final step co
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