Organic Field-Effect Transistors, Inverters, and Logic Circuits on Gate Electrets
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Organic Field-Effect Transistors, Inverters, and Logic Circuits on Gate Electrets Cheng Huang,1 James E. West,2 and Howard E. Katz1 Department of Materials Science and Engineering, 2 Department of Electrical and Computer Engineering, Whiting School of Engineering, Johns Hopkins University, Baltimore, MD 21218
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ABSTRACT By incorporating dielectrics with stored electric fields and organic semiconductors, new organic electronic components such as circuits with controlling voltages “prestored” for transistor tuning can be developed. We have successfully used excellent electret materials including charged and surface-treated silicon dioxide (SiO2) and silsesquioxane (SSQ) polymers as the dielectric layer in organic field-effect transistors (OFETs). Charge injection and quasipermanent charge storage induce threshold voltage shifts and current modulation, which results from the built-in electric fields in the conduction channels. Static and dynamic characteristics of organic thin-film transistors (OTFTs) such as charging conditions and voltage/current retention were evaluated. In addition, self-assembled monolayers (SAMs) of dipolar molecules have been utilized in the dielectric layer, with different mechanisms but similar effects compared to charged dielectrics. We also present new OFET unipolar inverters, comprised of only two simple OTFTs with enhancement-mode driver and depletion-mode load to implement full-swing organic logic circuits for process simplification of electronic components in organic electronics. INTRODUCTION Recent advances in new gate dielectric materials for incorporation into organic thin-film transistors (OTFTs) represent new opportunities for organic electronics as well as materials and device processing and physics of gate dielectric-organic semiconductor (OSC) interfaces [1,2]. Functional gate dielectric materials such as charged dielectrics (electrets), dipole dielectrics and ferroelectrics not only provide the normal gate dielectric materials for OFETs but also extend new applications of functional organic field-effect transistors for the simplification of processes and integration of logic circuits, memory circuits, and sensors with OTFT-compatible functional dielectrics. First attempts to use charge storage and polarizable gate insulators in combination with organic semiconductor for organic field-effect transistors were reported by Katz et al. at Bell Laboratories [3]. The intended applications for these polarized OSC devices were nonvolatile information storage and circuit tuning [4]. A schematic of the device, with two possible modes of charge storage (separated static charges and oriented dipoles), is shown in Figure 1. Most recently, the application range of electrets extended further, by combining them with organic semiconductors in functional organic field-effect transistors such as non-volatile organic memories and sensors [4,5]. Several groups have proposed charge-storing dielectrics that become polarized through varied mechanisms, and have offered various explanatio
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