Modeling of Polycrystalline Organic Thin-Film Transistor and Schottky Diode for the Design of Simple Functional Blocks
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Modeling of Polycrystalline Organic Thin-Film Transistor and Schottky Diode for the Design of Simple Functional Blocks M. Raja, D. Donaghy, R. Myers and W. Eccleston Organic Electronics Group, Department of Electrical Engineering & Electronics, University of Liverpool, Brownlow Hill, L69 3GJ. UK ABSTRACT We present analytical models for organic thin film transistors (OTFTs) and Schottky diodes based on polycrystalline semiconductors. The OTFT model is developed using a well-established approach previously developed for polysilicon, with slight modification for organics. The model predicts voltage and temperature dependencies on the various device and circuit parameters. A good agreement is obtained with experimental data of TIPS-based OTFTs. Essential parameters such as the characteristic temperature and Meyer-Neldel Energy extracted using the model with TIPS OTFTs data were in agreement with those obtained from Schottky diode measurements. INTRODUCTION The need for development of simple functional blocks through integration of several devices such as organic thin-film transistors (OTFTs), capacitors and rectifiers, built on a same flexible substrate is vastly growing. This essentially requires respective device models to be generated so as to facilitate their designs, and consequently perform accurate simulations prior to fabrication, thereby avoiding wasteful processing costs. The analytical model for polycrystalline OTFTs developed here adapts a well-established approach previously developed for polysilicon [1], with slight modification for organics. The conduction is assumed to occur through carrier hopping on microscopic scale between the more ordered grains, which are relatively free of traps. However it is also limited by the grain boundary which consists of disordered material, with a trap density assumed to be distributed exponentially in energy [2]. The distribution of such traps is attributed with a characteristic temperature Tc and also related to the so-called Meyer-Neldel Energy (MNE) [3], which is an essential modeling parameter. Such an exponential dependency is also revealed in the forward current of a Schottky diode, which is due to an increase in carrier concentration obtained from the product of the exponential dependency of the density of traps with occupancy. For the OTFT, the channel conduction, both in diffusion and drift regimes are considered, and consequently respective drain currents are found in terms of applied gate and drain voltages. The model is validated using experimental data of 6,13-bis(tri-isopropylsilylethynyl) pentacene OTFT. THEORY Polycrystalline semiconductors consist of ordered regions known as grain and disordered regions known as grain boundaries as illustrated in Figure 1. The conduction is largely dependent on grain sizes [4-5] however also limited by the grain boundaries [6-8]. The variation of the trap density in the boundaries is commonly expressed in term of a Gaussian distribution [9] or exponential [2] approximates at the Gaussian tail at low energies as given be
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