Modeling the Organic Thin Film Transistors
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Modeling the Organic Thin Film Transistors Viorel Olariu , Robert Rotzoll , Siddharth Mohapatra , Robert Wenz, Michelle Grigas, and Klaus Dimmler Organic ID, Inc., Colorado Springs, CO ABSTRACT Organic thin-film transistors (OTFTs) appear to have become strong contenders to silicon based MOSFET devices whenever low-cost and relatively low performance circuits are required in applications such as radio frequency identification (RFID) for large volume supply chains. In order to develop circuits based on OTFTs, circuit designers require circuit models that predict the operation of OTFT with a reasonable accuracy. Although, generally, OTFT operation is similar to ordinary silicon MOSFET devices, there are several characteristics that clearly differentiate them. One important difference between the operation of the OTFT and the silicon MOSFET (that is a direct consequence of the physical implementation of OTFT) is that the organic transistor is normally operated in the accumulation mode, while the silicon transistor regularly operates in the inversion mode. Due to the molecular nature of the semiconductor, the carrier mobility is orders of magnitude lower than for the silicon MOSFET. Variable carrier mobility law, low on/off ratio, and the Schottky barrier at the interface between the source/drain metal contact and the organic semiconductor are among other important effects that had to be considered for developing of an accurate circuit model of the organic transistor. The developed model has been used to simulate DC characteristics and also simple circuits such as logic gates, ring oscillators, rectifiers, etc. This paper presents the developed model as well as a comparison between the simulated data and the experimental data. The experimental circuits were fabricated on flexible plastic substrates and employed a solution-cast dielectric. Pentacene was the semiconductor of choice with carrier mobility in the range of 0.1 – 1.5 cm2/V⋅s. INTRODUCTION There are two main categories of organic semiconductors that can be used to build organic thin-film transistors: small-molecules and polymers. Polymers can generally be applied at room temperature and atmospheric pressure, opening the possibility of “printing” electronic circuits utilizing ink jets or printing press methods. Small-molecule transistors have higher mobility, but generally require non-ambient temperature and pressure deposition environments. These films are generally evaporated on a substrate through a shadow mask. Either way, the cost to process these organic materials is lower than silicon processing even at this stage of development. Organic thin-film transistors, fig. 1, contain either a molecular or polymeric channel connecting the source and drain contacts. The gate is first deposited onto an insulating substrate such as glass or plastic, followed by deposition of the gate insulator, which can be either an organic or inorganic dielectric film. Source and drain electrodes are deposited onto the gate dielectric, followed by the deposition of the thin
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