Device Model for Light-Emitting Field-Effect Transistors with Organic Semiconductor Channel
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1003-O04-08
Device Model for Light-Emitting Field-Effect Transistors with Organic Semiconductor Channel P. Paul Ruden1 and Darryl L. Smith2 1 University of Minnesota, Minneapolis, MN, 55455 2 Los Alamos National Laboratory, Los Alamos, NM, 87545 ABSTRACT We present a device model for light-emitting, ambipolar, organic field-effect transistors based on the gradual channel approximation. The model results are in very good agreement with recent experimental data. Trapping of injected carriers in localized states in the channel region is shown to be an important mechanism that can strongly affect the transfer characteristics and the light emission of these devices. INTRODUCTION Organic semiconductors, such as conjugated oligomers and polymers, have become materials for a wide range of electronic and optoelectronic devices. Organic light emitting diodes are already inserted into commercial systems and organic field-effect transistors (FETs) are in active development. Mobile charge carriers of either polarity may be injected from suitable contacts. However, the mobilities of these charge carriers are very low in comparison to inorganic semiconductors, and the transport mechanism is best described as a (polaron) hopping process. Several research groups have demonstrated light emission from organic field effect transistors.[1, 2, 3] Recently, clear evidence for ambipolar transport in the channel of organic FETs and light emission with voltage controlled position were reported by two groups.[4, 5] In both cases the (active) organic material layers are PPV derivatives (ëSuper Yellowí and OC1C10PPV in references 4 and 5, respectively). These light emitting field-effect transistors (LEFETs) operate with one electron injecting (e-source) and one hole injecting (h-source) contact. The electron injecting contact consists of a low work-function metal (Ca) and the hole injecting contact consists of a high work-function metal (Ag[4], or Au[5]). The e-source contact may also act as the drain for holes (h-drain) and the h-source contact may serve as the drain for electrons (e-drain). The channel is separated from the gate electrode by an insulating layer. With charge carrier trapping, particularly electron trapping, carefully minimized through suitable fabrication steps, light emission from a narrow zone in the channel between the source and drain contacts is observed.[4, 5] The location of this light-emitting zone is controlled by the two voltages applied between the three transistor terminals. In this work we present a charge control model for an ambipolar organic LEFET. A schematic diagram of the device structure is shown in Figure 1.
DESCRIPTION OF THE MODEL We assume that the organic semiconductor is undoped, and that the only charges present are those associated with injected electrons and holes. Not all of these charge carriers are necessarily mobile because organic semiconductor devices typically contain localized states that may trap electrons and holes. The field effect transistors considered are rather large in their late
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