Transient Charge Carrier Transport Effects in Organic Field Effect Transistor Channels
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1270-II01-08
Transient charge carrier transport effects in organic field effect transistor channels Hsiu-Chuang Chang1, P. Paul Ruden1, Yan Liang2, and C. Daniel Frisbie2 1 Department of Electrical and Computer Engineering, University of Minnesota, Minneapolis, MN 55455, U.S.A. 2 Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455, U.S.A. ABSTRACT We present device simulations exploring the effects of traps during transient processes in the conducting channel of organic semiconductor field effect transistors (OFETs). The device structure considered resembles a typical organic thin-film transistor with one of the channel contacts removed. However, the channel length is much longer than in typical OFETs in order to increase the transit time. By measuring the displacement current in these long-channel capacitors, transient effects in the carrier transport in organic semiconductors may be studied. When carriers are injected into the device, a conducting channel is established while traps, which are initially empty, are being populated. The filling of the traps then modifies the transport characteristics of the injected charge carriers. In contrast, DC experiments as they are typically performed to characterize the transport properties of organic semiconductor channels investigate steady states with traps partially filled. Numerical and approximate analytical models for the formation of the conducting channel and the resulting displacement current are discussed. We show that displacement current measurements on OFET structures provide unique opportunities for the study of trap dynamics. INTRODUCTION Organic semiconductors are among the most promising materials for novel electronics. Many electronic devices have been fabricated from conjugated organic hydrocarbons, and some are already commercialized [1]. For carrier transport in organic materials, the trapping/detrapping of mobile charge carriers is an important issue that can even play the dominant role in device performance. Frequently, organic field effect transistors (OFETs) are used to characterize the electrical properties of organic semiconductors (OSCs) [2,3]. Usually one measures the DC current through the conducting channel as a function of the applied gate-tosource and drain-to-source voltages and obtains a steady state charge carrier field effect mobility. The steady state mobility is measured with traps partially filled and the dynamic effects of traps do not manifest themselves directly in the experiments. Transient measurements of the displacement current [4,5,6] can provide information about the dynamics of carrier trapping phenomena. An effective mobility that characterizes transient experiments is measured when trapping effects are in process and the fractional trap filling changes. In the displacement current measurements, we use a two-terminal device structure resembling a simplified field effect transistor with one of the channel contacts removed, as shown in Figs. 1(a) and 1(b). Under suitable bi
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