Displacement Current and Transfer Curve Simultaneous Measurement in Bottom-Contact Organic Thin-film Transistors Seiichi Suzuki a , Taiyou Suzuki, Adhitya Bhaswara, and Yutaka Majima b Department of Physical Electronics, Tokyo Institute of Technology 2-12-1, Ookayama, Meguro-ku, Tokyo 152-8552, Japan Phone: +81-3-5734-2673 E-mail: a suzuki@nanoele.pe.titech.ac.jp b majima.y.aa@m.titech.ac.jp 1. Introduction Organic thin-film transistors (OTFTs) have a great potential for applications requiring large area, structural flexibility, and low-cost fabrication processes.[1] Many trials have been proposed and demonstrated such as flexible displays, electronic paper, and radio frequency identification tags. One of the issues for the practical use of OTFT, it is essential to understand the OTFT device operation mechanisms, including both charge injection properties and carrier transport properties. Displacement current measurement (DCM) is a direct method of determining carrier injection properties in organic materials.[2-5] We have developed a simultaneous measurement method for displacement current and transfer curve in top-contact OTFTs, which enable us to understand carrier injection properties under OTFT operation.[6,7] In this paper, we demonstrate charge sheet formation dynamics at the channel during the device operation using the simultaneous measurement method for displacement current and transfer curve in bottom-contact OTFTs. 2. Experiments The basic set-up of the simultaneous measurement method for displacement current and transfer curve in bottom-contact OTFT is simple and is illustrated in Fig. 1. The gate electrode is connected to the wave generator, which applies triangular wave of V GS . A constant source– drain bias voltage (V DS ) is applied, while source current (I S ) and drain current (I D ) are measured. When it is assumed that displacement current at the source (I disS ) and drain (I disD ) electrodes have the same value, then displacement current (I dis ) is expressed as  ) 1 ( , D S disD disS dis I I I I dt t dQ I      while channel current (I DS ) is given by ) 2 ( . 2 D S DS I I I    We use bottom-contact pentacene OTFTs with a dielectric double layer of SiO 2 and polyimide, and gold electrodes in the experiments. [6,7] Polyimide film was formed onto SiO 2 /n+Si substrate using spin-coating method. Au source and drain electrodes were thermally evaporated through a shadow mask, followed by a physical vapor deposition of pentacene thin-film with a shadow mask. The channel length and width were 400 and 1000 m, respectively. The long channel length allows easier observation of changes in displacement current at the channel. Simultaneous measurements for displacement current and transfer curve took place inside a nitrogen atmosphere at 300 K. I S and I D were measured using a digitizing storage oscilloscope (Hioki, 8855) while applying a constant drain bias voltage (V DS ) and a triangular-wave gate voltage (V GS ) in the bottom-contact pentacene OTFTs. Transfer characteristics and output characteristics were measured with semiconductor parameter analyzer (Agilent 4156C) for comparison. 3. Results and Discussions Figure 2 shows the typical experimental results of (a) square-root of channel current (I DS 1/2 ) – V GS and (b) displacement current (I dis ) – V GS characteristics at the V DS of 0 and -20 V. I dis and I DS were calculated from the simultaneously measured I S and I D of the first cycle of V GS using eqs. (1) and (2). From Fig. 2 (a), hysteresis was not observed between backward and forward bias sweep, and the field-effect mobility () and the threshold voltage (V th ) Fig. 1 Schematic diagram of simultaneous measurement method of I dis and I DS in bottom-contact OTFTs. I S and I D are simultaneously measured under the applications of a triangular-wave V GS and a constant V DS . -855- Extended Abstracts of the 2009 International Conference on Solid State Devices and Materials, Sendai, 2009, pp855-856 F-5-4