Three-Dimensional Organic Field-Effect Transistors: Charge Accumulation in their Vertical Semiconductor Channels
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Three-Dimensional Organic Field-Effect Transistors: Charge Accumulation in their Vertical Semiconductor Channels M. Uno1, 2, I. Doi3, K. Takimiya3, and J. Takeya1 1 Graduate School of Science, Osaka University, Toyonaka 560-0043, Japan. 2 Technology Research Institute of Osaka Pref., Izumi 594-1157, Japan. 3 Graduate School of Applied Chemistry, Hiroshima University 739-8527, Japan ABSTRACT Three-dimensional organic field-effect transistors are developed with multiple vertical channels of organic semiconductors to gain high output current and high on-off ratio. Highmobility and air-stable dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene thin films deposited on horizontally elongated vertical sidewalls have realized unprecedented high output current per area of 2.6 A/cm2 with the application of drain voltage -10 V and gate voltage -20 V. The on-off ratio is as high as 2.7x106. Carrier mobility of the organic semiconductor deposited on the vertical sidewalls is typically 0.30 cm2/Vs. The structure is built also on plastic substrates, where still considerable current modulation is preserved with high output current per area of 70 mA/cm2 and with high on-off ratio of 8.7x106. The performance exceeds practical requirements for applications in driving organic light-emitting diodes in active-matrix displays. The technique of gating with electric double layers of ionic liquid is also introduced to the threedimensional transistor structure. INTRODUCTION Organic electronics have attracted much attention as “post-silicon electronics” due to their mechanical flexibility, low cost, energy-saving and environmental-friendly fabrication processes. Organic field-effect transistors (OFETs) are the key devices in which further development is necessary for such applications as full-flexible displays, incorporated in their matrix-controlling elements. So far, though much effort have been devoted to material development, even the best value of the carrier mobility µ of organic semiconductor films remain in the order of 1 cm2/Vs [1-4], except for single-crystal devices [5-7]. Therefore, their performance in current amplification per pixel is not necessarily sufficient for practical use. Though decreasing the channel length L and increasing the width W can be another approach, this effect is limited if one cares the condition that the channels and the electrodes are in the same plane as the conventional OFETs. In this paper, we propose a three-dimensional organic field-effect transistor (3D-OFET) [8,9] to accumulate charge in its vertical semiconductor channels, so that space availability for the field-induced carriers is essentially enlarged. EXPERIMENT Figures 1(a) and 1(b) illustrates schematic cross-sectional views of conventional planartype and 3D-type OFETs, respectively. In 3D-OFET, the same metal-insulator-semiconductor structure as the planar OFET is included, so that output current is modulated by the gate electric field with the identical mechanism but in the different direction. In Fig. 1(b), gate electric field is
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