Very High On/Off Ratio in Vertical-Type Metal-Base Organic Transistors
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0965-S10-05
Very High On/Off Ratio in Vertical-Type Metal-Base Organic Transistors Ken-ichi Nakayama and Masaaki Yokoyama Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka, 565, Japan
ABSTRACT The on/off ratio of the vertical-type metal-base organic transistors was drastically improved by heat treatment in air. The heat treatment after deposition of the collector layer and base electrode reduced the leakage current between the base and collector, resulting in remarkable suppression of the off current. As a result, in addition to the advantage of low voltage and high current operation based on the vertical structure, very high on/off ratio exceeding 105 was achieved. INTRODUCTION Organic field effect transistors (OFETs) are the mainstream of organic transistors and being studied extensively. However, the lateral structure of OFETs (planar configuration of source and drain electrodes) requires long channel length about several microns or tens microns, which is not suitable for low voltage and high current operation. On the other hand, the verticaltype organic transistor is a promising device structure that can make the channel length much shorter, leading to low voltage operation and high frequency response. So far, several verticaltype devices have been proposed. For example, polymer grid triodes using a self-assembled polyaniline network as a control grid [1], organic static induction transistors based on a modulation of a depletion layer around the striped gate electrode [2], and unique architecture vertical transistors having a gate electrode behind the common source electrode [3]. The simple structure of vertical transistor using Si substrate as a collector was also proposed [4]. Recently, we have reported a high performance vertical transistor having a simple layered structure composed of organic/metal/organic layers [5,6]. This device was named a metal-base organic transistor (MBOT), because the inserted middle electrode behaves like a base layer in the bipolar transistors. So far, MBOTs achieved very high current density modulation exceeding 100 mA/cm2 with low voltage operation of several volts. Generally, such vertical-type organic transistors usually suffer from poor on/off ratio, because the short channel length tend to increase off current. The MBOT reported also showed low on/off ratio around 100. In this study, we succeeded in drastic improvement of the on/off ratio using heat treatment of the device in air. EXPERIMENT Figure 1 shows the device structure and measurement system. The device was fabricated by vacuum deposition. The first organic layer (collector layer) of perylenetetracarboxylic derivatives (Me-PTC) was deposited on an ITO glass substrate of a collector electrode. The base electrode of aluminum was deposited with a thickness of 20 nm. The second organic layer
(emitter layer) of C60 and the emitter electrode of Ag were deposited. The heating treatment was performed after deposition of the collector semiconductor and the base electrode. The device was kept at
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