Charge transport in ambipolar pentacene thin film transistors
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Charge transport in ambipolar pentacene thin film transistors Ronak Rahimi and D. Korakakis Lane Department of Computer Science and Electrical Engineering, West Virginia University, Morgantown, WV 26506-6109 ABSTRACT Ambipolar organic transistors are technologically interesting because of their potential applications in light-emitting field-effect transistors [1] and complementary-metal-oxidesemiconductor (CMOS) devices by providing ease of design, low cost of fabrication, and flexibility [2]. Although common organic semiconductors show either n- or p-type charge transport characteristic, organic transistors with ambipolar characteristics have been reported recently. In this work, we show that ambipolar transport can be achieved within a single transistor channel using LiF gate dielectric in the transistors with pentacene active layer. This ambipolar behavior can be controlled by the applied source-drain and gate biases. It was found that at low source-drain biases multistep hopping is the dominant conduction mechanism, while in high voltage regimes I-V data fits in Fowler-Nordheim (F-N) tunneling model. From the slope of the F-N plots, the dependency between field enhancement factor and the transition point in conduction mechanism upon gate bias has been extracted. The transition points show more dependency on gate voltage for negative biases compared to the positive biases. While sweeping negative gate voltages from -5 to -20 V, the source-drain voltages change from about 27 to 17 V. On the other hand, for positive gate voltages from 5 to 20 V, the value of the transition point stays at approximately 36 V. In order to further understand the transport mechanisms, new structures with an interface layer between dielectric and active layer have been fabricated and characterized. As expected, a significant decrease in the amount of the source-drain current has been observed after introducing the interface layer. INTRODUCTION The mechanical flexibility, light weight and low-cost processing of organic materials are some of the main reasons for considering organic materials as valuable alternatives to the more commonly used inorganic materials for applications in electronic devices [2]. In addition to these factors, organic semiconductor materials exhibit ambipolar charge carrier transport. This interesting feature provides ease of design and ability to fabricate low-cost and flexible electronic devices such as light-emitting field-effect transistors as well as organic complementary-metal-oxide-semiconductor (CMOS) devices. Ambipolar conduction can also provide important information that helps with fundamental understanding of the transport processes in organic materials [3]. Ambipolar transport can be achieved by using blends or bilayers of n-type or p-type organic materials [2, 4]. This conduction behavior can also be achieved in devices with a single layer organic material by employing a high or a low work function metal, respectively, for hole or electron injection [5, 6]. Engineering semiconductor-dielectric interfac
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