Organic Thin-Film-Transistors with High on/off Ratios
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ABSTRACT In this article a new procedure to obtain alpha-hexathienylene (ct-6T) thin-film-transistors (TFTs) with on/off ratios in excess of one million is reported. This procedure involves subjecting the TFTs to rapid thermal annealing. Previously, high on/off ratios have been achieved with improved device design and better chemical synthesis of Ct-6T oligomers. High on/off ratios, along with a switching time of - 10 pis, render ct-6T TFTs potential candidates as switching devices in active matrix displays. The experimental current-voltage (I-V) characteristics of cc6T TFTs with channel length L = 4 urm are also presented and a measured field effect mobility of 0.02 cm 2/V-s is extracted from these characteristics using an analytical model which we have developed for short-channel ct-6T TFTs.
INTRODUCTION The use of organic / polymeric materials in active devices such as light emitting diodes and transistors has been demonstrated by several groups [1, 2]. We have recently reported alphahexathienylene (cc-6T) thin-film-transistors (TFTs) with mobilities of 0.01 - 0.08 cm 2/V-s and switching times of about 10 pts [3]. The compatibility of thiophene oligomers with flexible plastic substrates is another important reason why such materials may be very useful in some types of display applications [1]. In addition, the cx-6T film deposition process is simple (relative to Sibased technologies) and can be performed at room temperature. c-6T films, deposited by sublimation at room temperature, show grain-like morphologies. These grains are typically 0.1 .tm and have a crystalline structure [4] with the oligomer chains preferentially oriented nearly normal to the substrate [4]. The residual conductivity is p-type (the typical concentration of holes is about 1017 cm"3) and arises from adventitious dopants [5] which might be either impurities or lattice defects. TFTs must have an on/off ratio of at least 106 to be considered for use in active matrix displays. This figure of merit in cc-6T TFTs depends on a number of factors, but careful attention to device design [3] and to the synthesis of the ot-6T oligomers [5] can result in TFTs with on/off ratios higher than 106. These large on/off ratios are a result of very low levels of residual doping in cx-6T films. In this paper we demonstrate another method to realize oc-6T TFTs with high on/off ratios when the starting oligomers result in p-type films. This method involves subjecting TFTs with as-deposited ot-6T to rapid thermal annealing (RTA). Since TFTs with channel length shorter than L=10 gtm are typically employed in active matrix displays, we also report the experimental I-V characteristics for both L = 4 l.m and L = 12 itm a-6T TFTs. These characteristics have also been modeled using a procedure outlined in Ref, 6.
695 Mat. Res. Soc. Symp. Proc. Vol. 377 ©1995 Materials Research Society
EXPERIMENTAL The cx-6T TFTs were fabricated on thermally oxidized Si substrates. The thickness of the oxide is d = 300 nm (capacitance per unit area C, = 10 nF/cm 2) and the resistivity
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