Conjugated Polymer Thin Film Transistors Constructed Using Ohmic and Schottky Source/Drain Contacts
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Conjugated Polymer Thin Film Transistors Constructed using Ohmic and Schottky Source/Drain Contacts Giles Lloyd and Bill Eccleston Department of Electrical Engineering and Electronics, University of Liverpool, Liverpool, L69 3BX, UK ABSTRACT Measurement and theory are presented for accumulation mode P3AT TFTs with Schottky and ohmic contacts. The polymer has been allowed to turn p type through exposure to air. Sample characteristics for gold contact TFTs are presented. Mobility values are 0.2 cm2/Vs and are comparable to the best reported. Using Schottky contacts as the source and drain electrodes can reduce off currents due to the existence of potential barriers at the contact. The presence, however, of the Schottky depletion region changes the mode of operation. A model is presented to describe the subthreshold and gradual channel regions. Devices constructed using titanium show good agreement with the model. Transconductance for these devices is reduced when compared to ohmic contact devices with calculated mobilities of 0.01 cm2/Vs. Much smaller channel lengths are, however, possible in this technology. Off currents are found to be completely independent of gate voltage indicating good control of the off current by the Schottky junctions. Little or no gate modulation has been observed, to date, for aluminium and chromium contact devices. INTRODUCTION High field-effect mobility thin film transistors (TFTs) have been reported using poly3alkylthiophenes (P3AT) as the semiconductor [1,2]. Mobility values reported are in the range 0.01-0.1 cm2/Vs, which is comparable to the value for hydrogenated amorphous silicon (a-Si:H). If polymer TFTs can yield similar or even better performance levels to a-Si:H TFTs the cost of production of large area electronics, such as flat panel displays could be significantly reduced. Recent developments in light emission and field emission from polymers, particularly field emission from P3AT [3] would enable complete flat panel displays to be produced from polymers. Development of the TFTs and circuitry is therefore very important. The one-dimensional nature of the polymer means that separation of generated electron-hole pairs is difficult. Therefore, inversion of the polymer is not possible and devices operate in accumulation mode. This is further complicated by the oxidation of the polymer when exposed to air thereby introducing high levels of p-type doping. Fabrication in ambient atmospheres and encapsulation of the polymer will reduce this unwanted dopant but can increase production costs. In the absence of a depletion region, high dopant densities produce large off-currents in accumulation mode devices thus reducing the on-off ratio of the TFT. This problem is particularly relevant in the case of ohmic source/drain contacts. The solution is to use blocking contacts for the source and drain to restrict the current flow in the off condition. Good Schottky contacts have been produced between various metals and poly(3hexylthiophene) (P3HT, a form of P3AT) [4] indicating the possibility
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