Dielectrics for Organic Transistors with Low Threshold Voltage
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Dielectrics for Organic Transistors with Low Threshold Voltage Jochen Brill, Silke Goettling and Eduardo Margallo Balbás Chair of Display Technology, University of Stuttgart, Allmandring 3b, D-70569 Stuttgart, Germany ABSTRACT Organic thin film transistors for display applications are investigated. Different dielectric materials – inorganic and organic – have been studied with respect to their electrical performance. It was found that anodic oxidation is an excellent process to achieve smooth high-k dielectrics with high breakthrough field strength. With the proposed electrolyte electrical properties were further improved. The alignment of pentacene on different insulators as well as transistors characteristics is presented. INTRODUCTION In comparison to the well-established technologies organic thin film transistors (OTFTs) offer appealing advantages: low process temperatures and low-cost devices will result in novel devices like flexible displays and RFID-tags. For display application the TFTs have to match the following requirements The carrier mobility µ has to be sufficiently large to allow fast pixel charging; for comparison the carrier mobility of amorphous silicon currently used in TFT-LCDs is in the range of ∼ 1cm2/Vs. A large number of rows demands an on/off-ratio of Ion/Io f f = 106 and finally the off-current has to be very small to avoid discharging of the pixel. In the practical application it is important to achieve small threshold voltages to allow for low voltage applications with low power consumption. From the MOSFET equation for the saturation current accessible parameters can be seen: w (1) Ion = IDsat = µCGS (VG −Vth )2 2l To achieve a large current at a low gate voltage VGS the mobility µ and the gate capacity CGS should be large and the threshold voltage Vth small enough. The gate capacity CGS = ε0 εr/d is determined by the thickness of the dielectric layer d and its dielectric constant εr . As these parameters can be influenced by the choice of material inorganic and organic insulators have been examined on their usability in organic TFTs. For our system we have chosen sublimed pentacene as organic semiconductor due to its high mobility recently reported [1] and availability. This work was done on glass substrates with the perspective of transferring the process to plastic substrates for future applications in flexible displays. EXPERIMENTAL DETAILS In table I the examined materials are listed with their deposition technique, dielectric constant and realized film thickness. Silicon oxide and nitride were deposited via classical plasma enhanced chemical vapor deposition at temperatures of 300◦ C resp. 280◦ C. These dielectrics were optimized for the application in polysilicon TFTs [3]. For the usage with plastic substrates the deposition temperature has to be lowered and the excellent electrical properties will degrade. Spin coating is a totally vacuum free deposition technique, the only drawbacks being the duration of the thermal annealing and also its possibly elevated curing temper
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