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Electronic active circuits based on organic thin film transistors M. Berliocchi, M.Manenti, A. Bolognesi, A. Di Carlo, P. Lugli INFM and Dept. of Electronic Engineering University of Rome “Tor Vergata” Via del Politecnico, 1 - 00133 Rome, Italy Simulation Model Abstract We simulate organic devices using an industry standard device simulation tool, namely ISETCADTM, a package able to resolve the standard drift-diffusion equations coupled with Poisson’s equation in two and three dimensions. ISETCADTM is not capable to handle charge transport in organic material, so we have implemented a new model based on the following properties: Organic field effect transistors have been realized and characterized since they are excellent candidates to study the transport properties of the organic material used as active layer. Basic circuits as inverters, in addition, were realized and have been investigated by experimental and numerical analysis. Devices are fabricated using the same manufacture processes in a bottom contact configuration. First of all, source and drain multifinger contacts were prepared by Cr/Au vacuum evaporation using a standard photolitographic process over thermally grown silicon dioxide substrates. After the contact deposition, we sublimated (in vacuum atmosphere) a thin layer (about 30 nm at a deposition rate of 0.2 Å/sec) of organic molecules. Moreover a hybrid logic family was demonstrated. •Organic/Inorganic Band Alignment J n q n qDnn J p q p qD p p ( 0 r ) q( p n N D N A ) n 1 J n (G R ) t q p 1 J p (G R ) t q •Density of States (equal to molecule density) To characterize the devices we performed two dimensional drift diffusion simulations taking into account field dependent mobility, trap states (both interface and bulk trap states) and surface charge density at the interface beetween organic material and silicon dioxide. Numerical calculations for the quasi static output characteristics of single organic transistor show a satisfactory agreement with experimental results and allow us to estabilish a procedure of extraction of field dependent mobility. With this informations we can simulate more complex configurations such as inverters and ring oscillators. Here the most interesting point is given by the time dependence of the gate current that is influenced strongly by (de)trapping of charge carriers. Drift Diffusion Equation •Monte Carlo extracted Field dependent Mobility (see panel above) Pentacene Band Structure •Trap states at the interface between organic material and silicon oxide (see panel below). Vacuum Level We apply this model to the simulation of pentacene organic thin film transistor. In particular we simulate a device with a channel length of 12 m and oxide thickness of 250 nm. The same device was realized and measured from our group. 2.6 eV Source/Drain Metal LUMO 0.1 eV 2.4 eV HOMO Pentacene TFT:Simulation and Experimental Device Geometric Characteristics Multifinger structure for source-drain contacts (5nm Cr; 45nm Au) Active layer material Measured and simulated output characteristic Pentacene ALDRICH All measurements were carried out in vacuum atmosphere at room temperature with Agilent Semiconductor Analyzer 4155C. Measured transfer characteristic @ Vds=-30 V Square root of saturation corrent From square root of IDSS is possible to calculate pentacene mobility using the expression: Inverters Conclusions Vcc Vout Vin -18,5 Rc -4 -19,0 -19,5 -6 -20,0 -8 Vout (V) Vout Voltage (V) Vin -2 -20,5 -21,0 Vin = 10V -21,5 -10 -12 -14 -22,0 -16 -22,5 -23,0 Vin: 250 mHz; Vpp=20 V Vin = -10V -18 -60 0 5 10 15 20 -40 -20 0 Vin (V) 25 Time (sec) Vcc: -20V Rc: 30 MΩ Channel lenght: 12 μm Vcc: -20V Rc: 50 MΩ Channel lenght: 12 μm Voltage (V) Vin: 250 mHz; Vpp=20 V Vout Vin 10 5 Vin: DC 0 Vcc: -20V -5 Rc: 100MW -10 Channel lenght: 20mm -15 -20 0 10 20 Time (sec) 30 40 20 We simulate organic devices using an industry standard device simulation tool, namely ISETCADTM. This package is able to resolve the standard drift-diffusion equations coupled with Poisson’s equation in two and three dimensions. We have realized and characterized single organic transistors and basic circuits like inverters using commercial Pentacene. We settled the basis for the development of a complete family of circuits like three stages ring oscillators.