Published in IET Circuits, Devices & Systems Received on 1st November 2010 Revised on 11th August 2011 doi: 10.1049/iet-cds.2010.0372 Special Section on Thin-Film-Transistor Modelling for Circuit Simulation ISSN 1751-858X Organic thin-film transistor bias-dependent capacitance compact model in accumulation regime A. Castro-Carranza 1 M. Estrada 2 J.C. Nolasco 1 A. Cerdeira 2 L.F. Marsal 1 B. In ˜ı ´guez 1 J. Pallare `s 1 1 Departament d’Enginyeria Electro ` nica, Ele ` ctrica i Automa ` tica, Universitat Rovira i Virgili, Av. Paı ¨sos Catalans 26, Tarragona 43007, Spain 2 Departamento de Ingenierı´a Ele ´ ctrica, CINVESTAV, Seccio ´ n de Electro ´ nica del Estado So ´ lido (SEES), Av. IPN No. 2508, Apto. Postal 14-740, DF 07300, Mexico E-mail: alejandra.castro@urv.cat Abstract: The authors present an analytical and continuous model for the total charges at the gate, drain and source electrodes for organic thin-film transistors (OTFTs), from which analytical expressions of the total capacitances are obtained. Under the quasi- static approximation, the model parameters are extracted using the previously developed unified model and parameter extraction method (UMEM). The capacitance model is valid above threshold voltage. It guarantees continuity of the expressions for the capacitance at the transition between linear and saturation regimes, as well as takes into account the overlap capacitance. Comparisons between modelled and experimental C GG values are shown. 1 Introduction Organic devices, such as thin film transistors (OTFTs) [1, 2], light-emitting diodes [3] and solar cells [4] have gained considerable attention over the last two decades. Many organic materials and blends have been used in these devices to improve their performance. OTFTs are interesting due to their potential applications in large and flexible areas, a relative low-cost circuit fabrication. From another point of view, to efficiently design circuit applications based on these OTFTs, the development of accurate current-voltage (IV), capacitance-voltage (CV) and mobility compact models that allow describing their electrical behaviour is particularly important and much effort has been dedicated on them during the last years [5–10]. Among capacitance models [8–10], not all the reported expressions for the capacitance guarantee continuity when passing from the linear to the saturation regime, nor consider the overlap capacitance, for example, [9, 10]. Another important issue is the procedure to extract precisely and as simple as possible model parameters. In this study, we present a compact capacitance model that is continuous in the entire above-threshold regime, considers the overlap capacitance and includes a simple and precise extraction method. In addition, most model parameters have a clear physical meaning and are related to the properties of the device materials. 2 Charge model The mobile charge as function of gate bias may be expressed as [11, 12] Q mob = C i (V GT - V ) 1+g Vaa g (1) where V is the potential applied along the channel of the OTFT, and V GT ¼ V GS – V T . V GS is the gate-source voltage, V T the threshold voltage; and C i is the insulator capacitance per unit area, g, which is related to the disorder within the material, and Vaa are parameters extracted by unified model and parameter extraction method (UMEM) [5] from the measured electrical characteristics of the device. It is assumed that the energy distribution of states in the material can be represented by an exponential function, where the characteristic temperature T 0 is related to the model parameter g as [13] g = 2 T 0 T - 1   (2) The drain current is calculated as I D = W m 0 Q mob dV dx (3) where W is the channel width, and dV/dx is the differential term of the potential dependent on the position along the channel length (L). From (3) an expression for dx is obtained dx = W m 0 Q mob I D dV (4) 130 IET Circuits Devices Syst., 2012, Vol. 6, Iss. 2, pp. 130–135 & The Institution of Engineering and Technology 2012 doi: 10.1049/iet-cds.2010.0372 www.ietdl.org