Introduction rganic electronics is the field which is fast developing in today's scenario. Organic semiconductors (OSC) have made the device low cost and made the field of organic electronics active. Organic transistors can be directly fabricated on flexible cheap substrates and it requires low temperature which makes the device cost efficient. Various researchers used the flexible substrates like glass [1] and plastic [2] which led the fabrication cost very low. Organic thin film transistors have been used in various applications like Organic light emitting diodes (OLEDs) [3], Organic displays [4], Organic radio frequency identification tags [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23], organic sensors [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] and many more high end applications. Several improvements have been made by researchers in geometry, materials, insulators and fabrication to make the device more reliable in performance issues and still it is needed to be improving to implement in basic electronic circuitry. Numerical simulation is very useful in understanding the sub threshold characteristics and electrical properties of a device which is also helpful in designing of a better model. 2D device simulator like ATLAS from Silvaco international would be suitable for the purpose. In this paper, numerical simulation of the device is done with top and bottom gate configurations to understand how the device behaves physically. A number of devices with different geometry were implemented in the structure of device and their performance was noted down. Pentacene organic semiconductor was used as an active layer of transistor because of its high mobility. In this paper, we model the characteristics of top and bottom gate configurations including top gate top contact (TGTC), top gate bottom contact (TGBC), bottom gate top contact (BGTC), bottom gate bottom contact (BGBC). The path of charge carriers changes in different geometries which possess difference in the electrical behavior of the devices. The performances of bottom and top gate pentacene based devices are compared and their performance parameters like mobility, threshold voltage, sub threshold slope, trans conductance, on off ratio are summarizing in detail. # II. # Experimental Setup # III. MODELLING AND NUMERICAL SIMULATION Numerical simulation of Electrical characteristics of the top and bottom gate configuration is measured using TCAD ATLAS by Silvaco International software. TCAD ATLAS by Silvaco International is physically based, numerical device simulator which predicts the electrical behavior of device and used to design a high performance device. This section describes the simulation procedure followed by ATLAS software. This software follows some fundamental equations that are linked with performance parameters. The equations used by the ATLAS to simulate the Device are Poisson's equation and Continuity equation which were used to measure the characterization of these two devices. [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] # a) Poisson's equation Poisson's Equation relates variations in the electrostatic potential to local charge densities. It is mathematically described by the following relation [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23], ? ? ? ? = ? ? ) .((1)( ) ? + ? + ? ? = ? ? a d N N n p q ) .( ? ? (2) Where ? is the electrostatic potential, ? is the local space charge density, ? is the local permittivity of the semiconductor (F/cm), p is the hole density (cm -3 ), n is the electron density (cm -3 ),N d + is the ionized donor density (cm -3 ) and N a + is the ionized acceptor density (cm -3 ). The reference potential is always taken as the intrinsic Fermi potential for simulations in ATLAS. The local space charge density is the sum of all contributions from all mobile and fixed charges, including electrons, holes and ionized impurities. # b) Continuity Equations For electrons and holes, the continuity equations are defined as follows: n n n R G J q t n ? + ? = ? ? . 1 (3) p p p R G J q t p ? + ? ? = ? ? . 1 (4) # c) Transport Equations These equations are to specify the physical models for electrons and holes current densities and generation (recombination) rates. The Current density equations are obtained by using the "drift-diffusion" charge transport model. The reason for this choice lies in the inherent simplicity and the limitation of the number of independent variables to just three ,? , n and p. The accuracy of this model is excellent for all technologically feasible feature sizes. The drift -diffusion model is described as follow: n qD qn J n n n n ? + ? = µ (5) p ? ? ? = p p p p qD qn J µ (6) where n and p are the electron and hole concentrations, J n and J p are the electron and hole current densities, G n (R n ) and G p (R p ) are the generation (recombination) rates for the electrons and holes, respectively and q is the fundamental electronic charge. ATLAS incorporates both eqns. In simulations, but, also gives the user an option to turn off one of the two equations and solve either the electron continuity equation. where ? n and ? p are the electron and hole mobilities, D n and D p are the electron and hole diffusion constants, En and Ep are the local electric fields for electrons and holes, respectively, and ? n and ? p are the three dimensional spatial gradient of n and p. The local electric fields are defined as follows: n E = - ie L n q kT ln ( + ? ? ) (7) p E = - ie L n q kT ln ( ? ? ? ) (8) Where n ie is the local effective intrinsic carrier concentration. For numerical simulation of OTFT device with top and bottom gate configuration, the Poole-Frenkel mobility model has been employed for Pentacene active channel and defines the dependency of mobility capability due to electric field, this model is expressed as [20][21][22][23], ??(??(??), ??) = ?? °(??) exp[??(??)???(??)](9) Here, in equation ( 9), ?? ° is zero field mobility, F is electric field, and ?? is characteristic parameter for the field dependence. IV. # The Density of Defect States The density of the defect states (DOS) g(E), which dominates the properties of amorphous or polycrystalline TFTs, is modeled as a combination of four components [3] , where E denotes the state energy. The equations describing these terms are given as follows [8] ? ? ? ? ? ? ? = TA C TA TA W E E N E g exp ) ( (10) ? ? ? ? ? ? ? = TD V TD TD W E E N E g exp ) ( (11) ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? = 2 exp ) ( GA GA GA GA W E E N E g (12) ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? = 2 exp ) ( GD GD GD GD W E E N E g (13) where E is the trap energy, E C is conduction band energy, E V is valence band energy, and the subscripts T, G ,A, D stand for tail, Gaussian (deep level), acceptor and donor states respectively. The exponential distribution of DOS is described by conduction and valence band intercept densities (N TA and N TD ), and by its characteristic decay energy (W TA and W TD ). For Gaussian distributions, the DOS is described by its total density of states (N GA and N GD ), its characteristic decay energy (W GA and W GD ), and its peak energy/peak distribution (E GA and E GD ). Input parameters used in the simulation of the OTFT devices with different geometries are summarized in Table II. V. RESULTS AND DISCUSSIONS All Organic thin film transistor devices were built up with technique of top gate and bottom gate configuration with top and bottom contacts. Electrical characterization and numerical simulation of the devices are measured using TCAD ATLAS by Silvaco International software and with the of characterization of devices, electrical performance parameters such as Mobility, Trans conductance, threshold voltage, Sub threshold sweep and on/off ratio were calculated. Mobility is the rate of flow of charge carriers in the electric field. It is the parameter which deals with processing speed of device. This mobility (µ) has been calculated using the following equations, µ= (L × g m ) / (W × C ox × V ds ) (10) g m = dI ds / dV gs (11) C ox = ? ox / d ox (12) Here, g m is the trans conductance which is calculated by transfer characteristics curve (I ds /V ds ) and calculation is done by equation (11). L and W are length and width of device respectively. C ox is the capacitance of oxide with is the ratio of permittivity of oxide and thickness of oxide, given by equation ( 12).V ds is drain voltage which is taken as 1V for all the devices. Minimum From above calculation, it was observed that bottom gate configuration perform better than top gate configuration in terms of mobility, sub threshold slope and with good on off ratio but top gate configuration have higher on off ratio as compared to bottom gate configuration which is in magnitude of 10 8 . # VI. CONCLUSION This paper presented the numerical simulation and characterization of bottom and top gate pentacene based OTFTs. The performances of these devices have been analyzed and their performance parameters like mobility, threshold voltage, sub threshold slope, trans conductance, on off ratio have been extracted and compared. It was observed that bottom gate configuration perform better than top gate configuration in terms of mobility, sub threshold slope and with good on off ratio but top gate configuration have higher on off ratio as compared to bottom gate configuration which is in magnitude of 10 8 . # Global Journal of Researches in Engineering ( ) Volume XIX Issue III Version I ![Top and bottom gate configurations of the Pentacene based Organic thin film transistor have been implemented and the schematic of the bottom gate configuration and top gate configuration are shown in Fig.(1) and Fig (2) respectively. For the fabrication of OTFT devices, Layer by Layer (LBL) technique is used in which the materials are evaporated in form of layers one by one.[7] Bottom and top contact structure are differentiated in terms of position of source and drain contacts with respect to active semiconductor layer and keeping gate at constant position. Fig.1. shows the top gate geometry with top contact fig.1 (a) and bottom contact fig.1 (b) used in the simulation.](image-2.png "") 1![Fig. 1: Schematic of Top gate configuration (a) top gate top contact (TGTC) (b) top gate bottom contact (TGBC)](image-3.png "Fig. 1 :") 2![Fig. 2: Schematic of bottom gate configuration (a) Bottom gate bottom contact (BGBC) (b) Bottom gate top contact (BGTC)](image-4.png "Fig. 2 :") ![voltage required for the device to be in ON state or the accumulation of charge carriers at gate dielectric-semiconductor interface is said as Threshold Voltage or Cut-in Voltage. Sub threshold sweep is ratio of change in gate biasing to change in logarithm scale of drain current. It can be expressed as, SS = ?V gs /? log 10 (I ds ) (13) a) Top gate configuration Fig. 2 (a) and (b) shows the output and transfer characteristics of top gate top contact configuration top gate bottom contact configuration. At high operating gate voltage, linear and saturation region are expect in the thin film transistor and the same is observed in the output graph for Top gate configuration at top contact and bottom contact. Similar characteristics behavior is also observed in am bipolar organic TFT reported in [10]. Figure 2(a) is the comparison of top gate top contact and top gate bottom configuration which tend to gain better characteristics in top gate top contact than top gate bottom contact. Transfer characteristics shows a good electrical performance with good electrical parameters with higher on-off ratio greater than 10 5 and sub threshold sweep of 0.11 in top gate bottom contact and 0.02 in top gate top contact shown in fig. 2(b).](image-5.png "") 23![Figure 3(a): Output (b) transfer charactristics curve (at V ds = -1V) of OTFT in BGTC and BGBC configuration](image-6.png "Fig. 2 :Figure 3") 1Device ParameterValue (µm)Channel Width (W)220Channel length10Gate thickness. T G0.02Dielectric thickness, t ox5.7 × 10 -3Organic semiconductor thickness, t osc0.03S/D contact thickness, t c0.03 2Year 20199Parameters Effective density of state in conduction band( N c ) Effective density of state in valence Dielectric constant for , Al 2 O 3 Electron gap at 300K N TD bandValues 1.0 × 10 21 cm -3 8.5 2.8 1.0×10 18 cm 3 /eV 1.0 ×10 21 cm -3Global Journal of Researches in Engineering ( ) Volume XIX Issue III Version I FN TA2.5×10 18 cm 3 /eVW TD0.5eVW TA0.129eVW GA0.15eV 3StructuresParametersBGBC BGTC TGBCTGTCV t (V)1.11.200On off ratio1.9 × 10 49.5 × 10 31.9 × 10 87.9×10 8 © 2019 Global Journals ## ACKNOWLEDGEMENT The authors are thankful to SERB, DST Government of India for the financial support under Early Career Research Award (ECRA) for Project No.ECR/2017/000179. * An Organic Active-Matrix Imager KNausieda IRyu AKymissis VIbitayo Akinwande CGBulovic Sodini IEEE Transactions on Electron Devices Feb. 2008 55 * SPICE Library for Low-Cost RFID Applications Based on Pentacene Organic FET SShen RTinivella MPirola GGhione VCamarchia 6th International Conference on Wireless Communications Networking and Mobile Computing (WiCOM) Chengdu 2010. 2010 * Integration of organic field-effect transistors and rubbery pressure sensors for artificial skin applications TSomeya IEEE International Electron Devices Meeting Washington, DC, USA 2003. 2003 * Integration of organic FETs with organic photodiodes for a large area, flexible, and lightweight sheet image scanners TSomeya Yusaku ShingoKato YoshiakiIba TsuyoshiNoguchi Hiroshi Kawaguchiand TakayasuSekitani Sakurai IEEE Transactions on Electron Devices Nov. 2005 52 * Analog and digital circuits using organic thin-film transistors on polyester substrates MGKane JCampi MSHammond FPCuomo BGreening CDSheraw JANichols DJGundlach JRHuang CCKuo LJia HKlauk TNJackson IEEE Electron Device Letters Nov. 2000 21 * Organic FET device as a novel sensor for cell bioelectrical and metabolic activity recordings SSpanu PLai ACosseddu MBonfiglio STedesco Martinoia 6th International IEEE/EMBS Conference on Neural Engineering (NER) San Diego, CA 2013. 2013 * Design for Mixed Circuits of Organic FETs and Plastic MEMS Switches for Wireless Power Transmission Sheet MTakamiya 2007 IEEE International Conference on Integrated Circuit Design and Technology Austin, TX 2007 * Low Power and Flexible Braille Sheet Display with Organic FET's and Plastic Actuators MTakamiya TSekitani YKato HKawaguchi TSomeya TSakurai 2006 IEEE International Conference on IC Design and Technology Padova 2006 * Detection of explosive vapors using organic thinfilm transistors EBentes HLGomes PStallinga LMoura Proceedings of IEEE Sensors IEEE Sensors 2004 2 * SPICE Optimization of Organic FET Models Using Charge Transport Elements VVaidya JKim JNHaddock BKippelen DWilson IEEE Transactions on Electron Devices Jan. 2009 56 * ATLAS User's Manual from SILVACO International * Characteristics of High-Performance Ambipolar Organic Field-Effect Transistors Based on a Diketopyrrolopyrrole-Benzothiadiazole Copolymer TJHa PSonar SPSingh ADodabalapur IEEE Transactions on Electron Devices May 2012 59 * High mobility of pentacene field-effect transistors with polyimide gate dielectric layers YKato SIba RTeramoto TSekitani TSomeya HKawaguchi TSakurai Appl. Phys. Lett 84 19 2004 * An all-organic soft thin film transistor with very high carrier mobility FGarnier GHorowitz ZPeng X DFichou Adv. Mater 2 12 1990 * The use of high K dielectric thin film prepared by RF sputtering as insulating layers for organic TFT devices EItoh TMurayama THighchi KMiyairi Proceedings of the IEEE International Conference on Solid Dielectrics the IEEE International Conference on Solid Dielectrics 2004. 2004. 2004 1 * Fabrication of pentacene organic field-effect transistors containing SiO 2 nanoparticle thin film as the gate dielectric TianhongCui GuirongLiang JingshiShi IEEE International Electron Devices Meeting Washington, DC, USA 2003. 2003 * Effects of the polymeric dielectric on OTFT performances MPetrosino ARubino RMiscioscia ADe Girolamo DelMauro CMinarini 3rd International Conference on Signals, Circuits and Systems (SCS) Medenine 2009. 2009 * Polymer gate dielectrics and conducting polymer contacts for high performance organic thin film transistors MHalik M HKlauk UZschieschang GSchmid WRadlik WWeber Adv. Mater 14 23 2002 * The work function of the elements and its periodicity HBMichaelson J. Appl. Phys 1977 11 * Modelling of organic field-effect transistors for technology and circuit design SMijalkovic 26th International Conference on Microelectronics (MIEL) Nis, Serbia May 11-2008 14 * GraciellaNall Organic Electronics 81157-27-5, 2011 First edition ISBN 978-93- * Numerical simulation of P3HT based Organic Thin Film Transistors (OTFTs) AD DDwivedi RajeevDharDwivedi RaghvendraDhar Dwivedi SumitVyas PChakrabarti International Journal of Microelectronics and Digital integrated circuits 1 2 2015 * Numerical Simulation and Spice Modeling of Organic Thin Film Transistors (OTFTs) DDDwivedi International Journal of Advanced Applied Physics Research 1 2014