Analytic model for organic thin film transistors (OTFTs): effect of contact resistances application to the octithiophene (original) (raw)
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Modeling of mobility in organic thin-film transistor based octithiophene (8T)
Synthetic Metals, 2010
Thin-film organic field-effect transistor was made with vapor-deposited polycrystalline octithiophene on silicon oxide insulator layers. In conventional field-effect transistors, the extracted mobility does not take into account the distribution of charge carriers. However, in disordered organic field-effect transistors, the local charge carrier mobility decreases from the semiconductors/insulator interface in to the bulk, due to its dependence on the charge carrier density. It is demonstrated that the conventional field-effect mobility is a good approximation for the local mobility of the charge carriers at the interface. In this paper we present a new approach to the mobility in organic thin-film transistor (OTFTs), and with a new procedure we extract the electrical parameters of organic TFTs that possible to reproduce very well the device characteristic and mobility.
Charge Distribution and Contact Resistance Model for Coplanar Organic Field-Effect Transistors
IEEE Transactions on Electron Devices, 2013
We propose a theoretical description of the charge distribution and the contact resistance in coplanar organic field-effect transistors (OFETs). Based on the concept that the current in organic semiconductors is only carried by injected carriers from the electrodes, an analytical formulation for the charge distribution inside the organic layer was derived. We found that the contact resistance in coplanar OFETs arises from a sharp low-carrier-density zone at the source/channel edge because the gate-induced channel carrier density is orders of magnitude higher than the source carrier density. This image is totally different from the contact resistance in staggered OFETs, in which the contact resistance mainly originates from the resistance through the semiconductor bulk. The contact resistance was calculated through charge-distribution functions and the model could explain the effect of the gate voltage and injection barrier on the contact resistance. Experimental data on pentacene OFETs were analyzed using the transmission-line method. We finally noticed that the gate-voltage dependent mobility is a critical factor for proper understanding of the contact resistance in real devices.
IET Circuits, Devices & Systems, 2014
This research study analyses the impact of semi-conductor (t osc ) and dielectric (t ox ) thicknesses on top contact (TC) and bottom contact (BC) organic thin film transistors (OTFTs) using Atlas 2-D numerical device simulation. Thickness of each layer is varied from 20 to 150 nm. The parameters such as electric field, charge carrier distribution and trap density are analysed from device physics point of view with variations in organic semi-conductor layer and dielectric thicknesses. A decrease of 22% in TC to BC current ratio is observed for maximum increase in t osc , whereas, it remains almost constant at unity with variations in t ox . Furthermore, the maximum mobility for TC is achieved at t osc of 20 nm and reduces monotonically with further increase in thickness because of lowering of average charge. However, its highest value is obtained at 60 nm for BC structure that declines with positive or negative change in t osc . Besides this, the threshold voltage (V t ) shows a reduction of 50% for both the structures on scaling down t ox from 150 to 20 nm. Furthermore, the ON to OFF current ratio is found to be more dependent on t osc as compared with t ox . This is because of a dominant impact of t osc reduction on OFF current as compared with impact of t ox reduction on the ON current. Additionally, a decrease in contact resistance (R C ) is observed in TC structure for thicker active layer while operating at lower V gs . However, at high gate voltage, t osc maps to the access resistance that results in higher R C values.
Modeling of static electrical properties in organic field-effect transistors
Journal of Applied Physics, 2011
A modeling of organic field-effect transistors' (OFETs') electrical characteristics is presented. This model is based on a one-dimensional (1-D) Poisson's equation solution that solves the potential profile in the organic semiconducting film. Most importantly, it demonstrates that, due to the common open-surface configuration used in organic transistors, the conduction occurs in the film volume below threshold. This is because the potential at the free surface is not fixed to zero but rather rises also with the gate bias. The tail of carrier concentration at the free surface is therefore significantly modulated by the gate bias, which partially explains the gate-voltage dependent contact resistance. At the same time in the so-called subthreshold region, we observe a clear charge trapping from the difference between C-V and I-V measurements; hence a traps study by numerical simulation is also performed. By combining the analytical modeling and the traps analysis, the questions on the C-V and I-V characteristics are answered. Finally, the combined results obtained with traps fit well the experimental data in both pentacene and bis(triisopropylsilylethynyl)pentacene OFETs.
An analytical solution for contact resistance of staggered organic field-effect transistors
Journal of Applied Physics, 2017
We have developed analytical models for bias dependent contact resistance (RC) and output characteristics of staggered organic field-effect transistors (OFETS) based on a bulk resistance-approximated and mobility-modified current-crowding method. Numerical evaluations of RC and its resistive components show that the bias dependency of the bulk resistance is negligible. Consequently, the properties of the active layer interfaces determine RC and its characteristics. Effective parameters include a normally constant charge injection barrier at the organic-metal interface (Eb) and a gate induced surface carrier-concentration (PS0) at the organic-insulator boundary. The energy barrier pertains to the fabrication process, and its related resistance (rc) can be determined as the fitting parameter of the theoretical model. However, PS0 is strongly gate bias dependent and the results of the numerical model indicate that the resulting component (rch) is dominant and has a considerable effect ...
Organic Thin-Film Transistors: Part II—Parameter Extraction
IEEE Transactions on Electron Devices, 2000
A parameter extraction methodology and a verification of a generic analytical model and a thin-film transistor (TFT) compact dc model for the current-voltage characteristics of organic TFTs are presented. The verification shows that the proposed models meet the requirements for compact modeling and for computer circuit simulators. The models are fully symmetrical, and the TFT compact dc model is validated in all regimes of operation-linear and saturation above threshold, subthreshold, and reverse biasing. Suitable characterization techniques for parameter extraction of mobility, threshold voltage, and contact resistance are provided. Approaches are elaborated for the essential practical feature of upgradability and reducibility of the TFT compact dc model, allowing for easier implementation and modification, as well as separation of characterization techniques.
Organic Thin-Film Transistors: Part II—Parameter Extraction
IEEE Transactions on Electron Devices, 2009
A parameter extraction methodology and a verification of a generic analytical model and a thin-film transistor (TFT) compact dc model for the current-voltage characteristics of organic TFTs are presented. The verification shows that the proposed models meet the requirements for compact modeling and for computer circuit simulators. The models are fully symmetrical, and the TFT compact dc model is validated in all regimes of operation-linear and saturation above threshold, subthreshold, and reverse biasing. Suitable characterization techniques for parameter extraction of mobility, threshold voltage, and contact resistance are provided. Approaches are elaborated for the essential practical feature of upgradability and reducibility of the TFT compact dc model, allowing for easier implementation and modification, as well as separation of characterization techniques.
Analytical modeling of contact resistance in organic transistors
CAS 2012 (International Semiconductor Conference), 2012
Organic thin film transistors (OTFTs) are of significant interest for the development of organic electronics. The devices described in this paper, through measurements, analytic extraction of parameters and simulations, are organic transistor having Polytriarylamine (PTAA) as semiconductor. This paper refers to the determination of the source and drain contact resistance of these PTAA organic transistors. The MATLAB Simulink model is based on an analytic model, for which DC Sweep and parametric simulations were employed in order to obtain the total resistance. The results are processed in accordance to the Transfer Line Method (TLM). This mathematical method uses results on the total channel resistance of the thin film transistors with various channel lengths in order to extrapolate the contact resistance. The TLM structures considered here are bottom contact top gate OTFTs with interdigitated electrodes of various channel lengths. The values of the contact resistance are considered equal due to the symmetry of the measured structure.
Charge transport limited by grain boundaries in polycrystalline octithiophene thin film transistors
The European Physical Journal Applied Physics, 2002
Organic filed-effect transistor (OFETs) based on polycrystalline "octithiophene" has been realized. The current-voltage characteristics at low drain voltage has been used to derive the mobility of organic field effects transistors (OFETs). It appears that the data must be corrected for the substantial source and drain contact resistance. The carrier mobility is found to increase quasi linearly with gate voltage at room temperature. The temperature dependent measurements show that the mobility is thermally activated and becomes practically temperature independent at low temperatures. A model based on trapping mechanism, in which it is assumed that charge transport is limited by grain boundaries, has been used to describe the carrier mobility in polycrystalline "octithiophene" thin film transistors measured at temperatures ranging from 10 K to 300 K.