Model For TFTs Research Articles

Drain Current Model for Antiambipolar Organic TFTs at Different Temperatures

Drain Current Model for Antiambipolar Organic TFTs at Different Temperatures

A Physics-Based Compact DC Model for AOS TFTs Considering Effects of Active Layer Thickness Variation

A Physics-Based Compact DC Model for AOS TFTs Considering Effects of Active Layer Thickness Variation

Analytical I–V and C–V models for symmetric double-gate AOSTFTs

In this article we present a full physical analytical model (FAM) for symmetric double-gate amorphous oxide semiconductor TFTs (DG AOSTFTs). The current–voltage (I–V) model is physically described for the above-threshold operation regime. In this case, the general expression for the field effect mobility is evaluated for the potential at the centre of the semiconductor layer at V G= 1 + V T, obtaining the value of the mobility parameter called µ 1DG for DG-AOSTFT. This parameter can now be used to represent the field effect mobility expression like in the model known as the unified model and extraction method, which is widely used for modelling amorphous devices. The proposed FAM model contains the analytical capacitances–voltage (C–V) model and considers the specific characteristics of DG structures, including the potential different from zero, at the centre of the semiconductor layer. The model was described in Verilog-A for introducing in Silvaco’s SmartSpice simulation program and was validated with simulated data and experimental characteristics of reported DG amorphous indium-galium-zinc oxide devices.

Comparison of electrical performances of water and organic solvent-based amorphous indium-gallium-zinc-oxide thin-film transistors

Water and organic-based amorphous InGaZnO thin-film transistors (a-IGZO TFT) were fabricated and their electrical performances were analyzed. By comparing the threshold voltage, subthreshold slope, and field-effect mobility with scaling characteristics, we found that the a-IGZO TFT performances were mainly dependent on the solvent types and stirring times of the precursor solution. In addition, it is shown that the field-effect mobility values follow the Poole–Frenkel mobility model for water-based TFTs when the stirring time is under 24 h indicating that there are large defect density of states that degrade the field-effect mobility. When the stirring time is sufficiently long for the water-based sample, the sample shows superior field-effect mobility compared to the organic-based one, which is attributed to the high annealing efficiency of the water-based IGZO active layer compared to the organic-based one.

A physical channel-potential and drain-current model for asymmetric dual-gate a-IGZO TFTs

A physical channel-potential and drain-current model for asymmetric dual-gate a-IGZO TFTs

Charge-Based Compact Model for DC Current in Organic TFT Including Non-Linear Injection Effects with a Close Link to Electrical Device Parameter

In this paper a closed-form current model for organic TFTs is presented which leads to a charge-based continuous current model with threshold voltage and slope as model parameters, while still keeping a single expression valid from below to above threshold operation. Most compact models for organic field-effect transistors (OFETs) are based on a two-piece description: a set of equations for above threshold operation is combined with a current model valid in subthreshold region by means of an analytical smoothing function [1]. Thereby the threshold voltage is introduced as fitting parameter without any relation to physical parameters. In [2] a charge-based current model with a continuous equation valid in all regions of operation has been presented. While solving Poisson’s equation the model preserves a close link to physical parameters like shallow and deep trap densities and avoids the introduction of a threshold voltage and subthreshold slope as model parameters. However, from a circuit designer’s perspective a model providing a close link to electrical device parameters is desirable because parameters as threshold voltage and slope can easily be extracted from I/V curves and are performance measures of a device. In this paper an improvement of the model from [2] is presented which leads to a charge-based C∞-continuous current model with threshold voltage and slope as model parameters, while still keeping a single expression valid from below to above threshold operation. To include the effect of hopping transport the empirical power-law mobility model [3] and a contact resistance [4] have been incorporated in the effective mobility. Channel length modulation effects have been included according to [5]. Furthermore, from physical parameters given by trap densities the threshold voltage and subthreshold slope of the device can be calculated. In the case of co-planar device structures the output characteristics in the linear operation regime show a superlinear behaviour due to non-linear injection effects. If the barrier height between source electrode and the channel material is not negligible the channel current is limited by the source injection current. The Schottky barrier height is modulated by the gate and drain potential. A strong electric field causes thinning of the barrier, allowing more carriers to enter channel. An analytical equation for the electric field at the injection barrier has been derived by applying the conformal mapping technique. The bias-dependent lowering of the Schottky barrier, and hence the maximum injection current can be calculated. An equivalent resistance of the Schottky barrier is calculated from the voltage drop Vds and the maximum possible injection current. To arrive at a closed-form current equation for the intrinsic device together with the injecting Schottky barrier this equivalent resistance is incorporated in the current equation by a first order approximation which is commonly used for the consideration of parasitic resistances. In this case the equivalent resistance is bias dependent and reduced with increasing Vds due to Schottky barrier lowering. The model has been validated vs. measurements on devices with small molecule organic semiconductors (Fig. 1, 2). The results show good agreement, even for the 1st and 2nd derivatives of the transfer characteristics. Acknowledgements: This project was funded by the German Federal Ministry of Education and Research contract No.03FH013PX2 (FhProfUnt). This project was also funded by the Horizon 2020 Programme of the European Union under contract RISE 2014 645760 ("DOMINO"). We acknowledge CEA-Liten (Grenoble) and Max Planck Institute for Solid State Research (Stuttgart) for providing measurement data. We thank AdMOS GmbH for support and Keysight Technologies for license donation of IC-CAP.

Analytical channel potential model of amorphous InGaZnO thin-film transistors with synchronized symmetric dual-gate

Oxide indium gallium zinc thin film transistor (IGZO TFT) is a promising candidate for mass production of next-generation flat panel display technology with high performance. This is due to many merits of IGZO TFTs, such as high mobility, excellent uniformity over large area, and low cost. In recent years, IGZO TFTs with dual gate structure have attracted enormous attention. Compared with the conventional single gate IGZO TFTs, the dual gate IGZO TFTs have many advantages including increased driving ability, reduced leakage current, and improved reliability for both negative biasing stressing and positive biasing stressing. Although the measurement results of fabricated circuit samples have proven that dual gate IGZO TFTs are beneficial for the integration of digital circuit and active matrix light emitting display with in-array or external compensation schematics, there has been no proper analytic model for dual gate IGZO TFTs to date. As the analytic model is crucial to circuit simulations, there are great difficulties in circuit designs by using dual gate IGZO TFTs. Although there are some similarities between the operating principal of the dual gate IGZO TFTs and that of the dual gate silicon-on-insulator devices, the complexity of conducting mechanism of IGZO TFTs is increased due to the existence of sub-gap density of states (DOS) in the IGZO thin film. In this paper, an analytical channel potential model for IGZO TFT with synchronized symmetric dual gate structure is proposed. Gaussian method and Lambert function are used for solving the Poisson equation. The DOS of IGZO thin film is included in the proposed model. Analytical expressions for the surface potential (S) and central potential (0) of the IGZO film are derived in detail. And the proposed channel potential model is valid for both sub-threshold and above-threshold region of IGZO TFTs. The influences of geometry of dual-gate IGZO TFT, including thickness values of gate oxide layer and IGZO layer, on the device performance are thoroughly discussed. It is found that in the case of small gate-to-source voltage (VGS), as the conducting of IGZO layer is weak, both S and 0 increase linearly with the increase of VGS due to the increase of voltage division between the oxide and IGZO layer. However, the increase of S and 0 starts to saturate once VGS is larger than threshold voltage due to the shielding of electrical field by the induced electron layer of IGZO surface. With the evolution of VGS, the calculated results of S and 0 by using the proposed dual gate IGZO TFT model are in good agreement with the numerical results by technology computer aided design simulation method. Therefore, the proposed model is promising for new IGZO TFT electronics design automation tool development.

A Compact SPICE Model for Organic TFTs and Applications to Logic Circuit Design

This work introduces a compact DC model developed for organic thin film transistors (OTFTs) and its SPICE implementation. The model relies on a modified version of the gradual channel approximation that takes into account the contact effects, occurring at nonohmic metal/organic semiconductor junctions, modeling them as reverse biased Schottky diodes. The model also comprises channel length modulation and scalability of drain current with respect to channel length. To show the suitability of the model, we used it to design an inverter and a ring oscillator circuit. Furthermore, an experimental validation of the OTFTs has been done at the level of the single device as well as with a discrete-component setup based on two OTFTs connected into an inverter configuration. The experimental tests were based on OTFTs that use small molecules in binder matrix as an active layer. The experimental data on the fabricated devices have been found in good agreement with SPICE simulation results, paving the way to the use of the model and the device for the design of OTFT-based integrated circuits.

Physical Modeling of Amorphous InGaZnO Thin-Film Transistors: The Role of Degenerate Conduction

In amorphous indium-gallium-zinc oxide thin-film transistors (a-IGZO TFTs), the electron mobility easily exceeds 10 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /Vs and degenerate band conduction is observed. On the other hand, the field-effect mobility is gate voltage-dependent. Here, we propose a physical model for a-IGZO TFTs accounting for both the non-degenerate and degenerate conductions of trapped and free charges. The comparison between the model and the measurements shows that: 1) the shape of the drain current is almost completely defined by the localized density of states and 2) a transition from non-degenerate-to-degenerate conductions is always observed. This explains the measured gate voltage-dependent field-effect mobility and provides a simple and unified physical picture of the charge transport in a-IGZO TFTs.

Compact Modeling and Contact Effects in Thin Film Transistors

A compact model for the current-voltage characteristics of organic thin-film transistors (OTFTs), which includes the effects of the contact regions, is proposed. Different physical and morphological aspects of contacts with organic or other emerging materials such as graphene, semiconducting dichalcogenides such as MoS2, or NW devices are described. The electrical behavior of the contacts is studied in OTFTs, and circuit models that describe them are reviewed. Two trends are observed in the current-voltage curves of the contacts of different OTFTs: linear and nonlinear, and different models are used to explain them. A unified model for the contact region that reproduces both trends and gathers the different physical and structural features of the contacts is developed. It is described by a single parameter and introduced in a generic analytical model for TFTs. The variability in OTFT structures, materials, and fabrication approaches gives rise to a strong variability in the values of the parameters of the model. In this regard, a characterization technique to determine the value of the parameters of the model from experimental data is also developed. Different physical tests are proposed to validate the results of the technique. The procedure is applied to recent experimental data for different pentacene-based transistors. The good agreement between the experimental data and our analytical results provides a way to relate the parameters of the model with the physical or geometrical origin of the contact effects in OTFTs.

Interface engineering for suppression of flat-band voltage shift in a solution-processed ZnO/polymer dielectric thin film transistor

Flexible and transparent thin film transistors (FTTFTs) can lead to next generation displays that involve large area, future-oriented flexible and transparent displays. In order to achieve stable FTTFTs, solution processes of organic and inorganic compounds have received significant attention. Above all, transparent oxide semiconductors such as ZnO have been studied to enhance flexibility with high electrical performance by integration with organic dielectrics. However, interfacial traps between inorganic and organic compounds are derived by interface dipole, which induce a considerable flat band shift. Herein, we have developed a self-assembled inorganic layer (SAIL) via the photo-induced transformation of a mono-poly(dimethylsiloxane) (PDMS) layer as interface engineering. Especially, the shifting of flat band voltage (VFB) was effectively suppressed by the SAIL process, which was analyzed with a single-piece analytical model for ZnO TFTs. In addition, flexible ZnO/SAIL/polymer dielectric TFTs with low process temperature as high as 200 °C exhibited a good field-effect mobility μ = 0.28 cm2 V−1 s−1, more than 106 on–off current ratio and excellent device operational stability and flexibility.

Unified Current, Mobility and Capacitance Compact Model for Polymeric TFTs

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Optimized parameter extraction using fuzzy logic

Precise extraction of transistor model parameters is of much importance for modeling and at the same time a difficult and time consuming task. Methods for parameter extraction can rely on purely mathematical basis, calling for intensive use of computational resources, or in human expertise to interpret results. In this work, we propose a method for parameter extraction based on fuzzy logic that includes a precise knowledge about the function of each parameter in the model to create a set of simple fitting rules that are easy to describe in human language. To simplify the computational effort, the parameter fitting rules work using only data at specific points (e.g. the distance between the calculated curve and the measured one at VDS corresponding to 50% of the maximum current). If necessary, a more accurate implementation can be used without altering the basic underlying philosophy of the method. In this work, the method is applied to extract model parameters required by Level 3 bulk MOS model and by a compact model for TFTs used in the Unified Model and Extraction Method (UMEM), which is based on an integral function. Results obtained show that the method is quite insensitive to the initial conditions and that it is also quite fast. Extension of this method for more complex models requires only the creation of the corresponding rule base, using the appropriate measurements. The method is especially useful for production testing or design.

Unified model for short-channel poly-Si TFTs

We present a new unified model for long and short-channel polysilicon thin-film transistors (poly-Si TFTs) suitable for circuit simulation. The model is based on the effective medium approximation and should be valid for transistors of channel lengths down to 1 μm. The model accounts for field effect mobility enhancement in the moderate inversion regime and for mobility degradation at high gate voltages, for DIBL effect, kink effect, off-state current and channel-length modulation. Good agreement between the model and the measurements was found for a wide range of channel lengths.

Consideration of feed-through voltage in amorphous-Si TFT's

Feedthrough voltage is an important factor in designing integrated data drive circuits and display area in thin-film-transistor liquid-crystal displays (TFT LCDs). With respect to feedthrough voltage, only the gate-source overlay capacitance has been considered in amorphous-Si (a-Si) TFTs, because of their staggered structure with overlap area. It is pointed out that, in a-Si TFTs designed as active elements, feedthrough voltage is mainly due to the carrier redistribution. The main reason is that, since the field-effect mobility is low (W/L>1), the leakage current must be kept low (L>or=10 mu m), and an active layer is inserted in the overlap area (unlike the case with MOS device structures), the area of the active layer is large. Taking the carrier redistribution into account, the maximum difference between the voltage obtained using the modified model and the experimental voltage is within 20%. By comparison, the results obtained using the previous model for TFTs are approximately three times smaller than the experimental results. >