A new a-Si:H TFT pixel circuit compensating the threshold voltage shift of a-Si:H TFT and OLED for active matrix OLED

We propose a new hydrogenated amorphous silicon thin-film transistor (a-Si:H TFT) pixel circuit for an active matrix organic light-emitting diode (AMOLED) employing a voltage programming. The proposed a-Si:H TFT pixel circuit, which consists of five switching TFTs, one driving TFT, and one capacitor, successfully minimizes a decrease of OLED current caused by threshold voltage degradation of a-Si:H TFT and OLED. Our experimental results, based on the bias-temperature stress, exhibit that the output current for OLED is decreased by 7% in the proposed pixel, while it is decreased by 28% in the conventional 2-TFT pixel.     

A novel current-scaling a-Si:H TFTs pixel electrode circuit for AM-OLEDs

Hydrogenated amorphous silicon thin-film transistor (a-Si:H TFT) pixel electrode circuit with a function of current scaling is proposed for active-matrix organic light-emitting displays (AM-OLEDs). In contrast to the conventional current mirror pixel electrode circuit, in this circuit a high data-to-organic light-emitting device (OLED) current ratio can be achieved, without increasing the a-Si:H TFT size, by using a cascade structure of storage capacitors. Moreover, the proposed circuit can compensate for the variations of TFT threshold voltage. Simulation results, based on a-Si:H TFT and OLED experimental data, showed that a data-to-OLED current ratio larger than 10 and a fast pixel programming time can be accomplished with the proposed circuit.     

A new a-Si:H thin-film transistor pixel circuit for active-matrix organic light-emitting diodes

We propose a new pixel circuit using hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs), composed of three switching and one driving TFT, for active-matrix organic light-emitting diodes (AMOLEDs) with a voltage source method. The circuit simulation results based on the measured threshold voltage shift of a-Si:H TFTs by gate-bias stress indicate that this circuit compensates for the threshold voltage shifts over 10000 h of operation.     

AMOLED电流镜像像素电路的稳定性分析

a—Si：HTFT在长时间施加直流栅偏压下将导致晶体管闽值电压漂移,造成OLED的发光亮度下降,影响其使用寿命。而多管的像素电路设计可以补偿或消除阂值电压的漂移。本文分析了电流控制电流镜像像素电路的工作原理。结合a—Si：HTFT阈值漂移模型仿真了电路在长时间工作下阈值漂移对驱动电流稳定性的影响,并提出了相应的解决办法。研究结果表明合理的像素电路设计可以有效改善驱动电流的稳定性。     

Gated-four-probe a-Si:H TFT structure: a new technique to measurethe intrinsic performance of a-Si:H TFT

In this letter, a new technique based on gated-four-probe hydrogenated amorphous silicon (a-Si:H) thin-film transistor (TFT) structure is proposed. This new technique allows the determination of the intrinsic performance of a-Si:H TFT without any influence from source/drain series resistances. In this method, two probes within a conventional a-Si:H TFT are used to measure the voltage difference within a channel. By correlating this voltage difference with the drain-source current induced by applied gate bias, the a-Si:H TFT intrinsic performance, such as mobility, threshold voltage, and field-effect conductance activation energy, can be accurately determined without any influence from source/drain series resistances     

a—SiNx：H薄膜对a—Si：H TFT阈值电压的影响

介绍了测定a－Si：HTFT闽值电压的实验方法。重点研究了改变a－SiNx：H薄膜淀积时反应气体NH3／SiH4流速比以及a－SiNx:H膜厚对a－Si：HTFT阈值电压的影响。对实验结果进行了分析。实验结果表明：a－Si:HTFT的阈值电压随a－SiNx：H的膜厚增加而增大；增大X－SiNx：H薄膜淀积时NH3／SiH4气体流速比，可明显减小a-Si:HTFT的阈值电压。     

Current-source a-Si:H thin-film transistor circuit for active-matrix organic light-emitting displays

In this letter, we describe a four thin-film-transistor (TFT) circuit based on hydrogenated amorphous silicon (a-Si:H) technology. This circuit can provide a constant output current level and can be automatically adjusted for TFT threshold voltage variations. The experimental results indicated that, for TFT threshold voltage shift as large as /spl sim/3 V, the output current variations can be less than 1 and 5% for high (/spl ges/0.5 /spl mu/A) and low (/spl les/0.1 /spl mu/A) current levels, respectively. This circuit can potentially be used for the active-matrix organic light-emitting displays (AM-OLEDs).     

Improved a-Si:H TFT pixel electrode circuits for active-matrixorganic light emitting displays

Two improved four thin-film-transistors (TFTs) pixel electrode circuits based on hydrogenated amorphous silicon (a-Si:H) technology have been designed. Both circuits can provide a constant output current level and can be automatically adjusted for TFT threshold voltage variations. The circuit simulation results indicate that an excellent linearity between the output current and input current can be established. An output current level higher than ~5 μA can be achieved with these circuits. This current level can provide a pixel electrode brightness higher than 1000 cd/m² with the organic light-emitting device (OLED) having an external quantum efficiency of 1%. These pixel electrode circuits can potentially be used for the active-matrix organic light-emitting displays (AM-OLEDs)     

Estimate Threshold Voltage Shift in a-Si:H TFTs Under Increasing Bias Stress

A method of estimating threshold voltage shift in hydrogenated amorphous silicon (a-Si:H) transistors under increasing bias stress is proposed. Although the threshold voltage shift in a-Si:H thin-film transistor (TFT) has been modeled well under constant bias stress, its property with increasing bias stress, which occurs in many a-Si:H-based compensating circuits, still cannot be quantified without any restriction, such as constant overdrive bias or short stressing time. In this paper, we propose a model which is effective under an arbitrary increasing stress for a prolonged time. The proposed model reduces to the constant bias model if the stress bias remains unchanged. With this method, the lifetime of most compensating circuits based on a-Si devices can be estimated completely.      

Amorphous silicon thin film transistor circuit integration for organic LED displays on glass and plastic

This paper presents design considerations along with measurement results pertinent to hydrogenated amorphous silicon (a-Si:H) thin film transistor (TFT) drive circuits for active matrix organic light emitting diode (AMOLED) displays. We describe both pixel architectures and TFT circuit topologies that are amenable for vertically integrated, high aperture ratio pixels. Here, the OLED layer is integrated directly above the TFT circuit layer, to provide an active pixel area that is at least 90% of the total pixel area with an aperture ratio that remains virtually independent of scaling. Both voltage-programmed and current-programmed drive circuits are considered. The latter provides compensation for shifts in device characteristics due to metastable shifts in the threshold voltage of the TFT. Various drive circuits on glass and plastic were fabricated and tested. Integration of on-panel gate drivers is also discussed where we present the architecture of an a-Si:H based gate de-multiplexer that is threshold voltage shift invariant. In addition, a programmable current mirror with good linearity and stability is presented. Programmable current sources are an essential requirement in the design of source driver output stages.     

High field-effect-mobility a-Si:H TFT based on high deposition-ratePECVD materials

The hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFT's) having a field-effect mobility of 1.45 ±0.05 cm² /V·s and threshold voltage of 2.0±0.2 V have been fabricated from the high deposition-rate plasma-enhanced chemical vapor deposited (PECVD) materials. For this TFT, the deposition rates of a-Si:H and N-rich hydrogenated amorphous silicon nitride (a-SiN_1.5 :H) are about 50 and 190 nm/min, respectively. The TFT has a very high ON/OFF-current ratio (of more than 10⁷), sharp subthreshold slope (0.3±0.03 V/decade), and very low source-drain current activation energy (50±5 meV). All these parameters are consistent with a high mobility value obtained for our a-Si:H TFT structures. To our best knowledge, this is the highest field-effect mobility ever reported for an a-Si:H TFT fabricated from high deposition-rate PECVD materials     

Dependence of Threshold Voltage of a-Si:H TFT on a-SiN_x:H Film

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Simulation of Active-Matrix Electrophoretic Display Response Time Optimization by Dual-Gate a-Si:H TFT With a Common Gate Structure

Large off-state drain-source current of the thin-film transistor (TFT) in active-matrix electrophoretic display (AMEPD) pixel leads to dramatic data voltage degradation, which causes severe crosstalk and undesired large response time. In this paper, the leakage current influence on response time is investigated and simulated. A compact model of response time t versus off-state drain-source current I _off is established. The simulation result induces that by reducing I _off the response time can be efficiently shorted. In order to reduce the off-state current, dual-gate amorphous silicon (a-Si:H) TFT with a common gate structure is discussed. Its current regulation mechanism is illustrated, and its fitness for driving the AMEPD pixel is explained. The SPICE simulation results prove that except reducing the crosstalk, dual-gate a-Si TFT can also significantly short the response time by cutting down the off-state current under the operation conditions of AMEPD application, while insignificantly reduces the on-state current.     

Dependence of Threshold Voltage of a-Si:H TFT on a-SiNx:H Film^①

The relation between threshold voltage for hydrogenated amorphous silicon thin film transistors(a-Si:HTFTs)and deposition conditions for hydrogenated amorphous silicon nitride(a-SiNx:H)films is investigated.It is observed that the threshold voltage,Vth,of a-Si:HTFT increases with the increase of the thickness of a-SiNx:H film,and the threshold voltage is reduced apparently with the increase of NH3/SiH4 gas flow rate ratio.     

Dynamic characterization of a-Si TFT-LCD pixels

A dynamic analysis of an amorphous silicon (a-Si) Thin-Film-Transistor-Liquid-Crystal-Display (TFT-LCD) pixel is presented using new a-Si TFT model and new Liquid Crystal (LC) capacitance models for SPICE simulators. This analysis is useful to all Active Matrix LCD designers for evaluating and predicting the performance of LCD's. The a-Si TFT model is developed to simulate important a-Si TFT characteristics such as off-leakage current, threshold voltage shift due to voltage stress and temperature, localized states behavior, and bias- and frequency-dependent gate to-source and gate-to-drain capacitance. In addition, the LC Capacitance model is developed using simplified empirical equations. The modeling procedure is useful to TFT and LCD designers who need to develop their own models. Since our experiments simulate critical TFT-LCD transient effects such as the voltage drop due to gate-to-source capacitance and dynamic off-leakage current, it is possible to accurately characterize TFT-LCD's in the time domain. The analysis and models are applicable to today's optical characterizations of Flat-Panel-Displays (FPD's)     

Electrical Stability of Hexagonal a-Si:H TFTs

In this letter, we study the current-temperature-stress-induced electrical instability of single and multiple hexagonal (HEX) hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs) connected in parallel. The influence of the threshold voltage shift of a single HEX TFT on the overall electrical performance of multiple HEX TFTs is discussed. The results indicate that a-Si:H HEX TFTs have an improved electrical stability and a threshold voltage shift linear dependence on a number of connected HEX-TFT units.     

Device and circuit level optimization for high performance a-Si:H TFT-based AMOLED displays

Active matrix organic light-emitting diode (AMOLED) displays with amorphous hydrogenated silicon (a-Si:H) thin-film transistor (TFT) backplanes are becoming the state of art in display technology. Though a-Si:H TFTs suffer from an intrinsic device instability, which inturn leads to an instability in pixel brightness, there have been many pixel driving methods that have been introduced to counter this. However, there are issues with these circuits which limit their applicability in terms of speed and resolution. This paper highlights these issues and provides detailed design considerations for the choice of pixel driver circuits in general. In particular, we discuss the circuit and device level optimization of the pixel driver circuit in a-Si:H TFT AMOLED, displays for high gray scale accuracy, subject to constraints of power consumption, and temporal and spatial resolution.     

Static characteristics of a-Si:H dual-gate TFTs

This paper examines the effect of the top gate on the static characteristics of dual-gate hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs). Both forward and reverse regimes of operation are considered. The top gate has a distinct effect on the threshold voltage, subthreshold slope, drive-current capability, and the leakage current of the TFT. In particular, the threshold voltage is found to linearly decrease with increasing top-gate bias. Specific bias configurations of the dual gate TFT critical to vertical integration of on-pixel electronics for imaging and display applications are also presented.     

Three-TFT image sensor for real-time digital X-ray imaging

A new current-programmed, current-output active TFT image sensor suitable for real-time X-ray imaging (e.g. fluoroscopy) using hydrogenated amorphous silicon (a-Si:H) thin-film-transistor (TFT) technology is introduced. The proposed pixel circuit can successfully compensate for characteristic variations such as mobility and threshold voltage shift in a-Si:H TFTs. Simulation and measurement results show that high on-pixel amplification can be accomplished with this pixel circuit.     

Level shifter embedded in drive circuits with amorphous silicon TFTs

A drive circuit embedded with a level shifter was fabricated with conventional a-Si:H thin-film transistor (TFT) process. Two cascaded bootstrapped inverters were fabricated for the level shifter. The proposed level shifter shifts an input signal to a high voltage, for example, from 5 to 30 V. The level shifter embedded driver with a-Si:H TFT operates well for the input 5-V start pulse and clocks, and the 4th output of the driver shows 30.3-V output. The level shifter is stable under operation even though there is a threshold voltage shift of a-Si:H TFT. The stability of the driver was also investigated.