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.     

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。     

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)     

A novel a-Si:H AMOLED pixel circuit based on short-term stress stability of a-Si:H TFTs

This letter presents a novel pixel circuit for hydrogenated amorphous silicon (a-Si:H) active matrix organic light-emitting diode displays employing the short-term stress stability characteristics of a-Si:H thin film transistors (TFTs). The pixel circuit uses a programming TFT that is under stress during the programming cycle and unstressed during the drive cycle. The threshold voltage shift (V/sub T/-shift) of the TFT under these conditions is negligible. The programming TFT in turn regulates the current of the drive TFT, and the pixel current therefore becomes independent of the threshold voltage of the drive TFT.     

A novel 3-TFT voltage driving method of compensating VTH shift for a-Si:H TFT and OLED degradation for AMOLED

This work demonstrates the feasibility of a novel pixel circuit by using three a-Si:H TFTs. The proposed circuit can stabilize the OLED current and provide an additional driving current to ameliorate the brightness degradation of the AMOLED. Measurement results indicate that the current degradation of the proposed circuit, caused by V_TH variations, is less than 5% over more than 50,000 s at 60 °C, whereas that of a conventional 2T1C pixel circuit is larger than 34%. Furthermore, to ameliorate the decrease in luminance owing to the OLED degradation, the OLED current can be increased by 10% by analyzing the current degradation and modulating the detected voltage appropriately.     

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 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.     

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。     

a—Si：H TFT自对准工艺的研究

实验研究了自对准结构的ａ－Ｓｉ：Ｈ ＴＦＴ的制备工艺，对其中关键的底部曝光和顶胶工艺进行了详细的研究和分析，对制备工艺和结构参数进行了合理的优化，成功地制备出自对准结构的ａ－Ｓｉ：Ｈ ＴＦＴ。对影响自对准结构ａ－Ｓｉ：Ｈ ＴＦＴ特性的主要因素进行了详细的分析，提出了一种新颖的双有源层结构的ａ－Ｓｉ：Ｈ ＴＦＴ，可以有效地改善ａ－Ｓｉ：Ｈ ＴＦＴ的开态特性，其通断电流比ＩＯＮ／ＩＯＦＦ〉１０＾５。     

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     

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.     

一种基于低温多晶硅薄膜晶体管的交流AMOLED像素补偿电路

This paper presents a new poly-Si pixel circuit employing AC driving mode for active matrix organic light-emitting diode （AMOLED） displays. The proposed pixel circuit, which consists of one driving thin-film tran- sistor （TFT）, three switching TFTs, and one storage capacitor, can effectively compensate for the threshold voltage variation in poly-Si and the OLED degradation. As there is no light emission, except for during the emitting period, and a small number of devices used in the proposed pixel circuit, a high contrast ratio and a high pixel aperture ratio can be easily achieved. Simulation results by SMART-SPICE software show that the non-uniformity of the OLED current for the proposed pixel circuit is significantly decreased （〈 10%） with an average value of 2.63%, while that of the conventional 2T1C is 103%. Thus the brightness uniformity of AMOLED displays can be improved by using the proposed pixel circuit.     

Low-Power Low-Cost Voltage-Programmed a-Si:H AMOLED Display for Portable Devices

This paper presents a driving scheme to achieve highly stable low-power amorphous silicon (a-Si:H) active-matrix organic light-emitting diode (AMOLED) displays. Although the conventional 2-thin-film transistor (TFT) a-Si:H AMOLED display has demonstrated interesting features, including simplicity, it is prone to growing nonuniformity due to the temporal instability of the a-Si:H material. Several compensating techniques have been proposed to control the nonuniformity, but they tend to compromise the key attributes of the simple 2-TFT display such as low power consumption, high yield, high aperture ratio, low implementation cost, and fast programming. For mobile applications which have tight constrains on power consumption, cost, and escalating resolution requirements, we propose a new driving and addressing scheme that not only improves the backplane stability, but also compensates for the OLED luminance degradation while maintaining the attractive features of the simple 2-TFT pixel circuit. The overhead in power consumption and implementation cost is reduced by over 90% compared to existing compensation driving schemes.     

基于低温多晶硅薄膜晶体管的AMOLED交流像素电路

This work presents a new voltage programmed pixel circuit for an active-matrix organic light-emitting diode(AMOLED) display.The proposed pixel circuit consists of six low temperature polycrystalline silicon thinfilm transistors(LTPS TFTs),one storage capacitor,and one OLED,and is verified by simulation work using HSPICE software.Besides effectively compensating for the threshold voltage variation of the driving TFT and OLED,the proposed pixel circuit offers an AC driving mode for the OLED,which can suppress the degradation of the OLED.Moreover,a high contrast ratio can be achieved by the proposed pixel circuit since the OLED does not emit any light except for the emission period.     

A New Poly-Si TFT Current-Mirror Pixel for Active Matrix Organic Light Emitting Diode

A new poly-Si thin-film-transistor (TFT) current-mirror-active-matrix-organic-light-emitting-diode (AMOLED) pixel, which successfully compensates for the variation of the threshold voltage as well as mobility in the excimer laser annealed poly-Si TFT pixel, is designed and fabricated. The OLED current$(I_ OLED)$of the proposed pixel does not depend on the operating temperature. When the temperature of pixel is increased from 27$^circhboxC$to 60$^circhboxC$, the$I_ OLED$of the new pixel circuit composed of four TFTs and one capacitor increases only about 1.5%, while that of a conventional pixel composed of two TFTs and one capacitor increases about 37%. At room temperature, nonuniformity of the$I_ OLED$in the proposed circuit was also considerably suppressed at around 9%. We have successfully fabricated a 1.2-in AMOLED panel$(hbox96 times hbox96 times hboxred green blue)$to evaluate the performance of the proposed pixel. A troublesome residual image caused by the hysteresis phenomenon of the poly-Si TFT was almost eliminated in the proposed AMOLED panel as a result of current programming.     

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

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a—Si：H TFT有源矩阵制造技术的研究

对ａ：Ｓｉ：Ｈ ＴＦＴ有源矩阵的一些关键制造工艺进行研究。研究了Ｔａ２Ｏ５／ａ－ＳｉＮｘ双绝缘层的制备技术，提出了一种新的双有源层结构ａ－Ｓｉ：Ｈ ＴＦＴ来降低背光源对器件特性的影响，研制的ａ－Ｓｉ：Ｈ ＴＦＴ有源矩阵实现了彩色视频信号的动态显示。     

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).     

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.     

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.