A Novel Voltage-Feedback Pixel Circuit for AMOLED Displays

This study presents a novel voltage-modulated pixel circuit for active-matrix organic light-emitting diode (AMOLED) consisting of five n-type thin-film transistors (TFTs), one additional control signal, and one storage capacitor. The proposed circuit, which can be implemented in all-n-type and all-p-type low temperature poly-silicon (LTPS) TFT technologies, successfully compensates for threshold voltage deviation of TFTs and facilitates correction of OLED degradation using a voltage feedback method. Simulation and experimental results for all-n-type TFTs indicate that the proposed pixel circuit reduced the nonuniformity brightness problem effectively by compensating for threshold voltage variation in TFTs and reduced the degradation of emission efficiency in OLEDs.     

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.     

Functional Pixel Circuits for Elastic AMOLED Displays

While fabrication of active matrix organic LED (AMOLED) displays on plastic substrates continues to face technological challenges, stable electrical operation of thin-film transistor (TFT) pixel circuits under mechanical stress induced by substrate bending remains a critical issue. This paper investigates strain-induced shifts in hydrogenated amorphous silicon TFT characteristics and the compound impact on TFT circuit behavior. Measurements show that the magnitude of the shifts is determined by the direction of current flow in the TFT with respect to the bending stress orientation as well as bias conditions. Physically based compact models are developed that relate device characteristics to material behavior for design and optimization of AMOLED pixel circuits that can maintain immunity to bending stress. In particular, current mirror-based pixel circuits are presented that compensate for the long term threshold voltage shift and instantaneous strain-induced shifts in device characteristics.     

A New Pixel Circuit Compensating for Brightness Variation in Large Size and High Resolution AMOLED Displays

A new pixel design and driving method for active-matrix organic light-emitting diode (AMOLED) display using low-temperature polycrystalline silicon thin-film transistor (LTPS-TFT) is proposed. The new circuit consists of five TFTs and one capacitor to eliminate the variation in the threshold voltage of the TFTs, and the drop in the supply voltage in a single frame operation. The proposed pixel circuit has been verified to realize uniform output current by the simulation work using HSPICE software. The simulated error rate of the output current is also discussed in this paper. The novel pixel design has great potential for use in large size and high resolution AMOLED displays.     

一种新型的电压型AMOLED像素补偿电路

针对常用3T1C结构的AMOLED像素补偿电路容易受到阈值电压漂移影响的问题,介绍了一种新型电压型4T2C结构的AMOLED像素补偿电路,阐明了该电路的工作原理,并对OLED的驱动电流作了定性分析。结果显示该驱动电流值只与驱动电压及电源电压有关,从而有效地解决了因阈值电压变化而造成的显示器亮度不均匀的问题。该电路已通过Spectre仿真软件验证了其有效性。     

A Novel Voltage Driving Method Using 3-TFT Pixel Circuit for AMOLED

A novel voltage driving method using three thin-film transistors (TFTs) for active-matrix organic light-emitting diodes (OLEDs) is presented and verified by automatic integrated circuit modeling SPICE simulation. The proposed novel 3-TFT pixel circuit, which successfully compensates for the threshold voltage variations, uses few TFTs with simplified control signals, and the current nonuniformity of the proposed circuit is 0.19% to 1.99% throughout the entire data range. To compensate for variations in OLED current, the proposed circuit utilizes a novel driving scheme that uses a diode connection current source with a biased voltage.     

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.     

Enhancement of Brightness Uniformity by a New Voltage-Modulated Pixel Design for AMOLED Displays

This letter presents a new pixel design and driving method for active-matrix organic light-emitting diode (AMOLED) displays using low-temperature polycrystalline silicon thin-film transistors (TFTs). The proposed pixel circuit consists of five TFTs and one capacitor to eliminate the variation in the threshold voltage of the TFTs, and the drop in the supply voltage in a single frame operation by the source-follower-type connection and the bootstrap. The proposed pixel circuit has been verified to realize uniform output current by the simulation work using the HSPICE software. The novel pixel design has great potential for use in large-size and high-resolution AMOLED displays.     

AMOLED Pixel Circuit With Electronic Compensation of Luminance Degradation

A new voltage-programmed pixel circuit using hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs) for active-matrix organic light-emitting diodes (AMOLEDs) is presented. In addition to compensating for the shift in threshold voltage of TFTs, the circuit is capable of compensating for OLED luminance degradation by employing the shift in OLED voltage as a feedback of OLED degradation     

A Novel LTPS-TFT Pixel Circuit Compensating for TFT Threshold-Voltage Shift and OLED Degradation for AMOLED

This letter presents a novel pixel circuit that uses low-temperature polycrystalline-silicon thin-film transistors (LTPS-TFTs) composed of one driving and four switching TFTs for active-matrix organic light-emitting diodes (AMOLEDs) with a voltage-source method. The proposed circuit effectively enables threshold-voltage-shift correction of the drive TFT and compensates for degradation of the OLED using a feedback structure     

Accelerated stress testing of a-Si:H pixel circuits for AMOLED displays

Electronics reliability testing is traditionally carried out by accelerating the failure mechanisms using high temperature and high stress, and then predicting the real-life performance with the Arrhenius model. Such methods have also been applied to organic light-emitting diode (OLED) testing to predict lifetimes of tens of thousands of hours. However, testing the active matrix OLED thin-film transistor (TFT) backplane is a unique and complex case where standard accelerated testing cannot be directly applied. This is because the failure mechanism of pixel circuits is governed by multiple material and device effects, which are compounded by the self-compensating nature of the circuits. In this paper, we define and characterize the factors affecting the primary failure mechanism and develop a general method for accelerated stress testing of TFT pixel circuits in a-Si AMOLED displays. The acceleration factors derived are based on high electrical and temperature stress, and can be used to significantly reduce the testing time required to guarantee a 30 000-h display backplane lifespan.     

Parallel Addressing Scheme for Voltage-Programmed Active-Matrix OLED Displays

This paper presents a novel parallel addressing scheme for voltage-programmed active-matrix organic light-emitting diode (OLED) displays which provides high precision recovery of the threshold voltage shift. As a result, the uniformity over the panel is significantly improved. In addition, a new pixel circuit is presented that is capable of providing a predictably higher current as the pixel ages, so as to compensate for the OLED luminance degradation     

Transparent Electronics for See-Through AMOLED Displays

Transparent thin-film-transistors (TFTs) with a channel semiconductor based on the zinc–tin–oxide (ZTO) system are presented. Specifically, the technological and material aspects of the plasma-assisted pulsed laser deposition of these materials are discussed. The supply of additional radical oxygen species will be evidenced to significantly reduce defects in the material and as a consequence allows for well-behaved n-channel TFTs with mobilities higher than 10 ${hbox{cm}}^{2} {hbox{V}} ^{-1} {hbox{s}} ^{-1}$ and a threshold voltage in the range of 0 V. In addition the devices are extremely stable versus bias/current stress, which is especially important for active matrix OLED applications. Based on a detailed understanding of the interaction of the TFT channels with oxygen a strategy for the thin-film encapsulation of the TFTs will be presented, which leaves their device characteristics unaffected.      

Stability analysis of current programmed a-Si:H AMOLED pixel circuits

In this paper, we present self-compensating current mirror-based pixel circuits, and analyze basic stability issues to provide a deeper understanding of circuit operation, and the impact of thin film transistor bias nonidealities, which can lead to the long-term (and gradual) instabilities in pixel drive current. The analysis also provides the circuit designer a means to tailor the pixel drive current stability to the long-term brightness degradation characteristics of the organic light-emitting diode.     

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.     

中小尺寸AMOLED面临的挑战

有源矩阵有机发光二极管显示器（AMOLED）在实现大规模生产的过程中仍然需要解决屏幕显示（front-of-screen，FOS）效果、基础设施的成本和良品率控制等问题。本文就上述问题提出了解决方案，包括实现更卓越的显示效果，降低制造成本以及增加良品率。     

A Current-Mode Comparator for Digital Calibration of Amorphous Silicon AMOLED Displays

We present a digital calibration driving scheme for stabilizing the uniformity of large area amorphous silicon active-matrix organic light emitting diode displays. A current-mode analog-to-digital converter (ADC) is used to extract the differential aging of the individual pixels. For the ADC, a configurable current comparator is designed that employs the output buffer of the source driver to reduce the die area. The comparator and pixel circuit were fabricated in 0.8-$mu$m high-voltage CMOS and amorphous silicon technologies, respectively. Analysis and measurement results show a calibration refresh time of 2 s for a high-definition display (1920$,times,$RGB$,times,$1080). Moreover, the pixel current is highly stable despite a 5-V shift in the threshold voltage of the thin-film transistor driver.      

Driving schemes for a-Si and LTPS AMOLED displays

Design of stable active matrix organic light-emitting diode (AMOLED) displays comes with significant challenges that stem from the electrical property of the backplane materials, line parasitics in the matrix, and the opto-electronic property of the organic light-emitting diode (OLED). This paper reviews voltage and current programming schemes for AMOLEDs. Following a systematic review of pixel circuits, design considerations are examined for both current and voltage schemes with focus on stability and programming speed for both amorphous silicon (a-Si) and low temperature polysilicon (LTPS) pixel circuits. In particular, spatial parameter variations and stability, which hinder reliable operation of AMOLED display backplanes, are discussed. Analysis shows that while driving schemes reported hitherto maybe suitable for small and medium size displays, new schemes are critically needed for large-area high-resolution AMOLED displays.     

具有容错功能的OLED有源驱动电路

以有机电致发光器件(OLED)为基础的显示或照明器件,通常会受到短路故障的影响,从而使得像素失效,降低面板的亮度,进而严重地影响亮度的均匀性,并且会产生大量的功耗.本文介绍了一种新的有源OLED驱动电路,以自动检测在OLED中发生的短路故障并切换至备用OLED.该电路采用p型低温多晶硅薄膜晶体管制造.当发生短路故障时,本电路可以在不改变驱动电流的情况下,保持OLED像素的亮度维持不变.实验结果表明,本电路不仅具有容错功能,而且与标准电路相比可以显著地降低功耗.     

基于改善AMOLED TFT均匀性和稳定性像素补偿电路

有机发光二极管显示器(OLED)正越来越多地用于中小尺寸的显示,但在大尺寸方面进展缓慢,因为在有源大尺寸方面对OLED的稳定性和均匀性要求较高,需要设计像素补偿电路。各研究机构提出了像素补偿电路用于改善OLED的均匀性和稳定性等问题,文中对目前采用有源OLED的α-Si TFT和p-Si TFT的各种像素补偿电路进行了分析。分析结果表明,文中设计方案取得了一定的效果,但尚存不足。