有源矩阵有机电致发光像素电路的研究进展

有源驱动方式的有机发光二极管（AMOLED）较之无源驱动方式易于实现高亮度和高分辨率、功耗更小,更适合大屏幕显示。但传统的两管驱动电路会出现驱动管阈值电压在整个屏幕上分布不均匀,或长时间加偏压后驱动管的阂值电压发生漂移。本文在两管驱动电路的基础上介绍了几种最近提出的补偿电路并描述了它们的改善效果及各自存在的问题。     

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.     

Mirrored OLED pixel circuit for threshold voltage and mobility compensation with IGZO TFTs

In this paper, pixel circuit using mirroring structure with Indium–Gallium–Zinc oxide (IGZO) thin film transistors (TFTs) for active matrix organic light emitting diode (AMOLED) display is proposed. This pixel circuit consists of only four TFTs, and one capacitor. Due to the mirroring structure, characteristic of the driving TFT can be precisely sensed by the sensing TFT, which is deployed in a discharging path for gate electrode of the driving TFT. This discharging process is strongly dependent on threshold voltage (V_T) and effective mobility of the sensing TFT. Circuit operating details are discussed, and compensation effects for threshold voltage shift and mobility variations are verified through numerical derivation and SPICE simulations. Furthermore, compared with conventional schematics, the proposed pixel circuit might have much simplified external driving circuits, and it is a promising alternative solution of high performance AMOLED display.     

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.     

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.     

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 Driving Scheme for Active-Matrix Organic Light-Emitting Diode Displays Based on Current Feedback

This paper presents a method of driving active-matrix organic light-emitting diode (AMOLED) displays with amorphous silicon (a-Si) thin-film transistors (TFTs). By using current feedback, the method effectively compensates for the effect of shift in the threshold voltage $(V_{T})$ of a-Si TFTs on the OLED current. A CMOS transresistance amplifier is used as the column driver to cancel the effect of large parasitic capacitance of data lines. An accelerating pulse is used at the start of the programming cycle to improve the settling at low currents. A detailed analysis has been done to investigate the effect of circuit components on the sensitivity of the OLED current to $V_{T}$ shift and the settling behavior of the circuit. Prototypes of pixel circuits and the transresistance amplifier were fabricated in an a-Si TFT process and a 0.8- $mu{hbox{m}}$ 20-V CMOS technology, respectively. Measurements show less than 5% change in the OLED current for 2.5-V shift in $V_{T}$ of TFTs. Settling times smaller than 50 $mu{hbox{s}}$ were achieved for parasitic capacitances of 50–200 pF and programming currents as small as 200 nA.      

Amorphous Silicon Display Backplanes on Plastic Substrates

Amorphous silicon (a-Si) thin-film transistor (TFT) backplanes are very promising for active-matrix organic light-emitting diode displays (AMOLEDs) on plastic. The technology benefits from a large manufacturing base, simple fabrication process, and low production cost. The concern lies in the instability of the TFTs threshold voltage (V_T) and its low device mobility. Although V_T-instability can be compensated by means of advanced multi-transistor pixel circuits, the lifetime of the display is still dependent on the TFT process quality and bias conditions. A-Si TFTs with field-effect mobility of 1.1 cm²/Vmiddots and pixel driver circuits have been fabricated on plastic substrates at 150 degC. The circuits are characterized in terms of current drive capability and long-term stability of operation. The results demonstrate sufficient and stable current delivery and the ability of the backplane on plastic to meet AMOLED requirements     

Self-Stabilization in Amorphous Silicon Circuits

Thin-film transistors (TFTs) based on disordered semiconductors such as amorphous hydrogenated silicon (a-Si:H) experience a threshold voltage (VT) shift with time in the presence of a gate bias. The VT shift needs to be compensated for circuit applications. We study an interesting property of self-compensation in fundamental analog TFT circuits with one a part of the circuit compensating for the effects of VT shift in the other and vice versa.      

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.     

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.     

AMOLED驱动管阈值的补偿方法综述

有机电致发光二极管(Organic Light Emitting Diode,OLED)在显示技术中得到越来越广泛的应用,特别是AMOLED已经成为新一代显示技术发展方向,但是一系列影响AMOLED显示质量和寿命的问题需要得到解决。其中,AMOLED的驱动TFT的阈值电压偏移严重影响了AMOLED的性能。该文综述了当前几种典型的AMOLED的像素驱动结构及其驱动TFT阈值电压的补偿方法,并详细介绍其补偿原理。     

High-speed low-power voltage-programmed driving scheme for AMOLED displays

A new voltage-programmed driving scheme named the mixed parallel addressing scheme is presented for AMOLED displays, in which one compensation interval can be divided into the first compensation frame and the consequent N-1 post-compensation frames without periods of initialization and threshold voltage detection. The proposed driving scheme has the advantages of both high speed and low driving power due to the mixture of the pipeline technology and the threshold voltage one-time detection technology. Corresponding to the proposed driving scheme, we also propose a new voltage-programmed compensation pixel circuit, which consists of five TFTs and two capacitors(5T2C). In-Zn-O thin-film transistors(IZO TFTs) are used to build the proposed 5T2C pixel circuit. It is shown that the non-uniformity of the proposed pixel circuit is considerably reduced compared with that of the conventional 2T1C pixel circuit. The number of frames(N) preserved in the proposed driving scheme are measured and can be up to 35 with the variation of the OLED current remaining in an acceptable range. Moreover, the proposed voltage-programmed driving scheme can be more valuable for an AMOLED display with high resolution, and may also be applied to other compensation pixel circuits.     

A new pixel circuit for driving organic light-emitting diode with low temperature polycrystalline silicon thin-film transistors

A new pixel circuit design for active matrix organic light-emitting diode (AMOLED), based on the low-temperature polycrystalline silicon thin-film transistors (LTPS-TFTs) is proposed and verified by SPICE simulation. Threshold voltage compensation pixel circuit consisting of four n-type TFTs, one p-type TFT, one additional control signal, and one storage capacitor is used to enhance display image quality. The simulation results show that this pixel circuit has high immunity to the variation of poly-Si TFT characteristics.     

Polysilicon thin-film transistors on polymer substrates

Different approaches to fabricate low-temperature polycrystalline silicon (LTPS) thin film transistors (TFTs) on polymer substrates are reviewed and the two main routes are discussed: (1) standard fabrication of LTPS TFTs on glass substrates followed by a transfer process of the devices on the polymeric substrate; (2) direct fabrication of the devices on the polymeric substrate. Among the different techniques we have described in more detail the process we have recently developed for the fabrication of LTPS TFTs directly on ultra-thin polyimide (PI) substrate. LTPS TFT technology is particularly suited for high performance flexible electronics applications, due to the excellent device characteristics, good electrical stability and CMOS technology. Flexible display application remains the most attractive application for LTPS technology, especially for AMOLED displays, where device stability and the possibility to integrate the driving circuits make LTPS technology superior to all the other competitive TFT technologies. Among the other applications, particularly promising is also the application to flexible smart sensors, where integration of a front-end electronics is essential. Some examples of flexible gas sensors and pressure sensors, integrated with simple readout electronics based on LTPS TFTs and fabricated on ultra-thin PI substrate, are presented.     

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.     

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.     

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.     

A Stable Voltage-Programmed Pixel Circuit for a-Si:H AMOLED Displays

Hydrogenated amorphous silicon (a-Si:H) active matrix organic light-emitting diode (AMOLED) displays are attractive given the potentially low manufacturing cost and ultimately low-temperature fabrication enabling using flexible substrates. Although the conventional two thin-film transistor (2-TFT) AMOLED voltage-programmed pixel circuit (VPPC) can provide high resolution and high yield, the 2-TFT VPPC is prone to image retention over time due to shift in the threshold voltage (V_T-shift) of a-Si:H TFTs. This paper presents a new driving scheme that not only preserves the simplicity of the 2-TFT VPPC, but also demonstrates high uniformity. Experimental results indicate that the current drop in the new driving scheme is less than 11% after 15 days of operation whereas it is over 50% for the conventional driving scheme. Moreover, the new driving scheme is less sensitive to temperature variations due to an internal feedback mechanism. After a 70% change in the temperature, the current in the conventional driving scheme increases by as much as 300%. However, the current in the driving scheme presented here is approximately constant     

Thick-layered etched-contact amorphous silicon transistors

We introduce a new thick-layered, etched-contact a-Si:H TFT (TLEC-TFT) structure which allows the use of thick a-Si:H layers without increasing the TFT contact resistance. This device facilitates the integration of high-performance TFTs and thick-layered photo-transistors in a-Si:H-based image sensors. The TLEC-TFT is fully compatible with the conventional TFT fabrication process and requires no extra masking steps. For low values of the drain-to-source voltage, our new TFT boosts the linear region current by two orders of magnitude over that of conventional TFTs with identically thick a-Si:H layers. By removing the adverse effects of contact resistance in transistors with thick a-Si:H layers, our TLEC-TFT design allows us to compare the performance of TFTs with thick and thin a-Si:H layers. We find that the width of the conduction-band tail decreases in thick-layered a-Si:H TFTs. This reduction in the width of the band tails results in an increase in the TFT mobility and subthreshold slope. Consequently, thick-layered, etched-contact TFTs possess higher overall current-drive capabilities compared to conventional, thin-layered TFTs. We present experimental evidence which correlates the width of the conduction-band tail to the density of as-deposited free carriers