Active-Matrix Amorphous-Silicon TFTs Arrays at 180$^circhboxC$on Clear Plastic and Glass Substrates for Organic Light-Emitting Displays

An amorphous-silicon thin-film transistor (TFT) process with a 180$^circhboxC$maximum temperature using plasma-enhanced chemical vapor deposition has been developed on both novel clear polymer and glass substrates. The gate leakage current, threshold voltage, mobility, and on/off ratio of the TFTs are comparable with those of standard TFTs on glass with deposition temperature of 300$^circhboxC$–350$^circhboxC$. Active-matrix pixel circuits for organic light-emitting displays (LEDs) on both glass and clear plastic substrates were fabricated with these TFTs. Leakage current in the switching TFT is low enough to allow data storage for video graphics array timings. The pixels provide suitable drive current for bright displays at a modest drive voltage. Test active matrices with integrated polymer LEDs on glass showed good pixel uniformity, behaved electrically as expected for the TFT characteristics, and were as bright as 1500$hboxcd/hboxm^2$.     

高性能钆铝锌氧薄膜晶体管的制备

本文研究并制备了钆铝锌氧薄膜和以钆铝锌氧为有源层的薄膜晶体管。钆铝锌氧薄膜材料的光致发光光谱和透过率说明钆铝锌氧薄膜在透明显示方向的应用潜力。透射电子显微镜揭示了钆铝锌氧薄膜的非晶态微观结构。钆铝锌氧薄膜晶体管显示了良好的转移特性和输出特性。器件开关比大于10~5、饱和迁移率约为10cm~2·V~(-1)·s~(-1)。实验结果表明,钆铝锌氧薄膜可用作氧化物薄膜晶体管的有源层材料;钆铝锌氧薄膜晶体管可作为像素电路的驱动器件。     

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.     

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。     

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 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.     

低温多晶硅TFT技术的发展

本文综述低温多晶硅（LTPS）TFT技术的最新进展情况。该技术目前的研究前沿是：（1）制作高性能的TFT；（2）在柔性衬底上制作LTPS TFT；（3）驱动有机发光器件的TFT；（4）LTPS TFT的新应用。同时还展望了LTPS TFT技术的未来发展趋势。     

Does TFT mobility impact pixel size in AMOLED backplanes?

Recent advances in organic light emitting diode (OLED) device efficiencies are making the amorphous silicon (a-Si) backplane a viable solution for a large range of display sizes. This paper presents the possibilities and design challenges of a-Si active-matrix organic light-emitting-diode (AMOLED) backplanes for applications ranging from small full color cell phone displays to HDTV screens. An analytical model for the minimum pixel-area, and hence the maximum resolution for both bottom and top emitting AMOLED architectures is presented in terms of the a-Si thin-film transistor (TFT) device parameters, the process design-rules, and the pixel circuit parameters. It is established that the lower device mobility of a-Si TFTs is no longer the limiting factor. For instance, in a 20' W/SXGA panel with full color red-green-blue subpixels, the state-of-the-art TFT processes yield a square pixel size of /spl sim/266 /spl mu/m. Further, quantitative analysis of charge-injection/charge-feedthrough error in the pixel, and the maximum allowable leakage current for the TFT is also presented.     

High-voltage TFT fabricated in recrystallized polycrystallinesilicon

High-voltage thin-film transistors (TFTs) fabricated using CW-Ar laser annealed polycrystalline silicon have an offset gate structure between the source and gate and between the gate and drain. The breakdown voltage, transconductance, and leakage current in various size TFTs are described. These TFTs exhibited n-channel enhancement characteristics with a low-threshold voltage, and a breakdown voltage above 100 V could be obtained at an offset gate length of 20 μm. Active TFT circuits were fabricated with these high-voltage Si TFTs. These high-voltage TFT circuits can drive thin-film EL (electroluminescent display) at low signal voltage     

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 current scaling pixel circuit for AMOLED

A new active-matrix organic light-emitting diode (AMOLED) pixel design, composed of four polycrystalline silicon thin-film transistor (poly-Si TFT) and one capacitor, is proposed by employing a novel current scaling scheme. The simulation results, based on the measured characteristics of an OLED and poly-Si TFTs, show that the proposed pixel design would scale down the data current more effectively, so as to guarantee a lower charging time compared with the conventional current mirror structure, as well as successfully compensate the variation of the electrical characteristics of the poly-Si TFTs, such as the threshold voltage and mobility.     

Stability of amorphous-silicon TFTs deposited on clear plastic substrates at 250/spl deg/C to 280/spl deg/ C

Amorphous-silicon (a-Si) thin-film transistors (TFTs) were fabricated on a free-standing new clear plastic substrate with high glass transition temperature (T/sub g/) of >315/spl deg/ C and low coefficient of thermal expansion of <10 ppm/ /spl deg/ C. Maximum process temperatures on the substrates were 250/spl deg/C and 280/spl deg/C, close to the temperatures used in industrial a-Si TFT production on glass substrates. The first TFTs made at 280/spl deg/C have dc characteristics comparable to TFTs made on glass. The stability of the 250/spl deg/C TFTs on clear plastic is approaching that of TFTs made on glass at 300/spl deg/C-350/spl deg/C. TFT characteristics and stability depend only on process temperature and not on substrate type.     

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.     

The Electrical Properties of Unidirectional Metal-Induced Lateral Crystallized Polycrystalline-Silicon Thin-Film Transistors

It has been known that adjacent Pd enhances the crystallization rate in Ni metal-induced lateral crystallization (Ni-MILC) and this knowledge has been used to fabricate the unidirectional MILC thin-film transistors (TFTs), which eliminate the boundary formed at the center of TFT channel in a normal MILC TFTs. It is discovered that the MILC/MILC boundary (MMB) is responsible for the high leakage current and low field- effect mobility. The electrical properties of unidirectional MILC TFTs (Width/Length = 10/10 mum) improved considerably comparing to those of MILC TFTs containing the MMB. The leakage current and field-effect mobility, which have been regarded as obstacles for industrialization of the MILC process, measure to be 2.1 X 10-11 A and 83 cm²/ V ldr s, respectively.     

用大电流驱动采用SELAX技术的高分辨率显示系统的整体可靠性

应用低压多晶硅TFT和高压多晶硅TFT组合开发成功高分辨率（〉300ppi）LCD／OEL显示系统，也就是说，将受激准分子激光结晶化（ELC）TFT用于耐高压显示电路，而将选择扩大激光结晶化（SELAX）TFT用于低功耗高性能显示电路。在这两种混合型的TFT中，扩大激光结晶化（SELAX）TFT成功地整合了显示系统大电流驱动的可靠性。     

Fully self-aligned tri-layer a-Si:H thin-film transistors withdeposited doped contact layer

We demonstrate a new self-aligned TFT process for hydrogenated amorphous silicon thin-film transistors (a-Si:H TFTs). Two backside exposure photolithography steps are used to fabricate fully self-aligned tri-layer TFTs with deposited n⁺ contacts. Since no critical data alignment is required, this simple process is well suited to fabrication of short channel TFTs. We have fabricated fully self-aligned tri-layer a-Si:H TFTs with excellent device performance, and contact overlaps <1 μm. For a 20-μm channel length TFT with an a-Si:H thickness of 13 nm, the linear region (V_DS=0.1 V) and saturation region (V_DS=25 V) extrinsic mobility values are both 1.2 cm²/V-s, the off currents are <1 pA, and the on/off current ratio is >10⁷     

High-performance poly-Si TFTs with ECR-plasma hydrogen passivation

High-performance thin-film transistors (TFTs) with electron-cyclotron resonance (ECR) plasma hydrogen passivation fabricated by the use of laser-recrystallized multiple-strip-structure poly-Si film are discussed. These TFTs have n-channel enhancement-mode characteristics with a large transconductance, a high switching ratio, and a threshold voltage as low as 0.4 v. The ECR-plasma hydrogen passivation of laser-recrystallized poly-Si, reduces the trap density of poly-Si and increases the carrier mobility thus, desirable TFT characteristics are obtained. This passivation increased the transconductance (g_m) of a TFT and decreased the leakage current between the source and the drain. As a result, a switching ratio as high as 2.5×10⁹ and very low leakage current of the order of 10¹⁴ A can be achieved by these high-performance TFTs     

N-channel thin-film transistors constructed on plastic by solution processes of HgSe nanocrystals

We demonstrate bottom-gate thin-film transistors (TFTs) based on solution-processed HgSe nanocrystals (NCs) on plastic substrates. Solid films made of spin-coated HgSe NCs were heated at a temperature of 150 °C for 15 min to maximize the magnitude of their current, and these films were utilized as the channel layers of TFTs. A representative TFT with a bottom-gate Al₂O₃ layer operated as a depletion-mode one with an n-channel, exhibiting a field effect mobility of 3.9 cm²/Vs and an on/off current ratio of about 10². In addition, the electrical characteristics of the TFT on bent substrates are briefly described.     

High-performance poly-Si TFTs on plastic substrates using a nano-structured separation layer approach

We demonstrate a manufacturable, large-area separation approach for producing high-performance polycrystalline silicon thin-film transistors on flexible plastic substrates. The approach allows the use of high growth-temperature gate oxides and removes the need for hydrogenation. The process flow starts with the deposition of a nano-structured high surface-to-volume ratio film on a reuseable "mother" substrate. This film functions as a sacrificial release layer and is Si-based for process compatibility. After high-temperature TFT fabrication (up to 1100/spl deg/C) is carried to completion on the sacrificial film coated mother substrate, a thick plastic top layer film is applied, and the sacrificial layer is removed by chemical attack. By using this separation process, the temperature, smoothness, and mechanical limitations posed by plastic substrates are completely circumvented. Both excellent n-channel and p-channel TFTs on plastic have been produced. We report here on p-channel TFTs on separated plastic with a linear field effect (hole) mobility of 174 cm/sup 2//V/spl middot/s, on/off current ratio of >10/sup 8/ at V/sub ds/=-0.1 V, off current of <10/sup -11/ A//spl mu/m-channel-width at V/sub ds/=-0.1 V, sub-V/sub t/ swing of /spl sim/200 mV/dec, and threshold voltage of -1.1 V.     

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.