A novel pixel memory using integrated voltage‐loss‐compensation (VLC) circuit for ultra‐low‐power TFT‐LCDs

Abstract— A novel pixel memory using an integrated voltage‐loss‐compensation (VLC) circuit has been proposed for ultra‐low‐power TFT‐LCDs, which can increase the number of gray‐scale levels for a single subpixel using an analog voltage gray‐scale technique. The new pixel with a VLC circuit is integrated under a small reflective electrode in a high‐transmissive aperture‐ratio (39%) 3.17‐in. HVGA transflective panel by using a standard low‐temperature‐polysilicon process based on 1.5‐μm rules. No additional process steps are required. The VLC circuit in each pixel enables simultaneous refresh with a very small change in voltage, resulting in a two‐orders‐of‐magnitude reduction in circuit power for a 64‐color image display. The advanced transflective TFT‐LCD using the newly proposed pixel can display high‐quality multi‐color images anytime and anywhere, due to its low power consumption and good outdoor readability.     

Amorphous‐oxide TFT backplane for large‐sized AMOLED TVs

Abstract— Amorphous‐oxide thin‐film‐transistor (TFT) arrays have been developed as TFT backplanes for large‐sized active‐matrix organic light‐emitting‐diode (AMOLED) displays. An amorphous‐IGZO (indium gallium zinc oxide) bottom‐gate TFT with an etch‐stop layer (ESL) delivered excel lent electrical performance with a field‐effect mobility of 21 cm²/V‐sec, an on/off ratio of >10⁸, and a subthreshold slope (SS) of 0.29 V/dec. Also, a new pixel circuit for AMOLED displays based on amorphous‐oxide semiconductor TFTs is proposed. The circuit consists of four switching TFTs and one driving TFT. The circuit simulation results showed that the new pixel circuit has better performance than conventional threshold‐voltage (VTH) compensation pixel circuits, especially in the negative state. A full‐color 19‐in. AMOLED display with the new pixel circuit was fabricated, and the pixel circuit operation was verified in a 19‐in. AMOLED display. The AMOLED display with a‐IGZO TFT array is promising for large‐sized TV because a‐IGZO TFTs can provide a large‐sized backplane with excellent uniformity and device reliability.     

Static active‐matrix OLED display without pixel refresh enabled by amorphous‐silicon non‐volatile memory

Abstract— An active‐matrix organic light‐emitting‐diode (AMOLED) display which does not require pixel refresh is demonstrated. This was achieved by replacing the thin‐film transistor (TFT) that drives the OLED with a non‐volatile memory TFT, in a 2‐transistor pixel circuit. The threshold voltage of the non‐volatile‐memory TFT can be changed by applying programming voltage pulses to the gate electrode. This approach eliminates the need for storage capacitors, increases the pixel fill factor, and potentially reduces power consumption. Each pixel can be individually programmed or erased using a standard active‐matrix addressing scheme. The programmed image is stored in the display even if power is turned off.     

A novel TFT‐OLED integration for OLED‐independent pixel programming in amorphous‐Si AMOLED pixels

Abstract— The direct voltage programming of active‐matrix organic light‐emitting‐diode (AMOLED) pixels with n‐channel amorphous‐Si (a‐Si) TFTs requires a contact between the driving TFT and the OLED cathode. Current processing constraints only permit connecting the driving TFT to the OLED anode. Here, a new “inverted” integration technique which makes the direct programming possible by connecting the driver n‐channel a‐Si TFT to the OLED cathode is demonstrated. As a result, the pixel drive current increases by an order of magnitude for the same data voltages and the pixel data voltage for turn‐on drops by several volts. In addition, the pixel drive current becomes independent of the OLED characteristics so that OLED aging does not affect the pixel current. Furthermore, the new integration technique is modified to allow substrate rotation during OLED evaporation to improve the pixel yield and uniformity. The new integration technique is important for realizing active‐matrix OLED displays with a‐Si technology and conventional bottom‐anode OLEDs.     

Electro‐optic properties of an intrinsic half‐V‐mode FLCD and its application to field‐sequential full‐color LCDs using a poly‐Si TFT matrix array

Abstract— An intrinsic half‐V‐mode ferroelectric liquid‐crystal display (FLCD) exhibiting a high contrast ratio (300:1), owing to defect‐free gray‐scale capability, with a high response speed (τ ? 400 μsec) and good switchability with TFTs, has been developed. Furthermore, this FLCD features high‐temperature reliability owing to the use of a special hybrid alignment technique. We successfully fabricated an active‐matrix poly‐Si TFT field‐sequential full‐color (FS FC) LCD with XGA specifications and a 0.9‐in. diagonal using a half‐V‐mode FLCD and an RGB light‐emitting‐diode (LED) array microdisplay. It is shown that the fabricated active‐matrix FS FCLCD exhibits good moving‐image performance with high full‐color display capability.     

Active‐matrix organic light‐emitting displays employing two thin‐film‐transistor a‐Si:H pixels on flexible stainless‐steel foil

Abstract— An active‐matrix organic light‐emitting diode (AMOLED) display driven by hydrogenated amorphous‐silicon thin‐film transistors (a‐Si:H TFTs) on flexible, stainless‐steel foil was demonstrated. The 2‐TFT voltage‐programmed pixel circuits were fabricated using a standard a‐Si:H process at maximum temperature of 280°C in a bottom‐gate staggered source‐drain geometry. The 70‐ppi monochrome display consists of (48 × 4) × 48 subpixels of 92 ×369 μm each, with an aperture ratio of 48%. The a‐Si:H TFT pixel circuits drive top‐emitting green electrophosphorescent OLEDs to a peak luminance of 2000 cd/m².     

Novel analog feedback current programming architecture compatible with 2‐transistor 1‐capacitor pixel for active matrix organic light‐emitting diode displays

A new feedback current programming architecture is described, which is compatible with active matrix organic light‐emitting diode (AMOLED) displays having the 2T1C pixel structure. The new pixel programming approach is compatible with all TFT technologies and can compensate for non‐uniformities in both threshold voltage and carrier mobility of the pixel OLED drive TFT. Based on circuit simulations, a pixel drive current of less than 10 nA can be programmed in less than 50 µ. This new approach can be implemented within an AMOLED external or integrated display data driver.     

Vt Compensated voltage‐data a‐Si TFT AMOLED pixel circuits

Abstract— Active‐matrix organic light‐emitting‐diode (AMOLED) displays are now entering the marketplace. The use of a thin‐film‐transistor (TFT) active matrix allows OLED displays to be larger in size, higher in resolutions and lower in power dissipation than is possible using a conventional passive matrix. A number of TFT active‐matrix pixel circuits have been developed for luminance control, while correcting for initial and electrically stressed TFT parameter variations. Previous circuits and driving methods are reviewed. A new driving method is presented in which the threshold‐voltage (V_t) compensation performance, along with various circuit improvements for amorphous‐silicon (a‐Si) TFT pixel circuits using voltage data, are discussed. This new driving method along with various circuit improvements is demonstrated in a state‐of‐the‐art 20‐in. a‐Si TFT AMOLED HDTV.     

A low‐cost low‐temperature thin‐film‐transistor backplane based on oxide semiconductor

Abstract— An indium‐gallium‐zinc‐oxide (IGZO) thin‐film transistor (TFT) based on an anodized aluminum‐oxide gate dielectric and photoresist passivation has been fabricated. The TFT showed a field‐effect mobility of as high as 18 cm²/V‐sec and a threshold voltage of only 0.5 V. A 50 × 50 AMOLED display based on this type of TFT was designed and fabricated. The average luminance of the panel was 150 cd/m², and the maximum pixel luminance was 900 cd/m².     

Dual‐plate OLED display (DOD) based on amorphous‐silicon (a‐Si) TFT backplane

Abstract— An improved AMOLED with an a‐Si TFT backplane based on a unique structure is reported. The new structure is refered to as a dual‐plate OLED display (DOD). While a top‐emission OLED array is directly fabricated on a TFT backplane, the DOD consists of an upper OLED substrate and a lower TFT substrate, which are independently fabricated. Because the OLED substrate, which is fabricated through the process flow of bottom emission, is attached to the TFT substrate, the light is emitted in the opposite direction to the TFT backplane. The DOD enables the design of large‐sized TFTs and a complicated pixel circuit. It can also not only achieve higher uniformity in luminance in large‐sized displays due to the low electrical resistance of the common electrode, but also wider viewing angles.     

Design of large‐area OLED displays utilizing seamless tiled components

Abstract— In recent years, the majority of R&D for large‐area displays has been to serve the production of large monolithic substrate technology such as plasma‐display panels (PDPs) and TFT‐LCD. While the pursuit of large displays for domestic and light‐industrial use benefits from the production of these high‐quality high‐pixel‐count technologies, there is still a need to produce displays in formats other than 4:3 or 16:9 and on a larger scale than currently available in single‐substrate displays. The options that exist for producing tiled displays from emerging technologies is examined and a practical technique for creating large‐area (1.8 × 1.2 m and larger) monochrome or color displays from tiling smaller units is discussed. This presents a cost‐effective approach for arranging small tiles to create a much larger screen and offers a simple way to address the market gap between large monolithic displays and small conventional LED video‐wall displays, in the size range of 1–10‐mm pixels for advertising and industrial use. By examining the requirements for pixel size and pitch against the range of viewing distances commonly associated with the target markets for these displays, it will be shown that complex manufacturing is not always required.     

Low‐temperature poly‐Si TFT‐LCD with an integrated analog circuit

We have developed a 6‐bit D/A converter and amplifier integrated low‐temperature poly‐Si TFT‐LCD in which an integrated signal‐line driver is driven by a 5‐V power supply. We have employed a D/A converter including a new capacitor array and an original amplifier comprised of serially connected comparators to achieve high accuracy. The D/A converter performs gamma correction using upper significant bits of input data. Control signals for these circuits were generated by the integrated timing circuit. These advances in integration have been achieved for the first time using 3‐μm design rule and improved LTPS TFT technologies and provide an advanced display system with lower power consumption, smaller module size, and higher durability.     

12.1‐in. WXGA AMOLED display driven by InGaZnO thin‐film transistors

Abstract— A full‐color 12.1‐in.WXGA active‐matrix organic‐light‐emitting‐diode (AMOLED) display was, for the first time, demonstrated using indium‐gallium‐zinc oxide (IGZO) thin‐film transistors (TFTs) as an active‐matrix backplane. It was found that the fabricated AMOLED display did not suffer from the well‐known pixel non‐uniformity in luminance, even though the simple structure consisting of two transistors and one capacitor was adopted as the unit pixel circuit, which was attributed to the amorphous nature of IGZO semiconductors. The n‐channel a‐IGZO TFTs exhibited a field‐effect mobility of 17 cm²/V‐sec, threshold voltage of 1.1 V, on/off ratio >10⁹, and subthreshold gate swing of 0.28 V/dec. The AMOLED display with a‐IGZO TFT array is promising for large‐sized applications such as notebook PCs and HDTVs because the a‐IGZO semiconductor can be deposited on large glass substrates (larger than Gen 7) using the conventional sputtering system.     

In‐cell capacitive‐type touch sensor using LTPS TFT‐LCD technology

Abstract— An LTPS TFT‐LCD with an in‐cell capacitive‐type touch sensor has been proposed and prototyped. The embedded sensor in the pixel was designed to amplify the voltage change caused by capacitive coupling between the detection electrode and conductive object (user's finger). No touch force is needed for sensor actuation and no extra electrical connection for the counter‐substrate is needed. The validity of the observed voltage difference of the sensor output on the TFT substrate was examined. The proposed architecture is considered to be applicable to larger LCDs for various applications such as smartphones, automotive navigation systems, and mobile internet devices.     

A fast 2‐transistor current‐mode AMOLED pixel

Abstract— Active‐matrix OLEDs are thinner and potentially more energy efficient than AMLCDs; however, most current AMOLED pixel designs are also excessively complicated. This paper compares the operating principles and performance of the four basic types of OLED pixels: converter pixels, system compensation pixels, compensated pixels, and current‐mode pixels. A new current‐mode pixel is described that is fast, provides excellent compensation for processing variations, and yet retains the simplicity and manufacturing advantages of the simplest 2‐transistor OLED pixels. Like other current‐mode pixels, this sequential current mirror pixel provides excellent compensation for variations in TFT V^t, TFT mobility, and non‐uniformities in the OLED itself. However, unlike other current‐mode pixels that are too slow for use in large displays, this sequential current mirror pixel can operate with a voltage precharge in a superlinear mode to reduce data line settling delays to less than 3 μsec. Like the simplest uncompensated pixels, the compensated sequential current mirror pixel requires only two TFTs, a single data line, and a single select line.     

TFT‐LCD driver IC with embedded non‐volatile memory for portable applications

Abstract— To improve the display quality and yield of the TFT‐LCD driver IC, non‐volatile multiple‐time‐programmable (MTP) memory, which consists of an EEPROM cell and our proposed sense amplifier and power control circuit (SP), was integrated into a TFT‐LCD driver IC. The proposed SP has a 30% smaller layout area and a 18% faster response time compared to that of the conventional SP. The proposed SP also has lower power consumption because it does not use a static current. The TFT‐LCD quality was also improved by tuning the characteristics of the driver IC and the panel with the VREF, OSC, and VCOM blocks, using non‐volatile MTP memory. When the display quality improved, the yield also improved, along with a reduction in the failure ratio of the display module, which consists of the driver IC and the panel. As a result, the TFT‐LCD driver IC with the non‐volatile MTP memory demonstrated improved display quality and a higher yield compared to conventional driver ICs without such a memory.     

Compact modeling of amorphous‐silicon thin‐film transistors with BSIM3

Abstract— A novel approach of modeling a‐Si:H TFTs with the industry‐standard BSIM3 compact model is presented. The described approach defines the a‐Si:H TFT drain current and terminal charges as explicit functions of terminal voltages using a minimum set of BSIM3 parameters. The set of BSIM3 parameters is chosen based on the electrical and physical characteristics of the a‐Si:H TFT and their values extracted from measured data. By using the selected BSIM3 model parameters, the a‐Si:H TFT is simulated inside SPICE to fit the simulated I‐V and C‐V curves with the measured results. Finally, the extracted BSIM3 model is validated by simulating the kickback voltage effect in an AMLCD pixel array.     

Novel a‐Si:H TFT pixel circuit for electrically stable top‐anode light‐emitting AMOLEDs

Abstract— A novel pixel circuit for electrically stable AMOLEDs with an a‐Si:H TFT backplane and top‐anode organic light‐emitting diode is reported. The proposed pixel circuit is composed of five a‐Si:H TFTs, and it does not require any complicated drive ICs. The OLED current compensation for drive TFT threshold voltage variation has been verified using SPICE simulations.     

Threshold‐voltage‐compensation methods for AMOLED pixel and analog buffer circuits

Abstract— New pixel‐circuit designs for active‐matrix organic light‐emitting diodes (AMOLEDs) and a new analog buffer circuit for the integrated data‐driver circuit of active‐matrix liquid‐crystal displays (AMLCDs) and AMOLEDs, based on low‐temperature polycrystalline‐silicon thin‐film transistors (LTPS‐TFTs), were proposed and verified by SPICE simulation and measured results. Threshold‐voltage‐compensation pixel circuits consisting of LTPS‐TFTs, an additional control signal line, and a storage capacitor were used to enhance display‐image uniformity. A diode‐connected concept is used to calibrate the threshold‐voltage variation of the driving TFT in an AMOLED pixel circuit. An active load is added and a calibration operation is applied to study the influences on the analog buffer circuit. The proposed circuits are shown to be capable of minimizing the variation from the device characteristics through the simulation and measured results.     

A pixel structure for simultaneous programming and emission method for shutter‐glasses‐type stereoscopic 3‐D AMOLED displays

Abstract— A pixel structure for shutter‐glasses‐type stereoscopic 3‐D active‐matrix organic light‐emitting‐diode (AMOLED) displays is proposed. The proposed pixel programs data to the pixel during the light‐emission time of an OLED. Because the emission time of the proposed pixel is extended, it is expected that the proposed pixel not only decreases the peak current of the OLED during the emission period but also reduces flicker. Moreover, the aperture ratio of the proposed pixel is 58.69% for a 50‐in. full‐high‐definition (FHD) condition by minimizing the number of thin‐film transistors (TFTs), capacitors, and control signal lines as seven TFTs, two capacitors, two power lines, and four control lines per unit pixel. Simulation results show that the error in the emission current of the proposed pixel is from ?0.82% to +0.90% when the threshold‐voltage variation of the driving TFT is ±1.00 V, and the maximum variation of the emission current is ?1.35% when a voltage drop in the power line is ?0.50 V on a full‐white‐image display.