Low‐power‐consumption TFT‐LCD with dynamic memory embedded in the pixels

A low‐power‐consumption thin‐film‐transistor liquid‐crystal display (TFT‐LCD) with dynamic memory cells embedded in each pixel using low‐temperature poly‐Si technology has been developed. By holding data in the memory, the operating rate of the data driver can be dramatically reduced to 4 Hz. Eight levels of gray scale with low power consumption can be achieved by using the area‐ratio gray‐scale method. This TFT‐LCD can be used for displaying fine still images, with low power consumption.     

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.     

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.     

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.     

Critical point defect detection for small‐pixel thin‐film transistor array applied to medical display

Thin‐film transistor (TFT) array testing technique has been used, which provides defect detection capability to control the yield of the TFT process. In the past, several defect inspection technologies have been developed and applied for the TFT array testing. When the TFT array pixel size is getting smaller and the resolution is higher, they also encounter the performance limitation on detecting the critical defect in this small‐pixel TFT array and facing a limited testing requirement. For medical display applications, the display pixels on an array panel are getting smaller and smaller; therefore, defect detection is getting more important and critical for managing yield with quality. In this study, a novel approach for defect detection was proposed. Here, the proposed voltage imaging technique is used for the TFT array test, and it provides better small‐pixel TFT array defect detection capability. The experimental results show that by using the voltage imaging technique, detecting critical point defect of TFT array can be effectively improved. And the detected small‐pixel size of TFT array panels can be smaller than 55 µm of an advanced medical display.     

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.     

An active‐matrix organic light‐emitting‐diode pixel circuit for the compensation of I × R voltage drop in power line

Abstract— A new voltage‐driving active‐matrix organic light‐emitting diode (AMOLED) pixel circuit is proposed to improve the display image‐quality of AMOLED displays. Because OLEDs are current‐driven devices, the I × R voltage drop in the power lines is evitable. Accordingly, the I × R voltage‐drop compensation scheme should be included in the pixel‐driving method when a voltage‐compensation method is used. The proposed pixel was designed for the compensation of an I × R voltage drop in the power lines as well as for the compensation of the threshold‐voltage non‐uniformity of low‐temperature polycrystalline‐silicon thin‐film transistors (LTPS TFTs). In order to verify the compensation ability of the proposed pixel, SPICE simulation was performed and compared with those of other conventional pixels. When the Vss voltage varies from 0 to 1 V, the drain current of the proposed pixel decreased by under 1% while that of conventional Vth compensation methods without Vss compensation decreased by over 60%. 2.2‐in. QCIF⁺ full‐color AMOLED displays, which employ the proposed pixel, have been also developed. It was verified by comparison of the display image quality with a conventional panel that our proposed panel successfully overcame the voltage‐drop problems in the power lines.     

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 reduced‐voltage differential signaling (RVDS) interface for chip‐on‐glass TFT‐LCD applications

Abstract— Reduced‐voltage differential signaling (RVDS) is a novel interface for TFT‐LCD panels with a chip‐on‐glass (COG) structure, which has a point‐to‐point topology and a voltage mode differential signaling scheme. The voltage‐driving interface scheme has advantages in high‐speed operation owing to its relatively small time constant for the resistive channel condition. And reduced‐voltage signaling can reduce the power consumption of a transmitter. The display source driver IC with an RVDS interface, which is fabricated by using a 0.25‐μm CMOS process with a 2.5‐V logic supply voltage, offers a high data rate up to 500 Mbps, low‐current consumption of 2.2 mA, and good EMI characteristics. Also, an RVDS interface has programmable options that control the bandwidth, system power, and EMI performance. Therefore, the RVDS interface is a competitive solution for low‐power, low‐cost, and slim notebook applications.     

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.     

Robust integrated shift register circuit over clock noises for in‐cell touch applications

This paper proposes an integrated shift register circuit for an in‐cell touch panel that is robust over clock noises. It is composed of 10 thin film transistors and 1 capacitor, and the time division driving method is adopted to prevent the negative effect of display signals on the touch sensing. Two pre‐charging nodes are employed for reducing the uniformity degradation of gate pulses over time. In particular, the proposed circuit connects a drain of the first pre‐charging node's pull‐up thin film transistor (TFT) to the positive supply voltage instead of clock signals. This facilitates to lower coupling noises as well as to clock power consumption. The simulation program with an integrated circuit emphasis is conducted for the proposed circuit with low temperature poly‐silicon TFTs. The positive threshold voltage that shifts up to 12 V at the first pre‐charging pull‐up TFT can be compensated for without the uniformity degradation of gate pulses. For a 60‐Hz full‐HD display with a 120‐Hz reporting rate of touches, the clock power consumption of the proposed gate driver circuit is estimated as 7.13 mW with 160 stages of shift registers. In addition, the noise level at the first pre‐charging node is lowered to ?28.95 dB compared with 2.37 dB of the previous circuit.     

Improved gray‐scale monochrome and color AMEL displays using analog pixel

Abstract— Active‐matrix electroluminescent (AMEL) microdisplays have been known for their numerous beneficial characteristics such as low weight, compactness, high brightness, and high contrast ratio. Beside these desirable characteristics, some of their drawbacks include difficulty in obtaining a high number of gray‐scale levels or a large number of colors, and interface‐electronics complexity. To address these drawbacks, AMEL displays using an analog addressing architecture have been developed. Utilizing this new driving scheme, 256 monochrome levels or 16 million colors are obtainable. Gray shade is proportional to an analog voltage stored on the hold node of each pixel. For color displays, each pixel is comprised of red, green, and blue subpixels arranged in vertical stripes, and can be sized independently to achieve the appropriate white balance. With the integration of control‐signal circuit blocks on the same substrate as the microdisplay, the number of input control signals is minimized and the display can be driven with very simple interface electronics. This results in low overall system cost, compact electronic packaging, and low power consumption. To accommodate most optical orientations, the display has built‐in modes to flip the image both vertically and horizontally. Additionally, the display supports multiple interlace addressing modes.     

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.     

Clamped‐inverter circuit architecture for luminescent‐period‐control driving of active‐matrix OLED displays

Abstract— An innovative pixel‐driving technology for high‐performance active‐matrix OLED flat‐panel displays is described. Called “clamped‐inverter circuit architecture,” it uses luminescent‐period‐control driving to reduce the inter‐pixel non‐uniformity caused by the device‐to‐device variability of low‐temperature poly‐Si TFTs. A prototype full‐color display shows a luminous deviation of less than 1.6%, which corresponds to only the LSB‐error in 6‐bit gray‐scale.     

Low‐temperature poly‐Si TFT data drivers for an SVGA a‐Si TFT‐LCD panel

Low‐temperature poly‐Si TFT data drivers for an SVGA a‐Si TFT‐LCD panel have been developed. The data drivers include shift registers, sample‐and‐hold circuits, and operational amplifiers, and drive LCD panels using a line‐at‐a‐time addressing method. To reduce the power consumption of the shift register, a dot‐clock control circuit has been developed. Using this circuit, the power consumption of the shift register has been reduced to 36% of that of conventional circuits. To cancel the offset voltage generated by the operational amplifier, an offset cancellation circuit for low‐temperature poly‐Si TFTs has been developed. This circuit is also able to avoid any unstable operation of the operational amplifier. Using this circuit, the offset voltage has been reduced to one‐third of the value without using the offset cancellation circuit. These data drivers have been connected to an LCD panel and have realized an SVGA display on a 12.1‐in. a‐Si TFT‐LCD panel.     

LTPS circuit integration for system‐on‐glass LCDs

Abstract— A 3.5‐in. QVGA‐formatted driving‐circuit fully integrated LCD has been developed using low‐temperature poly‐Si (LTPS) technology. This display module, in which no external ICs are required, integrates all the driving circuits for a six‐bit RGB digital interface with an LTPS device called a “FASt LDD TFT” and achieves a high‐quality image, narrow frame width, and low power consumption. The LTPS process, device, and circuit technologies developed for system‐on‐glass LCD discussed. The development phase of LTPS circuit integration for system‐on‐glass LCDs is also reviewed.     

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.     

Fast full‐color reflective display via photoluminescent enhancement

Reflective displays are advantageous in applications requiring low power or daylight readability. However, there are no low‐cost reflective technologies capable of displaying bright colors. By employing photoluminescence to more efficiently use ambient light, we created a prototype display that provides bright, full color in a simple, low‐cost architecture. This prototype includes a novel electrokinetic shutter, a layer that incorporates patterned luminescent red, green, and blue sub‐pixel elements, and a novel optical out‐coupling scheme. The luminescent elements convert otherwise‐wasted portions of the incident spectrum to light in the desired color band, resulting in improved color saturation and lightness. This prototype provides a color gamut that is superior to competing reflective display technologies that utilize color filters in single‐layer side‐by‐side sub‐pixel architectures. The current prototype is capable of switching in <0.5 s; future displays based on an alternative electro‐optic shutter technology should achieve video rate operation. A transflective version of this technology has also been prototyped. The transflective version utilizes its backlight with a power efficiency that is at least three times that of a conventional liquid crystal display. These photoluminescence‐based technologies enable a host of applications ranging from low‐power mobile products and retail pricing signage to daylight readable signage for outdoor advertising segments.     

New approaches to process simplification for large‐area high‐resolution TFT‐LCDs

Abstract— Novel process architectures are proposed for fabricating large‐area high‐resolution TFT‐LCDs with a minimal number of process steps. A low contact resistance between Al bus lines and the transparent conductive oxide layer, necessary for large‐area panels, is obtained by inducing a self‐formed inter‐metallic compound layer at the interface without using any additional buffer or capping layers. For enhanced brightness and resolution, a new TFT array structure integrated on a color‐filter substrate, referred to as an Array on Color Filter (AOC) structure, has been developed. Good‐quality TFTs were successfully constructed on the newly developed color filter for AOC within a sufficiently wide process margin. By adopting these novel technologies, a 15.0‐in. XGA prototype panel was fabricated and shows good display performance. Thus, these novel technologies have improved cost efficiency and productivity for TFT‐LCD manufacturing, and can be applied to the development of TFT‐LCDs of extended display area and enhanced resolution, benefiting from the low resistance bus lines, the high aperture ratio, and reduction in total process steps.     

High‐reliability gate driver circuit to prevent ripple voltage

In this paper, a high‐reliability gate driver circuit is proposed to prevent multiple outputs. The proposed circuit ensures reliability of the pull‐up thin‐film transistor (TFT) by periodically discharging the Q node voltage to the low‐level voltage (VGL) in the off stage. In addition, the output node is composed of two pull‐down TFTs that are driven alternately to ensure stability against bias stress. Thus, because the reliabilities of the pull‐up and pull‐down TFTs can be guaranteed simultaneously, the stability of the entire circuit is improved. Based on the simulation results, the rising and falling times of the output pulse are stable within 1.77 and 1.28 μs, respectively, even when the threshold voltage of the entire TFT is shifted by +10.0 V. In addition, the ripple voltage of the proposed circuit is almost eliminated and is within 0.79% of the total swing voltage. Moreover, through current is prevented in the proposed circuit because the turn‐on durations of the pull‐up and pull‐down units are completely nonoverlapping, which suggests that unnecessary power consumption can be eliminated. Therefore, based on 2,160 stages, the total power consumption of the proposed circuit is reduced by 34.7 mW from 276.3 to 241.6 mW.