High‐speed pixel circuits for large‐sized 3‐D AMOLED displays

Abstract— Large‐sized active‐matrix organic light‐emitting diode (AMOLED) displays require high‐frame‐rate driving technology to achieve high‐quality 3‐D images. However, higher‐frame‐rate driving decreases the time available for compensating Vth in the pixel circuit. Therefore, a new method needs to be developed to compensate the pixel circuit in a shorter time interval. In this work, image quality of a 14‐in. quarter full‐high‐definition (qFHD) AMOLED driven at a frame rate of over 240 Hz was investigated. It was found that image degradation is related to the time available for compensation of the driving TFT threshold voltage. To solve this problem, novel AMOLED pixel circuits for high‐speed operation are proposed to compensate threshold‐voltage variation at frame rates above 240 Hz. When Vth is varied over ±1.0 V, conventional pixel circuits showed current deviations of 22.8 and 39.8% at 240 and 480 Hz, respectively, while the new pixel circuits showed deviations of only 2.6 and 5.4%.     

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.     

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.     

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.     

Fast voltage‐driving scheme for AMOLED displays based on internal feedback

Abstract— A new driving scheme for active‐matrix organic light‐emitting diodes (AMOLED) displays based on voltage programming is proposed. While conventional voltage drivers have a trade‐off between speed and accuracy, the new scheme is inherently fast and accurate. Based on the new driving scheme, a fast pixel circuit is designed using amorphous‐silicon (a‐Si) thin‐film transistors (TFTs). As the simulation results indicate, this pixel circuit can compensate the threshold‐voltage shift (VT shift) of the driver transistors. This pixel can be programmed in just 10 μsec, and it can compensate the threshold‐voltage shifts over 5 V with an error rate of less than 5% for a 1 ‐μA pixel current.     

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.     

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 averaging pixel structure using microcrystalline‐silicon films prepared at high temperature for AMOLED displays

Abstract— A new voltage‐addressed pixel using a multiple drive distribution has been developed to improve, in a simple way, the brightness uniformity of active‐matrix organic light‐emitting‐diode (AMOLED) displays. Moreover, circuits were realized using microcrystalline‐silicon (μc‐Si) films prepared at 600°C using a standard low‐pressure CVD system. The developed p‐channel TFTs exhibit a field‐effect mobility close to 6 cm²/V‐sec. The experimental results show that the proposed spatial distribution of driving TFTs improves the uniformity of current levels, in contrast to the conventional two‐TFT pixel structure. Backplane performances have been compared using circuits based on μc‐Si and furnace‐annealed polysilicon materials. Finally, this technology has been used to make an AMOLED demonstration unit using a top‐emission OLED structure. Thus, by combining both an μc‐Si active‐layer and a current‐averaging driver, an unsophisticated solution is provided to solve the inter‐pixel non‐uniformity issue.     

Characterization and compensation of OLED aging in a digital AMOLED system

Organic light‐emitting diode (OLED) aging is the root cause for image sticking artifact and considered as the toughest problem besides the low yield problem of active‐matrix organic light‐emitting diode (AMOLED) displays. Digital driving can eliminate Mura artifact and allow a similar yield like LCD. However, it is more prone to OLED aging than analog driving, so that the lifetime will become shorter. In this paper, we pursue the approach to measure the pixel current and compensate OLED I–V drift. Information gained from electrical measurements during the lifetime of the display may be correlated to electro‐optical drift, particularly the current efficiency. The aging model has to consider the dependence of I–V drift and efficiency loss on the operation point/voltage for each subframe. Specific compensation algorithms have been developed. Two AMOLED prototypes (1.5 and 2.8 in.) were validated. Burned‐in pattern can be compensated, so the concept has been proven as effective. With the method described in this paper, digital AMOLED may reach a similar and even significantly higher lifetime than an analog AMOLED.     

Active‐matrix organic light‐emitting diode using inverse‐staggered poly‐Si TFTs with a center‐offset gated structure

Abstract— A low‐cost active‐matrix backplane using non‐laser polycrystalline silicon (poly‐Si) having inverse‐staggered TFTs with amorphous‐silicon (a‐Si) n⁺ contacts has been developed. The thin‐film transistors (TFTs) have a center‐offset gated structure to reduce the leakage current without scarifying the ON‐currents. The leakage current of the center‐offset TFTs at Vg = ?10 V is two orders of magnitude lower than those of the non‐offset TFTs. The center‐offset length of the TFTs was 3 μm for both the switching and driving TFTs. A 2.2‐in. QQVGA (1 60 × 1 20) active‐matrix organic light‐emitting‐diode (AMOLED) display was demonstrated using conventional 2T + 1C pixel circuits.     

Novel voltage programming n-channel TFT pixel circuit for low power and high performance AMOLED displays

This paper proposes a novel pixel circuit for high resolution, high frame rate, and low power AMOLED displays that is implemented with one driving n-channel TFT, six switching n-channel poly-Si TFTs, and a storage capacitor. The proposed pixel circuit adopts the voltage programming scheme for threshold voltage compensation. Because the whole line time is in use only for charging the data voltage, this pixel circuit is applicable to high resolution and frame rate displays. In addition, it compensates voltage variation of OLEDs and voltage drop of supply lines at lower power consumption. On the average, the non-uniformity of a proposed circuit is reduced to 2.5%, compared to 7.1% of the previous one at a 240 Hz full-HD display. On the other hand, the compensation voltage error, which is caused by feed-through and charge injection noises from falling control signals of switching TFTs, is much less in the proposed scheme than in the previous 5T2C structure. The average error of the proposed circuit is reduced to 0.18 V, compared to 0.75 V of the previous one. The initialization power consumption of the 7T1C circuit is reduced to 98 mW, compared to 530 mW of the 5T2C circuit and the average dynamic power saving ratio of data drivers is estimated in the 7T1C pixel as 98.7% over the 5T2C one for 24 test images.     

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.     

Voltage‐programming method with transimpedance‐feedback technique for threshold voltage and mobility compensations in large‐area high‐resolution AMOLED displays

Abstract— A voltage‐programming method with transimpedance‐feedback control technique is proposed for compensating threshold voltage and mobility variations of driving thin‐film transistors (TFTs) in large‐area high‐resolution polycrystalline‐silicon (poly‐Si) active‐matrix organic light‐emitting‐diode (AMOLED) displays. Those electrical characteristic variations of TFTs throughout a large‐area high‐resolution panel result in picture‐quality non‐uniformity of AMOLED displays. The simulation and experimental results of the proposed method show that the maximum emission‐current error for 30‐in. full‐high‐definition television (HDTV) applications is less than 1.9% when the mobility variation and the threshold‐voltage variation are ±12.5% and ±0.3 V, respectively. The proposed method is the best programming method for large‐area high‐resolution AMOLEDs among the published methods.     

High‐luminance 1.8‐mm‐pixel‐pitch CNT‐FED for ubiquitous color character displays

Abstract— A high‐luminance 1.8‐mm‐pixel‐pitch CNT‐FED for color character displays has been developed. The display panel has 32 × 256 color pixels, and the subpixel size is 0.6 × 1.8 mm. The display panel can provide good visibility when installed even in outdoor locations. The power consumption is low enough for the display to be battery driven. The practical application is the display of important messages regarding the evacuation from disaster areas, even under emergent no‐power conditions similar to the messages on vending machines.     

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.     

A new threshold‐voltage compensation technique of poly‐Si TFTs for AMOLED display pixel circuits

Abstract— A new threshold‐voltage compensation technique for polycrystal line‐silicon thin‐film transistors (poly‐Si TFTs) used in active‐matrix organic light‐emitting‐diode (AMOLED) display pixel circuits is presented. The new technique was applied to a conventional 2‐transistor—1‐capacitor (2T1C) pixel circuit, and a new voltage‐programmed pixel circuit (VPPC) is proposed. Theoretically, the proposed pixel is the fastest pixel with threshold‐voltage compensation reported in the literature because of the new compression technique implemented with a static circuit block, which does not affect the response time of the conventional 2T1C pixel circuit. Furthermore, the new pixel exhibits all the other advantages of the 2T1C pixel, such as the simplicity of the peripheral drivers and improves other characteristics, such as its behavior in the temperature variations. The verification of the proposed pixel is made through simulations with HSpice. In order to obtain realistic simulations, device parameters were extracted from fabricated low‐temperature poly‐Si (LTPS) TFTs.     

Active‐matrix organic light‐emitting diode displays with indium gallium zinc oxide thin‐film transistors and normal,inverted, and transparent organic light‐emitting diodes

Abstract— Active‐matrix organic light‐emitting diode (AMOLED) displays have gained wide attention and are expected to dominate the flat‐panel‐display industry in the near future. However, organic light‐emitting devices have stringent demands on the driving transistors due to their current‐driving characteristics. In recent years, the oxide‐semiconductor‐based thin‐film transistors (oxide TFTs) have also been widely investigated due to their various benefits. In this paper, the development and performance of oxide TFTs will be discussed. Specifically, effects of back‐channel interface conditions on these devices will be investigated. The performance and bias stress stability of the oxide TFTs were improved by inserting a SiO^x protection layer and an N²O plasma treatment on the back‐channel interface. On the other hand, considering the n‐type nature of oxide TFTs, 2.4‐in. AMOLED displays with oxide TFTs and both normal and inverted OLEDs were developed and their reliability was studied. Results of the checkerboard stimuli tests show that the inverted OLEDs indeed have some advantages due to their suitable driving schemes. In addition, a novel 2.4‐in. transparent AMOLED display with a high transparency of 45% and high resolution of 166 ppi was also demonstrated using all the transparent or semi‐transparent materials, based on oxide‐TFT technologies.     

PECVD‐based nanocrystalline‐silicon TFT backplanes for large‐sized AMOLED displays

Abstract— A 14.1‐in. AMOLED display using nanocrystalline silicon (nc‐Si) TFTs has been developed. Nanocrystalline silicon was deposited using conventional 13.56‐MHz plasma‐enhanced chemical vapor deposition (PECVD). Detailed thin‐film characterization of nc‐Si films was followed by development of nc‐Si TFTs, which demonstrate a field‐effect mobility of about 0.6–1.0 cm²/V‐sec. The nc‐Si TFTs show no significant shift in threshold voltage when over 700 hours of constant current stress is applied, indicating a stable TFT backplane. The nc‐Si TFTs were successfully integrated into a 14.1‐in. AMOLED display. The display shows no significant current decrease in the driving TFT of the 2T‐1cap circuit because the TFTs are highly stable. In addition to the improved lifetime of AMOLED displays, the development of nc‐Si TFTs using a conventional 13.56‐MHz PECVD system offers considerable cost advantages over other laser and non‐laser polysilicon‐TFT technologies for large‐sized AMOLEDs.     

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.     

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.