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.     

Oxide‐TFT technologies for next‐generation AMOLED displays

Abstract— High‐mobility high‐reliability low‐RC‐delay oxide TFTs have been developed. Their performances are good enough for AMOLED displays even for the large‐sized super‐high‐resolution, or high‐frame‐rate displays. In this paper, the status of oxide‐TFT development and the issues for the mass‐production of next‐generation AMOLED displays will be discussed, and three types of AMOLED displays using different oxide materials and TFT structures will be demonstrated.     

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.     

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.     

Solution‐processed oxide semiconductors for low‐cost and high‐performance thin‐film transistors and fabrication of organic light‐emitting‐diode displays

Abstract— High‐performance solution‐processed oxide‐semiconductor (OS) thin‐film transistors (TFTs) and their application to a TFT backplane for active‐matrix organic light‐emitting‐diode (AMOLED) displays are reported. For this work, bottom‐gated TFTs having spin‐coated amorphous In‐Zn‐O (IZO) active layers formed at 450°C have been fabricated. A mobility (μ) as high as 5.0 cm²/V‐sec, ?0.5 V of threshold voltage (VT), 0.7 V/dec of subthreshold swing (SS), and 6.9 × 10⁸ of on‐off current ratio were obtained by using an etch‐stopper (ES) structure TFT. TFTs exhibited uniform characteristics within 150 × 150‐mm² substrates. Based on these results, a 2.2‐in. AMOLED display driven by spin‐coated IZO TFTs have also been fabricated. In order to investigate operation instability, a negative‐bias‐temperature‐stress (NBTS) test was carried out at 60°C in ambient air. The IZO‐TFT showed ?2.5 V of threshold‐voltage shift (ΔVT) after 10,800 sec of stress time, comparable with the level (ΔVT = ?1.96 V) of conventional vacuum‐deposited a‐Si TFTs. Also, other issues regarding solution‐processed OS technology, including the instability, lowering process temperature, and printable devices are discussed.     

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%.     

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.     

Flexible AMOLEDs with low‐temperature‐processed TFT backplane technologies

Developments of backplane technologies, which are one of the challenging topics, toward the realization of flexible active matrix organic light‐emitting diodes (AMOLEDs) are discussed in this paper. Plastic substrates including polyimide are considered as a good candidate for substrates of flexible AMOLEDs. The fabrication process flows based on plastic substrates are explained. Limited by the temperature that plastic substrates can sustain, TFT technologies with maximum processing temperature below 400 °C must be developed. Considering the stringent requirements of AMOLEDs, both oxide thin‐film transistors (TFTs) and ultra‐low‐temperature poly‐silicon TFTs (U‐LTPS TFTs) are investigated. First, oxide TFTs with representative indium gallium zinc oxide channel layer are fabricated on polyimide substrates. The threshold voltage shifts under bias stress and under bending test are small. Thus, a 4.0‐in. flexible AMOLED is demonstrated with indium gallium zinc oxide TFTs, showing good panel performance and flexibility. Further, the oxide TFTs based on indium tin zinc oxide channel layer with high mobility and good stability are discussed. The mobility can be higher than 20 cm²/Vs, and threshold voltage shifts under both voltage stress and current stress are almost negligible, proving the potential of oxide TFT technology. On the other hand, the U‐LTPS TFTs are also developed. It is confirmed that dehydrogenation and dopant activation can be effectively performed at a temperature within 400 °C. The performance of U‐LTPS TFTs on polyimide is compatible to those of TFTs on glass. Also, the performance of devices on polyimide can be kept intact after devices de‐bonded from glass carrier. Finally, a 4.3‐in. flexible AMOLED is also demonstrated with U‐LTPS TFTs.     

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 AMOLED pixel circuit for high image quality of 1000 ppi mobile displays in AR and VR applications

In this paper, an active‐matrix organic light‐emitting diode pixel circuit is proposed to improve the image quality of 5.87‐in. mobile displays with 1000 ppi resolution in augmented and virtual reality applications. The proposed pixel circuit consisting of three thin‐film transistors (TFTs) and two capacitors (3T2C) employs a simultaneous emission driving method to reduce the number of TFTs and the emission current error caused by variations in threshold voltage (Vth) and subthreshold slope (SS) of the low‐temperature polycrystalline silicon TFTs. Using the simultaneous emission driving method, the compensation time is increased to 90 μs from 6.5 μs achieved in the conventional six TFTs and one capacitor (6T1C) pixel circuit. Consequently, the emission current error of the proposed 3T2C pixel circuit was reduced to ±3 least significant bit (LSB) from ±12 LSB at the 32nd gray level when the variations in both the Vth and SS are ±4σ. Moreover, both the crosstalk errors due to the parasitic capacitances between the adjacent pixel circuits and due to the leakage current were achieved to be less than ±1 LSB over the entire gray level. Therefore, the proposed pixel circuit is very suitable for active‐matrix organic light‐emitting diode displays requiring high image quality.     

Solution‐processed and low‐temperature metal oxide n‐channel thin‐film transistors and low‐voltage complementary circuitry on large‐area flexible polyimide foil

High‐performance solution‐based n‐type metal oxide thin‐film transistors (TFTs), fabricated directly on polyimide foil at a post‐annealing temperature of only 250 °C, are realized and reported. Saturation mobilities exceeding 2 cm²/(Vs) and on‐to‐off current ratios up to 10⁸ are achieved. The usage of these oxide n‐type TFTs as the pixel drive and select transistors in future flexible active‐matrix organic light‐emitting diode (AMOLED) displays is proposed. With these oxide n‐type TFTs, fast and low‐voltage n‐type only flexible circuitry is demonstrated. Furthermore, a complete 8‐bit radio‐frequency identification transponder chip on foil has been fabricated and measured, to prove that these oxide n‐type TFTs have reached already a high level of yield and reliability. The integration of the same solution‐based oxide n‐type TFTs with organic p‐type TFTs into hybrid complementary circuitry on polyimide foil is demonstrated. A comparison between both the n‐type only and complementary elementary circuitry shows the high potential of this hybrid complementary technology for future line‐drive circuitry embedded at the borders of flexible AMOLED displays.     

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.     

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.     

Alternative approach to large‐sized AMOLED HDTV

Abstract— In this work, alternative approaches to existing technologies for the fabrication of large‐sized AMOLEDs, such as non‐laser crystallization methods for poly‐Si TFT fabrication and color patterning using laser‐induced thermal imaging (LITI), is proposed. In particular, it was found that the super grain crystallization (SGS) method resulted in high‐performance TFTs in terms of mobility and off‐current. The feasibility of these techniques for large‐sized AMOLEDs is demonstrated by 17‐in. UXGA AMOLED displays which show good brightness uniformity.     

Novel self‐aligned top‐gate oxide TFT for AMOLED displays

Abstract— A novel highly reliable self‐aligned top‐gate oxide‐semiconductor thin‐film transistor (TFT) formed by using the aluminum (Al) reaction method has been developed. This TFT structure has advantages such as small‐sized TFTs, lower mask count, and small parasitic capacitance. The TFT with a 4‐μm channel length exhibited a field‐effect mobility of 21.6 cm²/V‐sec, a threshold voltage of ?1.2 V, and a subthreshold swing of 0.12 V/decade. Highly reliable TFTs were obtained after 300°C annealing without increasing the sheet resistivity of the source/drain region. A 9.9‐in.‐diagonal qHD AMOLED display was demonstrated with self‐aligned top‐gate oxide‐semiconductor TFTs for a low‐cost and ultra‐high‐definition OLED display. Excellent brightness uniformity could be achieved due to small parasitic capacitance.     

Modeling of threshold‐voltage‐shift dependency on drain bias in amorphous‐silicon thin‐film transistors in active‐matrix organic light‐emitting‐diode displays

Abstract— A theoretical model to interpret appearances of the threshold voltage shift in hydrogenated amorphous‐silicon (a‐Si:H) thin‐film transistors (TFTs) is developed to better understand the instability of a‐Si:H TFTs for the driving transistors in active‐matrix organic light‐emitting‐diode (AMOLED) displays. This model assumes that the defect creation at channel in a‐Si:H is proportional to the carrier concentration, leading to the defect density varying along the channel depending on the bias conditions. The model interprets a threshold‐voltage‐shift dependency on the drain‐stress bias. The model predicts the threshold voltage shift stressed under a given gate bias applying the drain saturation voltage is 66% of that with zero drain bias, and it even goes down to 50–60% of that when stressed by applying twice the drain saturation voltage.     

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.     

Development of 8‐in. oxide‐TFT‐driven flexible AMOLED display using high‐performance red phosphorescent OLED

An 8‐in. flexible active‐matrix organic light‐emitting diode (AMOLED) display driven by oxide thin‐film transistors (TFTs) has been developed. In‐Ga‐Zn‐O (IGZO)‐TFTs used as driving devices were fabricated directly on a plastic film at a low temperature below 200 °C. To form a SiO_x layer for use as the gate insulator of the TFTs, direct current pulse sputtering was used for the deposition at a low temperature. The fabricated TFT shows a good transfer characteristic and enough carrier mobility to drive OLED displays with Video Graphic Array pixels. A solution‐processable photo‐sensitive polymer was also used as a passivation layer of the TFTs. Furthermore, a high‐performance phosphorescent OLED was developed as a red‐light‐emitting device. Both lower power consumption and longer lifetime were achieved in the OLED, which used an efficient energy transfer from the host material to the guest material in the emission layer. By assembling these technologies, a flexible AMOLED display was fabricated on the plastic film. We obtained a clear and uniform moving color image on the display.     

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.     

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.