High refresh rate and low power consumption AMOLED panel using top‐gate n‐oxide and p‐LTPS TFTs

A pixel circuit and a gate driver on array for light‐emitting display are presented. By simultaneously utilizing top‐gate n‐type oxide and p‐type low‐temperature polycrystalline silicon (LTPS) thin‐film transistors (TFTs), the circuits provide high refresh rate and low power consumption. An active‐matrix LED (AMOLED) panel with proposed circuits is fabricated, and driving at various refresh rate ranging from 1 to 120 Hz could be achieved.     

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.     

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 55” 4K UHD OLED TV employing the internal gate IC with high reliability and short channel IGZO TFTs

We investigated oxide TFT backplane technology to employ the internal gate driver IC (GIP circuit) on 55” 4K OLED TV panel. For the GIP circuit, we developed the high reliability oxide TFTs, especially only ?0.4 V Vth degradation under 100‐h long‐term PBTS stress and the short channel length TFTs (L = 4.5um) for narrow bezel. Consequently, we demonstrated the 55‐in 4K OLED TV employing the internal gate IC with high reliability and short channel IGZO TFTs.     

Multioutputs single‐stage gate driver on array with wide temperature operable thin‐film‐transistor liquid‐crystal display for high resolution application

A hydrogenated amorphous silicon (a‐Si:H) thin‐film transistor (TFT) gate driver with multioutputs (eight outputs per stage) for high reliability, 10.7‐inch automotive display has been proposed. The driver circuit is composed of one SR controller, eight driving TFTs (one stage to eight outputs) with bridging TFTs. The SR controller, which starts up the driving TFTs, could also prevent the noise of gate line for nonworking period. The bridging TFT, using width decreasing which connects between the SR controller and the driving TFT, could produce the floating state which is beneficial to couple the gate voltage, improves the driving ability of output, and reaches consistent rising time in high temperature and low temperature environment. Moreover, 8‐phase clocks with 75% overlapping and dual‐side driving scheme are also used in the circuit design to ensure enough charging time and reduce the loading of each gate line. According to lifetime test results, the proposed gate driver of 720 stages pass the extreme temperature range test (90°C and ?40°C) for simulation, and operates stably over 800 hours at 90°C for measurement. Besides, this design is successfully demonstrated in a 10.7‐inch full HD (1080 × RGB×1920) TFT‐liquid‐crystal display (LCD) panel.     

Highly robust oxide TFT with bulk accumulation and source/drain/active layer splitting

We report stable and high performance amorphous indium‐gallium‐zinc oxide (a‐IGZO) thin‐film transistor (TFT) by using bulk‐accumulation (BA) and split active/source/drain layers. The a‐IGZO TFTs exhibit the mobility over 80 cm²/Vs and extremely stable under bias and mechanical stresses. We demonstrated a 4‐inch semitransparent AMOLED using the oxide TFT backplane with the gate driver integrated.     

In‐pixel temperature sensor for high‐luminance active matrix micro‐light‐emitting diode display using low‐temperature polycrystalline silicon and oxide thin‐film‐transistors

We propose an in‐pixel temperature sensor using low‐temperature polycrystalline silicon and oxide (LTPO) thin‐film transistor (TFTs) for high‐luminance active matrix (AM) micro‐light‐emitting diode (LED) displays. By taking advantage of the different off‐current characteristics of p‐type LTPS TFTs and n‐type a‐IGZO TFTs under temperature change, we designed and fabricated a temperature sensor consists of only LTPO TFTs without additional sensing component or material. The fabricated sensor exhibits excellent temperature sensitivity of up to 71.8 mV/°C. In addition, a 64 × 64 temperature sensor array with 3T sensing pixel and integrated gate driver has also been fabricated, which demonstrates potential approach for maxing out the performance of high‐luminance AM micro‐LED display with real‐time in‐pixel temperature monitoring.     

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.     

Design of a low‐power‐consumption a‐IGZO TFT‐based Vcom driver circuit with long‐term reliability

Abstract— An amorphous‐InGaZnO (a‐IGZO) thin‐film transistor (TFT)‐based Vcom driver circuit that has long‐term reliability and can be integrated with the pixel array on a panel has been designed. Owing to the Vcom inversion, the power consumed by the proposed driving scheme is 40% less than that consumed by the conventional line‐inversion method. The high mobility (>10 cm²/V‐sec) of the a‐IGZO TFTs allows the integration of devices with small channel widths (<750 μm) and thus keeps the overall device size small, which is important for displays with narrow bezels. The lifetime of the Vcom driver is improved by AC driving (by clocking the n‐th and (n + 1)‐th frame with 20 and 0 V, respectively) of the buffer TFTs.     

Transparent AMOLED display driven by split oxide TFT backplane

We have developed stable and high performance etch‐stopper amorphous indium–gallium–zinc oxide thin‐film transistor (TFT) by using split active oxide semiconductor. The amorphous indium–gallium–zinc oxide TFTs exhibit the mobility as high as over 70 cm²/Vs and the stable operation under positive bias temperature stress. In this work, we demonstrated a 4‐in. transparent active‐matrix organic light‐emitting diode display using oxide TFT backplane with split active layer, where the gate driver is integrated.     

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.     

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.     

Development of 55‐in. 8K AMOLED TV based on coplanar oxide thin‐film transistors and inkjet printing process

In this paper, we presented 55‐in. 8K4K AMOLED TV employing coplanar oxide thin‐film transistor (TFT) backplane, top emissive inkjet‐printing organic light‐emitting diode (OLED) device, gate driver on array (GOA), and compensation technologies. It is so far the largest prototype AMOLED TV fabricated by using inkjet printing process with 8K resolution. It shows the stunning display quality, thanks to the high resolution and fast refresh frequency. It proves that the inkjet printing process is not only cost competitive but also can deliver premium display.     

Flexible AMOLED displays on stainless‐steel foil

Abstract— The world's thinnest flexible full‐color 5.6‐in. active‐matrix organic‐light‐emitting‐diode (AMOLED) display with a top‐emission mode on stainless‐steel foil was demonstrated. The stress in the stainless‐steel foil during the thermal process was investigated to minimize substrate bending. The p‐channel poly‐Si TFTs on stainless‐steel foil exhibited a field‐effectmobility of 71.2 cm²/N‐sec, threshold voltage of ?2.7 V, off current of 6.7 × 10¹³ A/μm, and a subthreshold slope of 0.63 V/dec. These TFT performances made it possible to integrate a scan driver circuit on the panel. A top‐emission EL structure was used as the display element, and thin‐film encapsulation was performed to realize a thin and flexible display. The full‐color flexible AMOLED display on stainless‐steel foil is promising for mobile applications because of its thin, light, rugged, and flexible properties.     

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.     

Polysilicon TFT technology on flexible metal foil for AMPLED displays

Abstract— A top‐emitting 230‐dpi active‐matrix polymer light‐emitting diode (AMPLED) display, having a VGA format and a 3.3‐in.‐diagonal size, on a flexible stainless‐steel‐foil substrate is reported. The active‐matrix array was fabricated with laser‐crystallized polysilicon TFTs at a maximum process temperature of 700°C. The top‐emitting PLED diodes were prepared by spin‐casting organic light‐emitting polymers. This work demonstrates the compatibility of polysilicon‐TFT technology with flexible metal‐foil substrates for active‐matrix organic light‐emitting‐diode (AMOLED) display applications.     

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.     

A new current‐mirror pixel circuit employing poly‐Si TFTs for active‐matrix organic light‐emitting‐diode displays

Abstract— A novel active‐matrix organic light‐emitting‐diode (AMOLED) display employing a new current‐mirror pixel circuit, which requires four‐poly‐Si TFTs and one‐capacitor and no additional signal lines, has been proposed and sucessfully fabricated. The experimental results show that a new current mirror can considerably compensate luminance non‐uniformity and scale down a data current more than a conventional current‐mirror circuit in order to reduce the pixel charging time and increase the minimum data current. Compared with a conventional two‐TFT pixel, the luminance non‐uniformity induced by the grain boundaries of poly‐Si TFTs can be decreased considerably from 41% to 9.1%.     

A novel gate driver circuit for depletion‐mode a‐IGZO TFTs

In this paper, a novel gate driver circuit, which can achieve high reliability for depletion mode in a‐InGaZnO thin‐film transistors (TFTs), was proposed. To prevent the leakage current paths for Q node effectively, the new driving method was proposed by adopting the negative gate‐to‐source voltage (V_GS) value for pull‐down units. The results showed all the V_OUT voltage waveforms were maintained at VGH voltage despite depletion‐mode operation. The proposed circuit could also obtain stable V_OUT voltage when the threshold voltage for all TFTs was changed from ?6.5 to +11.5 V. Therefore, the circuit can achieve high reliability regardless of threshold voltage value for a‐IGZO TFTs. In addition, the output characteristics and total power consumption were shown for the alternating current (AC)–driven and direct current (DC)–driven methods based on 120‐Hz full‐HD graphics (1920 × 1080) display panel. The results showed that the AC‐driven method could achieve improved V_OUT characteristics compared with DC‐driven method since the leakage current path for Q node can be completely eliminated. Although power consumption of the AC‐driven method can be slightly increased compared with the DC‐driven method for enhancement mode, consumption can be lower when the operation has depletion‐mode characteristics by preventing a leakage current path for pull‐down units. Consequently, the proposed gate driver circuit can overcome the problems caused by the characteristics of a‐IGZO TFTs.     

Design of high speed gate driver employing IZO TFTs

This paper presents a new bi-side gate driver integrated by indium-zinc-oxide thin film transistors (IZO TFTs). Our optimized operate method can achieve high speed performance by employing a lower duty ratio (25%) CK2 with its pulse located in the middle of the pulse of CK2L to fully use the bootstrapped high voltage of node Q. In addition, the size of devices is optimized by calculation and simulation, and the function of the proposed gate driver is predicted by the circuit simulation. Furthermore, the proposed gate driver with 20 stages is fabricated by the IZO TFTs process. It is shown that a 2.6 μs width pulse with good noise-suppressed characteristic can be successfully output at the condition of R_load = 6 kΩ and C_load = 150 pF. The power consumption of the proposed gate driver with 20 stages is measured as 1 mW. Hence, the proposed gate driver may be applied to the display of 4K resolution (4096 × 2160) at a frame rate of 120 Hz. Moreover, there is a good stability for the proposed gate driver under 48 h operation.