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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Characteristics improvement of top‐gate self‐aligned amorphous indium gallium zinc oxide thin‐film transistors using a dual‐gate control

In this work, we have reported dual‐gate amorphous indium gallium zinc oxide thin‐film transistors (a‐IGZO TFTs), where a top‐gate self‐aligned TFTs has a secondary bottom gate and the TFT integration comprises only five mask steps. The electrical characteristics of a‐IGZO TFTs under different gate control are compared. With the enhanced control of the channel with two gates connected together, parameters such as on current (I_ON), sub‐threshold slope (SS^?1), output resistance, and bias‐stress instabilities are improved in comparison with single‐gate control self‐aligned a‐IGZO TFTs. We have also investigated the applicability of the dual‐gate a‐IGZO TFTs in logic circuitry such as 19‐stage ring oscillators.     

Novel back‐channel‐etch process flow based a‐IGZO TFTs for circuit and display applications on PEN foil

In this study, we report high‐quality amorphous indium–gallium–zinc‐oxide (a‐IGZO) thin‐film transistors (TFTs) fabricated on a polyethylene naphthalate foil using a new back‐channel‐etch (BCE) process flow. The BCE flow allows a better scalability of TFTs for high‐resolution backplanes and related circuits. The maximum processing temperature was limited to less than 165 °C in order to ensure good overlay accuracy (<1 µm) on foil. The presented process flow differs from the previously reported flow as we define the Mo source and drain contacts by dry etch prior to a‐IGZO patterning. The TFTs show good electrical performance, including field‐effect mobilities in the range of 15.0 cm²/(V·s), subthreshold slopes of 0.3 V/decade, and off‐currents <1.0 pA on foil. The threshold voltage shifts of the TFTs measured were less than 1.0 V after a stressing time of 10⁴ s in both positive (+1.0 MV/cm) and negative (?1.0 MV/cm) bias directions. The applicability of this new BCE process flow is demonstrated in a 19‐stage ring oscillator, demonstrated to operate at a supply voltage of 10 V with a stage delay time of 1.35 µs, and in a TFT backplane driving a 32 × 32 active‐matrix organic light‐emitting diode display.     

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.     

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.     

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.     

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.     

Circuits using uniform TFTs based on amorphous In‐Ga‐Zn‐O

Abstract— High‐performance and excellent‐uniformity thin‐film transistors (TFTs) having bottom‐gate structures are fabricated using an amorphous indium‐gallium‐zinc‐oxide (IGZO) film and an amorphous‐silicon dioxide film as the channel layer and the gate insulator layer, respectively. All of the 94 TFTs fabricated with an area 1 cm² show almost identical transfer characteristics: the average saturation mobility is 14.6 cm²/(V‐sec) with a small standard deviation of 0.11 cm²/(V‐sec). A five‐stage ring‐oscillator composed of these TFTs operates at 410 kHz at an input voltage of 18 V. Pixel‐driving circuits based on these TFTs are also fabricated with organic light‐emitting diodes (OLED) which are monolithically integrated on the same substrate. It is demonstrated that light emission from the OLED cells can be switched and modulated by a 120‐Hz ac signal input. Amorphous‐IGZO‐based TFTs are prominent candidates for building blocks of large‐area OLED‐display electronics.     

Oxide TFT with multilayer gate insulator for backplane of AMOLED device

Abstract— An indium gallium zinc oxide (IGZO) film with an amorphous phase was deposited and had a very flat morphology with a RMS value of 0.35 nm. IGZO TFTs were fabricated on a glass substrate by conventional photolithography and wet‐etching processes. IGZO TFTs demonstrated a high mobility of 124 cm²/V‐sec, a high on/off ratio of over 10⁸, a desirable threshold voltage of 0.7 V, and a sub‐threshold swing of 0.43 V/decade. High mobility partially resulted from the fringing‐electric‐field effect that leads to an additional current flow beyond the device edges. Therefore, considering our device geometry, the actual mobility was about 100 cm²/V‐sec, and had a very low dependence on the variation of W/L (channel width and length) and thickness of the active layer. IGZO TFTs were also fabricated on a flexible metal substrate for a conformable display application. TFT devices showed an actual mobility of 72 cm²/V‐sec, a high on/off ratio of ～10⁷, and a sub‐threshold swing of 0.36 V/decade. There was no significant difference before, during, or after bending. Moreover, an IGZO TFT array was fabricated and a top‐emitting OLED device was successfully driven by it. Therefore, the oxide TFT could be a promising candidate as a backplane for OLED devices.     

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%.     

Process development of inverted‐staggered amorphous InGaZnO thin‐film transistors with wet‐etched electrodes

Process development of inverted‐staggered amorphous InGaZnO thin‐film transistors (a‐IGZO TFTs) with wet‐etched electrodes was employed in this paper. Five metals (Al, Cu, Ti, Ta, and Cr) as well as various etchants were comparatively investigated, indicating H₂O₂ based solution etched Ta films were good candidates for the wet‐etched electrodes of a‐IGZO TFTs. The aforementioned findings along with other improving attempts successfully established inexpensive processing steps and conditions with which stable a‐IGZO TFTs were finally fabricated. The device performance was reasonably good enough (μ_FE of 6.0 cm²/V·s, V_th of 2.5 V, SS of 1.8 V/decade, and I_on/I_off of 10⁶) to meet the requirements of applications especially for small‐sized flat panel displays.     

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.