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.     

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.     

Comparison of a‐Si and Poly‐Si for AMOLED displays

Abstract— The characteristics of OLED backplanes including the intrinsic properties of a‐Si TFTs and LTPS TFTs will be reviewed. While LTPS TFTs reveal satisfactory stability in AMOLED‐display applications, a‐Si AMOLEDs show better uniformity and are capable of driving OLEDs. However, the stability of a‐Si TFTs under long‐term operation is still unacceptable and remains to be the key issue constraining the commercialization of a‐Si TFT AMOLEDs.     

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.     

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 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.     

High‐mobility self‐aligned top‐gate oxide TFT for high‐resolution AM‐OLED

High‐mobility and highly reliable self‐aligned top‐gate oxide thin‐film transistor (TFTs) were developed using the aluminum reaction method. Al diffusion to the oxide semiconductor and homogenization of the oxygen concentration in the depth direction after annealing were confirmed by laser‐assisted atom probe tomography. The high mobility of the top‐gate TFT with amorphous indium tin zinc oxide channel was demonstrated to be 32 cm²/V s. A 9.9‐in. diagonal qHD active‐matrix organic light‐emitting diode (AM‐OLED) display was fabricated using a five‐mask backplane process to demonstrate an applicable solution for large‐sized and high‐resolution AM‐OLEDs.     

Highly reliable a‐IGZO TFTs on a plastic substrate for flexible AMOLED displays

We have successfully reduced threshold voltage shifts of amorphous In–Ga–Zn–O thin‐film transistors (a‐IGZO TFTs) on transparent polyimide films against bias‐temperature stress below 100 mV, which is equivalent to those on glass substrates. This high reliability was achieved by dense IGZO thin films and annealing temperature below 300 °C. We have reduced bulk defects of IGZO thin films and interface defects between gate insulator and IGZO thin film by optimizing deposition conditions of IGZO thin films and annealing conditions. Furthermore, a 3.0‐in. flexible active‐matrix organic light‐emitting diode was demonstrated with the highly reliable a‐IGZO TFT backplane on polyimide film. The polyimide film coating process is compatible with mass‐production lines. We believe that flexible organic light‐emitting diode displays can be mass produced using a‐IGZO TFT backplane on polyimide films.     

Power saving through state retention in IGZO‐TFT AMOLED displays for wearable applications

We present a qHD (960 × 540 with three sub‐pixels) top‐emitting active‐matrix organic light‐emitting diode display with a 340‐ppi resolution using a self‐aligned IGZO thin‐film transistor backplane on polyimide foil with a humidity barrier. The back plane process flow is based on a seven‐layer photolithography process with a CD = 4 μm. We implement a 2T1C pixel engine and use a commercial source driver IC made for low‐temperature polycrystalline silicon. By using an IGZO thin‐film transistor and leveraging the extremely low off current, we can switch off the power to the source and gate driver while maintaining the image unchanged for several minutes. We demonstrate that, depending on the image content, low‐refresh operation yields reduction in power consumption of up to 50% compared with normal (continuous) operation. We show that with the further increase in resolution, the power saving through state retention will be even more significant.     

High‐mobility oxide TFT for circuit integration of AMOLEDs

Abstract— A high‐mobility and high‐reliability oxide thin‐film transistor (TFT) that uses In‐Sn‐Zn‐O (ITZO) as a channel material has been developed. The mobility was 30.9 cm²/V‐sec and the threshold voltage shift after 20,000 sec of a bias‐temperature‐stress (BTS) test (with a stress condition of Vg = 15 V, Vd = 15 V, and T = 50°C) was smaller than 0.1 V. In addition, a method of obtaining a stable enhancement‐type TFT, which realizes circuit integration for active‐matrix organic light‐emitting diode (AMOLED) displays has been developed.     

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.     

Development of flexible displays using back‐channel‐etched In–Sn–Zn–O thin‐film transistors and air‐stable inverted organic light‐emitting diodes

We developed flexible displays using back‐channel‐etched In–Sn–Zn–O (ITZO) thin‐film transistors (TFTs) and air‐stable inverted organic light‐emitting diodes (iOLEDs). The TFTs fabricated on a polyimide film exhibited high mobility (32.9 cm²/Vs) and stability by utilization of a solution‐processed organic passivation layer. ITZO was also used as an electron injection layer (EIL) in the iOLEDs instead of conventional air‐sensitive materials. The iOLED with ITZO as an EIL exhibited higher efficiency and a lower driving voltage than that of conventional iOLEDs. Our approach of the simultaneous formation of ITZO film as both of a channel layer in TFTs and of an EIL in iOLEDs offers simple fabrication process.     

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.     

Investigation of the hysteresis phenomenon of an a‐Si:H TFT at an elevated temperature for AMOLED displays

Abstract— The temperature dependence of the hysteresis of an a‐Si:H TFT has been investigated. An a‐Si:H TFT pixel driving scheme has been proposed and investigated. This scheme can eliminate changes in the organic light‐emitting diode (OLED) current caused by hysteresis of an a‐Si:H TFT. The VTH of the a‐Si:H TFT was changed according to the gate‐voltage sweep direction because of the hysteresis of the a‐Si:H TFT. The variation of VTH for a a‐Si:H TFT decreased from 0.41 to 0.17 V at an elevated temperature of 60°C because the sub‐threshold slope (s‐slope) of the a‐Si:H TFT, in the reverse voltage sweep direction, increased more than in the forward voltage sweep direction due to a greater increase in the initial electron trapped charges than the hole charges. Although the OLED current variation caused by hysteresis decreased (～14%) as the temperature increased, the error in the OLED current needed to be improved in order to drive the pixel circuit of AMOLED displays. The proposed pixel circuit can apply the reset voltage (?10 V) before the data voltage for the present frame that was written to fix the sweep direction of the data voltage. The variation in the OLED current caused by hysteresis of the a‐Si:H TFT was eliminated by the fixed voltage sweep direction in the proposed pixel circuit regardless of operating temperature.     

The study of oxide‐interface and grain‐boundary traps in poly‐Si TFT characteristics and its application to fabrication process diagnosis

Abstract— The study of oxide‐interface and grain‐boundary traps in poly‐Si TFT characteristics is reviewed. The subthreshold swing and threshold voltage mainly depend on the density of the oxide‐interface traps (Dit), while the transistor mobility mainly depends on the density of grain‐boundary traps (Dgb). These device properties are applied to diagnose two fabrication processes: plasma treatment and gate‐oxide deposition. It is found that oxygen (O) plasma treatment reduces Dit. This seems to be because O plasma treatment has the ability to terminate the dangling bonds, but O plasma species do not diffuse into the poly‐Si. A model is proposed by comparing the bond energy of the Si‐H and Si‐O‐H. On the other hand, plasma‐enhanced chemical‐vapor deposition (PECVD) of tetraethylorthosilicate (TEOS) for gate oxide increases Dgb. This seems to be because hydrogen (H) plasma species in the TEOS‐PECVD damage the grain boundaries. A model is proposed by considering the reaction processes: hydrolysis, dehydration and bonding, and H plasma species generated during the dehydration.     

Fabrication of 5.8‐in. OTFT‐driven flexible color AMOLED display using dual protection scheme for organic semiconductor patterning

Abstract— A 5.8‐in. wide‐QQVGA flexible color active‐matrix organic light‐emitting‐diode (AMOLED) display consisting of organic thin‐film transistors (OTFTs) and phosphorescent OLEDs was fabricated on a plastic film. To reduce the operating voltage of the OTFTs, Ta₂O₅ with a high dielectric constant was employed as a gate insulator. Pentacene was used for the semiconductor layer of the OTFTs. This layer was patterned by photolithography and dry‐etched using a dual protection layer of poly p‐xylylene and SiO₂ film. Uniform transistor performance was achieved in the OTFT backplane with QQVGA pixels. The RGB emission layers of the pixels were formed by vacuum deposition of phosphorescent small molecules. The resulting display could clearly show color moving images even when it was bent and operated at a low driving voltage (below 15 V).     

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.     

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.     

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.     

IGZO‐TFT technology for large‐screen 8K display

We succeeded in G8 factory for mass production of Indium–Gallium–Zink–Oxide thin‐film transistor (IGZO‐TFT) for the first time in the world. The initial TFT process was an etching stop‐type TFT, but now, we are mass producing channel etching‐type TFTs. And, its application range is smartphones, tablets, PCs, monitors, TV, and so on. In particular, because of recent demands for high‐resolution and narrow frame, our IGZO display has been advanced in technology development with gate driver in panel. In this paper, we report development combining low resistance technology and the latest IGZO‐TFT (IGZO5) for large‐screen 8K display.