a‐InGaZnO thin‐film transistors for AMOLEDs: Electrical stability and pixel‐circuit simulation

Abstract— Inverted‐staggered amorphous In‐Ga‐Zn‐O (a‐InGaZnO) thin‐film transistors (TFTs) were fabricated and characterized on glass substrates. The a‐InGaZnO TFTs exhibit adequate field‐effect mobilities, sharp subthreshold slopes, and very low off‐currents. The current temperature stress (CTS) on the a‐InGaZnO TFTs was performed, and the effect of stress temperature (TSTR), stress current (ISTR), and TFT biasing condition on their electrical stability was investigated. Finally, SPICE modelling for a‐InGaZnO TFTs was developed based on experimental data. Several active‐matrix organic light‐emitting‐display (AMOLED) pixel circuits were simulated, and the potential advantages of using a‐InGaZnO TFTs were discussed.     

Back‐channel‐etched thin‐film transistor using c‐axis‐aligned crystal In–Ga–Zn oxide

Our crystalline In–Ga–Zn oxide (IGZO) thin film has a c‐axis‐aligned crystal (CAAC) structure and maintains crystallinity even on an amorphous base layer. Although the crystal has c‐axis alignment, its a‐axis and b‐axis have random arrangement; moreover, a clear grain boundary is not observed. We fabricated a back‐channel‐etched thin‐film transistor (TFT) using the CAAC‐IGZO film. Using the CAAC‐IGZO film, more stable TFT characteristics, even with a short channel length, can be obtained, and the instability of the back channel, which is one of the biggest problems of IGZO TFTs, is solved. As a result, we improved the process of manufacturing back‐channel‐etched TFTs.     

Interface and bulk effects for bias—light‐illumination instability in amorphous‐In—Ga—Zn—O thin‐film transistors

Abstract— Positive‐current‐bias (PB) instability and negative‐bias—light‐illumination (NBL) instability in amorphous‐In—Ga—Zn—O (a‐IGZO) thin‐film transistors (TFTs) have been examined. The channel‐ thickness dependence indicated that the Vth instability caused by the PB stress is primarily attributed to defects in the bulk a‐IGZO region for unannealed TFTs and to those in the channel—gate‐insulator interface for wet‐annealed TFTs. The interface and bulk defect densities (Dit and Nss, respectively) are Dit = 4.8 × 10¹¹ cm^?2/eV and Nss = 7.0×10¹⁶ cm^?3/eV for the unannealed TFT, which increased to 5.2×10¹¹ cm^?2/eV and 9.8×10¹⁶ cm^?3/eV, respectively, by the PB stress test. These are reduced significantly to Dit = 0.82×10¹¹ cm^?2/eV and Nss = 3.2×10¹⁶ cm^?3/eV for the wet‐annealed TFTs and are unchanged by the PB stress test. It was also found that the photo‐response of a‐IGZO TFTs begins at 2.3 eV of photon excitation, which corresponds to subgap states observed by photoemission spectroscopy. The origin of the NBL instability for the wet‐annealed TFTs is attributed to interface effects and considered to be a trap of holes at the channel‐gate—insulator interface where migration of the holes is enhanced by the electric field formed by the negative gate bias.     

Driver‐circuits‐integrated LCDs based on novel amorphous In‐Ga‐Zn‐oxide TFT

Abstract— A liquid‐crystal panel integrated with a gate driver and a source driver by using amorphous In—Ga—Zn‐oxide TFTs was designed, prototyped, and evaluated. By using the process of bottom‐gate bottom‐contact (BGBC) TFTs, amorphous In—Ga—Zn‐oxide TFTs with superior characteristics were provided. Further, for the first time in the world, a 4‐in. QVGA liquid‐crystal panel integrated with a gate driver and a source driver was developed by using BGBC TFTs formed from an oxide semiconductor. By evaluating the liquid‐crystal panel, its functionality was successfully demonstrate. Based on the findings, it is believed that the novel BGBC amorphous In—Ga—Zn‐oxide TFT will be a promising candidate for future large‐screen backplanes having high definition.     

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 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.     

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.     

Novel top‐gate zinc oxide thin‐film transistors (ZnO TFTs) for AMLCDs

Abstract— High‐performance top‐gate thin‐film transistors (TFTs) with a transparent zinc oxide (ZnO) channel have been developed. ZnO thin films used as active channels were deposited by rf magnetron sputtering. The electrical properties and thermal stability of the ZnO films are controlled by the deposition conditions. A gate insulator made of silicon nitride (SiN_x) was deposited on the ZnO films by conventional P‐CVD. A novel ZnO‐TFT process based on photolithography is proposed for AMLCDs. AMLCDs having an aperture ratio and pixel density comparable to those of a‐Si:H TFT‐LCDs are driven by ZnO TFTs using the same driving scheme of conventional AMLCDs.     

Comparative study of a‐IGZO TFTs with direct current and radio frequency sputtered channel layers

In this work, a comparative study of electrical properties and gate‐bias stress stability between direct current (DC)‐sputtered and radio frequency (RF)‐sputtered amorphous indium–gallium–zinc oxide thin film transistors (a‐IGZO TFTs) is conducted. The RF‐sputtered a‐IGZO TFTs show higher field‐effect mobility and steeper sub‐threshold slope. The DC‐sputtered ones show a better uniformity of threshold voltage, enhanced stability under both positive bias stress and negative bias illumination stress. The X‐ray photoelectron spectroscopy characterization of the a‐IGZO films reveals that the concentration of oxygen vacancies and electron density in the RF‐sputtered a‐IGZO film is higher than that in the DC‐sputtered one, which probably accounts for the differences of electrical properties between the RF‐sputtered and DC‐sputtered a‐IGZO 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.     

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.     

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.     

Toward sub‐micron oxide thin‐film transistors for digital holography

High‐performance 2‐μm‐channel oxide thin‐film transistors (TFT) on glass substrate for a 7‐μm‐pixel‐pitch spatial light modulator panel for digital holography applications were fabricated using a two‐step source/drain etching process. It showed a μ_FE of 45.5 cm²/Vs, SS of 0.10 V/dec, and Von of near zero voltage. Furthermore, we succeeded in the demonstration of sub‐micron TFTs, which is an indispensable route to next‐generation spatial light modulation devices with near 1‐μm pixel pitch. The issue of short‐channel transistors for display applications is also introduced. Finally, the digital holographic demonstration results based on the fabricated backplane are presented.     

Comparative study of source–drain contact metals for amorphous InGaZnO thin‐film transistors

In this work, we compared the thin‐film transistor (TFT) characteristics of amorphous InGaZnO TFTs with six different source–drain (S/D) metals (MoCr, TiW, Ni, Mo, Al, and Ti/Au) fabricated in bottom‐gate bottom‐contact (BGBC) and bottom‐gate top‐contact (BGTC) configurations. In the BGTC configuration, nearly every metal can be injected nicely into the a‐IGZO leading to nice TFT characteristics; however, in the BGBC configuration, only Ti/Au is injected nicely and shows comparable TFT characteristics. We attribute this to the metal‐containing deposits in the channel and the contact oxidation during a‐IGZO layer sputtering in the presence of S/D metal. In bias‐stress stability, TFTs with Ti/Au S/D metal showed good results in both configurations; however, in the BGTC configuration, not all the TFTs showed as good bias results as Ti/Au S/D metal TFTs. We attribute this to backchannel interface change, which happened because of the metal‐containing deposits at the backchannel during the final the SiO₂ passivation.     

Amorphous‐silicon thin‐film transistors made at 280°C on clear‐plastic substrates by interfacial stress engineering

Abstract— A process temperature of ～300°C produces amorphous‐silicon (a‐Si) thin‐film transistors (TFTs) with the best performance and long‐term stability. Clear organic polymers (plastics) are the most versatile substrate materials for flexible displays. However, clear plastics with a glass‐transition temperature (T_g) in excess of 300°C can have coefficients of thermal expansion (CTE) much larger than that of the silicon nitride (SiN^x) and a‐Si in TFTs deposited by plasma‐enhanced chemical vapor deposition (PECVD). The difference in the CTE that may lead to cracking of the device films can limit the process temperature to well below that of the T_g of the plastic. A model of the mechanical interaction of the TFT stack and the plastic substrate, which provides design guidelines for avoid cracking during TFT fabrication, is presented. The fracture point is determined by a critical interfacial stress. The model was used to successfully fabricate a‐Si TFTs on novel clear‐plastic substrates with a maximum process temperature of up to 280°C. The TFTs made at high temperatures have higher mobility, lower leakage current, and higher stability than TFTs made on conventional low‐T_g clear‐plastic substrates.     

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.     

Single‐source dual‐layer amorphous IGZO thin‐film transistors for display and circuit applications

In this study, the authors report on high‐quality amorphous indium–gallium–zinc oxide thin‐film transistors (TFTs) based on a single‐source dual‐layer concept processed at temperatures down to 150°C. The dual‐layer concept allows the precise control of local charge carrier densities by varying the O₂/Ar gas ratio during sputtering for the bottom and top layers. Therefore, extensive annealing steps after the deposition can be avoided. In addition, the dual‐layer concept is more robust against variation of the oxygen flow in the deposition chamber. The charge carrier density in the TFT channel is namely adjusted by varying the thickness of the two layers whereby the oxygen concentration during deposition is switched only between no oxygen for the bottom layer and very high concentration for the top layer. The dual‐layer TFTs are more stable under bias conditions in comparison with single‐layer TFTs processed at low temperatures. Finally, the applicability of this dual‐layer concept in logic circuitry such as 19‐stage ring oscillators and a TFT backplane on polyethylene naphthalate foil containing a quarter video graphics array active‐matrix organic light‐emitting diode display demonstrator is proven.     

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.     

Uniformity and bias‐temperature instability of bottom‐gate zinc oxide thin‐film transistors (ZnO TFTs)

Abstract— Short‐range uniformity and bias‐temperature (BT) instability of ZnO TFTs with SiO^x/SiN^x stacked gate insulators which have different surface treatments have been investigated. The short‐range uniformity of ZnO TFTs was drastically improved by N²O plasma treatment of the gate insulator. The variation in the gate voltage where a drain current of 1‐nA flows (Vgs at an Ids of 1 nA) was dramatically reduced from ±1.73 V to ±0.07 V by N²O plasma treatment of the gate insulator. It was clarified that the variations in the subthreshold characteristics of the ZnO TFTs could be reduced by N²O plasma treatment of the gate insulator due to a decrease in the variation of trap densities in deep energy levels from 0.9–2.0 × 10¹⁷ to 1.2–1.3×10¹⁷ cm^?3‐eV^?1. From the BT stress tests, a positive shift of Vgs at an Ids of 1 nA could be reduced by N²O plasma treatment of the gate insulator due to a decrease in the charge traps in the gate insulator. When the gate‐bias stress increases, state creation occured in the ZnO TFTs in addition to the charge trapping in the gate insulator. However, N²O plasma treatment of the gate insulator has little effect on the suppression of the state creation in ZnO TFTs under BT stress. The surface treatment of the gate insulator strongly affects the short‐range uniformity and the BT instability of Vth in the ZnO TFTs.     

Impact of bending on flexible metal oxide TFTs and oscillator circuits

Thin‐film circuits on plastic capable of high‐frequency signal generation have important applications in large‐area, flexible hybrid systems, enabling efficient wireless transmission of power and information. We explore oscillator circuits using zinc‐oxide thin‐film transistors (ZnO TFTs) deposited by the conformal, layer‐by‐layer growth technique of plasma‐enhanced atomic layer deposition. TFTs on three substrates—glass, 50‐µm‐thick freestanding polyimide, and 3.5‐µm‐thick spin‐cast polyimide—are evaluated to identify the best candidate for high‐frequency flexible oscillators. We find that TFTs on ultrathin plastic can endure bending to smaller radii than TFTs on commercial 50‐µm‐thick freestanding polyimide, and their superior dimensional stability furthermore allows for smaller gate resistances and device capacitances. Oscillators on ultrathin plastic with minimized parasitics achieve oscillation frequencies as high as 17 MHz, well above the cutoff frequency f_T. Lastly, we observe a bending radius dependence of oscillation frequency for flexible TFT oscillators and examine how mitigating device parasitics benefits the oscillator frequency versus power consumption tradeoff.