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.     

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.     

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.     

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.     

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.     

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.     

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.     

Development of rollable silicon thin‐film‐transistor backplanes utilizing a roll‐to‐roll continuous lamination process

Abstract— Rollable silicon thin‐film‐transistor (TFT) backplanes utilizing a roll‐to‐roll process have been developed. The roll‐to‐roll TFT‐backplane technology is characterized by a glass‐etching TFT transfer process and a roll‐to‐roll continuous lamination process. The transfer process includes high‐rate, uniform glass‐etching to transfer TFT arrays fabricated on a glass substrate to a flexible plastic film. In the roll‐to‐roll process, thinned TFT‐glass sheets (0.1 mm) and a base‐film roll are continuously laminated using a permanent adhesive. Choosing both an appropriate elastic modulus for the adhesive and an appropriate tension strength to be used in the process is the key to suppressing deformation of the TFT‐backplane rolls caused by thermal stress. TFT backplanes that can be wound, without any major physical damage such as cracking, on a roll whose core diameter is approximately 300 mm have been sucessfully obtained. Incorporating the TFT‐backplane rolls into other roll components, such as color‐filter rolls, will make it possible to produce TFT‐LCDs in a fully roll‐to‐roll manufacturing process.     

Modeling of current—voltage characteristics for double‐gate a‐IGZO TFTs and its application to AMLCDs

Abstract— The equations for the transfer characteristics, subthreshold swing, and saturation voltage of double‐gate (DG) a‐IGZO TFTs, when the top‐ and bottom‐gate electrodes are connected together (synchronized), were developed. From these equations, it is found thatsynchronized DG a‐IGZO TFTs can be considered as conventional TFTs with a modified gate capacitance and threshold voltage. The developed models were compared with the top or bottom gate only bias conditions. The validity of the models is discussed by using the extracted TFT parameters for DG coplanar homojunction TFTs. Lastly, the new pixel circuit and layout based on a synchronized DG a‐IGZO TFT is introduced.     

Effect of source/drain overlap region on device performance in a‐IGZO thin‐film transistors

Abstract— In this paper, the effect of source/drain overlap length on the amorphous indium gallium zinc oxide (a‐IGZO) TFT performance has been investigated. Results of this paper show that as source/drain overlap length decreases to a negative value forming S/D offset, the threshold voltage and S parameters of a‐IGZO TFTs increased and the field‐effect mobility decreased. The V_T variation increases sharply as the channel length decreases because of the large resistance R_offset when it is formed at a‐IGZO source/drain. In the experiment, R_offset of each 1 μm, evaluated from the transfer length method (TLM), shows approximately 54–66 kΩ. This means thatthe source/drain overlap length is a very important control parameter for uniform device characteristics of a‐IGZO TFTs.     

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.     

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.     

Novel mobile TFT‐LCDs based on SLS technology

Abstract— Single‐crystal‐like silicon (SLS) technology is the most cost‐effective laser‐crystallization process ever invented. The throughput of the SLS process is about two times higher than that of the conventional excimer‐laser annealing (ELA) method. In addition, the performance of the TFTs fabricated by the SLS process is among the best utilized in mass production. Various TFT‐LCDs employing SLS technology, which included a 1.02‐in. full SOG LCD using an icon display for the sub‐display of cellular phones, a 1.9‐in. qVGA TFT‐LCD with a low‐power analog interface employing a low‐voltage driving scheme, and a 3.0‐in. VGA TFT‐LCD compatible with the 480i data format without additional signal processing were developed. Because the SLS process enables us to achieve highly uniform and reliable transistors, it can be effectively utilized in the mass production of mobile TFT‐LCDs with low power consumption and enhanced image quality.     

Performance improvements of IGZO and ZnO thin‐film transistors by laser‐irradiation treatment

Abstract— Zinc oxide (ZnO) and indium gallium zinc oxide (IGZO) thin films subjected to laser irradiation were investigated. The structural, optical, and electrical properties of the as‐deposited and laser‐irradiated films at different laser dosages were studied. The crystallinity of the structure increased after laser treatment. The transmittances without/with laser irradiation had a net rise of 85–92% and 80–95% (@550 nm) for 250‐nm ZnO and IGZO films, respectively. Thin‐film transistors (TFTs) with ZnO and IGZO as the active layer were fabricated. The as‐deposited ZnO/IGZO TFT devices had a field‐effect mobility of 0.19 and 1.3 cm²/V‐sec, respectively. The electrical characteristics increased by more than 2.8 times for ZnO and by 5.8 times for IGZO with laser treatment. The field‐effect mobility of ZnO and IGZO are 0.5 and 7.65 cm²/V‐sec.     

A design method for flicker‐free liquid crystal display panels based on indium‐gallium‐zinc‐oxide thin‐film transistors

This paper proposes a design method to reduce the flicker of liquid crystal display panels based on indium‐gallium‐zinc‐oxide (IGZO) thin‐film transistors (TFTs). The proposed design method employs a human factor model to convert the flicker measured at low frame frequency (F _FRAME) to a modification value of the measured flicker (MVMF ) having a frequency sensitivity of flicker, which can distinguish between no blinking and weak blinking. To investigate the causes and characteristics of flicker, the frequency component and increase factor of flicker are analyzed using the checkerboard and solid images. The increase factor in flicker is examined using IGZO TFTs with different antenna ratios (AR s) that cause the variation in threshold voltage of IGZO TFT. To verify the proposed design method, two test panels are implemented with asymmetric and symmetric AR s. The MVMF s of the 15 Hz component at a low F _FRAME of 30 Hz show that the solid image with a symmetric AR has an MVMF of ?62.9 dB, which is improved by 24.3 dB compared to that with an asymmetric AR . Therefore, the proposed method is applicable for a flicker‐free liquid crystal display panels at a low F _FRAME.     

Current and future technology of low‐temperature poly‐Si TFT‐LCDs

Abstract— The state of the art of large‐area low‐temperature TFT‐LCDs will be reported in this paper. High‐performance poly‐Si TFTs are expected to realize various applications such as system display where various signal‐processing functions are added to the display. In the past few years, low‐temperature poly‐Si thin‐film‐transistor (LTPS TFT) technology has made great progress, especially in the areas of excimer laser annealing (ELA) of high‐quality poly‐Si film, ion doping for large‐area doping, and high‐quality gate SiO₂ film formation by using the low‐temperature PE‐CVD method. Also, technology trends and possible applications, such as a system displays, will be discussed.     

Latest developments for “system‐on‐glass” displays using low‐temperature poly‐Si TFTs

Abstract— A 2.3‐in.‐diagonal QVGA‐formatted “System‐On‐Glass” display has been developed by using low‐temperature poly‐Si TFT‐LCD technology. This display fully integrates 6‐bit RGB digital interface drivers as well as all the power supply circuitry to drive the LCD, which requires neither external driver ICs nor power‐supply ICs. This paper discusses the newly developed TFT circuit technologies used in this LCD. The development trend of the “System‐On‐Glass” display is also reviewed.     

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.     

Negative bias illumination stress assessment of indium gallium zinc oxide thin‐film transistors

Electrical performance stability of indium gallium zinc oxide (IGZO) thin‐film transistors (TFTs) is evaluated under negative bias illumination stress (NBIS). A bottom‐gate IGZO TFT whose top surface is passivated with zinc tin silicon oxide (ZTSO) exhibits a dramatic improvement in NBIS stability compared with that of an unpassivated, bottom‐gate IGZO TFT. Oxygen chemisorption/desorption at the channel layer top surface is proposed to explain why an unpassivated TFT exhibits significantly more NBIS than a passivated TFT.     

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.