Tatsuya Takei

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

— 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).

A 5.8‐in. phosphorescent color AMOLED display fabricated by ink‐jet printing on plastic substrate

— A flexible phosphorescent color active-matrix organic light-emitting-diode (AMOLED) display on a plastic substrate has been fabricated. Phosphorescent polymer materials are used for the emitting layer, which is patterned using ink-jet printing. A mixed solvent system with a high-viscosity solvent is used for ink formulation to obtain jetting reliability. The effects of evaporation and the baking condition on the film profile and OLED performances were investigated. An organic thin-film-transistor (OTFT) backplane, fabricated using pentacene, is used to drive the OLEDs. The OTFT exhibited a current on/off ratio of 106 and a mobility of 0.1 cm2/V-sec. Color moving images were successfully shown on the fabricated display.

16.4: Low‐Temperature Fabrication of Flexible AMOLED Displays Using Oxide TFTs with Polymer Gate Insulators

We have developed InGaZnO4 TFTs with polymer gate insulators that can be formed by spin-coating on plastic substrates at temperatures below 130 °C. A 5-inch QVGA flexible OLED display has been fabricated by means of ink-jet printing on a TFT backplane, and it has successfully displayed clear color video images while in a bent state.

A 40-inch-diagonal HDTV DC plasma display

A new 40-inch-diagonal color DC plasma display is proposed on the basis of our previous 20 to 33-inch PDPs. To achieve high performance, we developed various precision fabrication technologies and a new panel driving technique to ensure sufficient memory margin for the pulse memory drive scheme. Experimental investigations result in stable reproduction of high-definition TV (HDTV) pictures with a peak white luminance of 93 cd/m/sup 2/. Several hybrid ICs for the new electrode drive technique contribute to reducing the display thickness to 8 cm. >

Influence of Oxide Semiconductor Thickness on Thin-Film Transistor Characteristics

We discuss here the influence of oxide semiconductor thickness on thin-film transistor (TFT) characteristics. We have determined this influence by measuring the transfer characteristics of amorphous InGaZnO (IGZO) TFTs having various IGZO thicknesses and using a simple method to calculate the depletion width in IGZO films. ON current was nearly constant with respect to IGZO thickness because it depended on a high electron density in an accumulation region sufficiently thinner than the IGZO film. The threshold voltage shifted negatively with increasing IGZO thickness, which indicates that a thicker IGZO film requires a higher negative gate voltage for it to be fully depleted. Calculation results suggest that threshold voltage variation due to IGZO thickness variation increases with increasing donor density and IGZO thickness.

Fabrication method for self-aligned bottom-gate oxide thin-film transistors by utilizing backside excimer-laser irradiation through substrate

A method for fabricating self-aligned bottom-gate InGaZnO (IGZO) thin-film transistors (TFTs) with low parasitic capacitance by utilizing backside excimer-laser irradiation through a substrate is proposed and experimentally validated. Irradiation from the backside of the glass substrate using gate electrode as a mask reduces resistance of the IGZO film selectively for their application as source/drain regions in bottom-gate IGZO-TFTs. This method offers a wide process margin with respect to laser energy density and is applicable to large-area processing.

Analysis of the Influence of Sputtering Damage to Polymer Gate Insulators in Amorphous InGaZnO4 Thin-Film Transistors

We have fabricated InGaZnO4 (IGZO) thin-film transistors (TFTs) with olefin-type polymer gate insulators formed at 130 °C and compared the transfer characteristics of top- and bottom-gate structures. We have investigated the mechanism behind the differences in the characteristics, focusing on the influence of IGZO sputtering damage to the polymer insulators. Bottom-gate IGZO-TFTs showed significant threshold voltage decreases attributable to positive fixed charges in the polymer gate insulators. These charges were generated by diffused metal ions during IGZO sputtering. Contrastingly, there was no sputtering damage to the polymer gate insulators in top-gate IGZO-TFTs, and these showed good switching performance.

Efficient Simulation Model for Amorphous In-Ga-Zn-O Thin-Film Transistors

A computationally efficient simulation model for the drain current characteristics of long-channel amorphous In-Ga-Zn-O (a-IGZO) thin-film transistors (TFTs) is developed. This model uses numerical solutions of the one-dimensional Poisson equation to significantly reduce the calculation time compared to a widely used two-dimensional approach. Moreover, for accurate simulation, the model takes into account the influence of trap states in the band gap, which makes it possible to reproduce the gradual increase of the drain current in the subthreshold region. The model also includes both drift and diffusion components of the drain current and so can describe the drain current in all regions of device operation, i.e., the subthreshold, linear, and saturation regions, by using a unified current equation without introducing the threshold voltage as an input parameter. Calculations using the model provide results that are in good agreement with the measured drain current characteristics of a-IGZO TFTs over a wide range of gate and drain voltages. The presented model is expected to enable faster and accurate characteristic analysis and structure design for a-IGZO TFTs.

Improvement in image quality of a 5.8-in. OTFT-driven flexible AMOLED display

— The image quality of an OTFT-driven flexible AMOLED display has been improved by enhancing the performance of OTFTs and OLEDs. To reduce the operating voltage of OTFTs on a plastic film, Ta2O5 with a high dielectric constant was used as a gate insulator. The organic semiconductor layer of the OTFT was successfully patterned by a polymer separator, which is an isolating wall structure using an organic material. The OTFT performance, such as its current on/off ratio, carrier mobility, and spatial uniformity on the backplane, was enhanced. A highly efficient phosphorescent OLED was used as a light-emission device. A very thin molybdenum oxide film was introduced as a carrier-injection layer on a pixel electrode to reduce the operating voltage of the OLED. After an OTFT-driven flexible AMOLED display was fabricated, the luminance and uniformity on the display was improved. The fabricated display also showed clear moving images, even when it was bent at a low operating voltage.

Development of Back-Channel Etched In-W-Zn-O Thin-Film Transistors

Back-channel etched (BCE) thin-film transistors (TFTs) are developed using a novel oxide semiconducting material, In-W-Zn-O (IWZO). A bi-layer structure for the IWZO oxide semiconductor layer is proposed to realize both high resistance to back-channel etching damage and high TFT mobility. The developed IWZO BCE-TFTs exhibit high mobilities of up to 20.2 cm2/V·s.