Transparent electronic devices formed on flexible substrates are expected to meet emerging technological demands where silicon-based electronics cannot provide a solution. Examples of active flexible applications include paper displays and wearable computers. So far, mainly flexible devices based on hydrogenated amorphous silicon (a-Si:H) and organic semiconductors have been investigated. However, the performance of these devices has been insufficient for use as transistors in practical computers and current-driven organic light-emitting diode displays. Fabricating high-performance devices is challenging, owing to a trade-off between processing temperature and device performance. Here, we propose to solve this problem by using a novel semiconducting material--namely, a transparent amorphous oxide semiconductor from the In-Ga-Zn-O system (a-IGZO)--for the active channel in transparent thin-film transistors (TTFTs). The a-IGZO is deposited on polyethylene terephthalate at room temperature and exhibits Hall effect mobilities exceeding 10 cm2 V(-1) s(-1), which is an order of magnitude larger than for hydrogenated amorphous silicon. TTFTs fabricated on polyethylene terephthalate sheets exhibit saturation mobilities of 6-9 cm2 V(-1) s(-1), and device characteristics are stable during repetitive bending of the TTFT sheet.

Recently, we have demonstrated the potential of amorphous oxide semiconductors (AOSs) for developing flexible thin-film transistors (TFTs). A material exploration of AOSs desired as the channel layer in TFTs is most important for developing high-performance devices. Here, we report our concept of material exploration for AOSs in high-performance flexible and transparent TFTs from the viewpoints of chemical bonding and electronic structure in oxide semiconductors. We find that amorphous In-Ga-Zn-O (a-IGZO) exhibits good carrier transport properties such as reasonably high Hall mobilities (>10 cm 2 ÁV À1 Ás À1 ) and a good controllability of carrier concentration from <10 15 to 10 20 cm À3 . In addition, a-IGZO films have better chemical stabilities in ambient atmosphere and at temperatures up to 500 C. The flexible and transparent TFT fabricated using a-IGZO channel layer at room temperature operated with excellent performances, such as normally-off characteristics, on/off current ratios ($10 6 ) and field-effect mobilities ($10 cm 2 ÁV À1 Ás À1 ), which are higher by an order of magnitude than those of amorphous Si:H and organics TFTs.

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Abstruct-The wavelength dependence of waveguide-type directional couplers is investigated with respect to waveguide pattern symmetry in the coupling region. A rough approximation of the dependence is determined based on simplified transfer matrix multiplications, followed by a more detailed computer calculation using the beam propagation method (BPM). Wavelength-flattened coupling characteristics over a wide range, for example, from 1.3 to 1.55 pm are predicted for several types of directional coupler with waveguide width asymmetry or a tapered width structure in the coupling region. These novel channel-type directional couplers are experimentally fabricated on silicon substrates with silicabased planar lightwave circuit (PLC) technology, which consists of a combination of flame hydrolysis deposition (FHD) and reactive ion etching (RIE). As a result, we show that experimental results are in good agreement with theoretical predictions, as expected. It also becomes clear for the first time that these couplers can be classified into four typical patterns with respect to their minimum and maximum coupling ratios.