We developed a high-mobility oxide semiconductor material and made a 5.46ƍƍ prototype of 806-ppi 4K2K liquid crystal display using CAAC-OS FET that has a top-gate structure. The top-gate structure caused the parasitic capacitance to be small, resulting in a bezel width of 0.675 mm.
Author Keywordscrystalline OS, LCD, narrow bezel, high mobility, ultra-high resolution, top-gate self-aligned (TGSA) type
BackgroundRecently the demand for high-resolution narrow-bezel panels has increased due to their use in small and intermediate-sized displays such as smartphones. Low-temperature polysilicon (LTPS) is often used for the backplanes of such displays. LTPS transistors possess high mobilities and low driving voltages. However, LTPS has several disadvantages including high power consumption when displaying still images due to high leakage current (I off ), high cost, multiple processes, and the use of special equipment such as laser crystallization apparatus that use linear laser beams, and ion-doping apparatus. The I off of LTPS transistors are larger than those of amorphous silicon (a-Si) and oxide semiconductor (OS) transistors. Therefore, it is necessary to reduce the resistance or reduce I off by splitting the voltage between the drain and the source. Furthermore, because the grain boundary may induce high I off , the transistors must be made in series to reduce the possibility of dot defects. Therefore, double-gate structures are adopted for the pixel circuits of displays. Consequently, the aperture ratio is small at higher resolutions.Currently, a-Si is used as the backplane for large displays such as TVs. To achieve the resolution required for ultra-high definition (4 or 8K) and a driving frequency of 120 Hz, a replacement material for a-Si must be identified; one candidate replacement material is OS. Transparent amorphous oxide semiconductors were proposed by Kamiya, Hosono, et al., who claimed that amorphous structures are ideal [6-8], whereas we have developed crystalline IGZO.We reported c-axis-aligned crystalline IGZO (CAAC-IGZO) and nanocrystalline IGZO (nc-IGZO), which have crystal morphologies different from those of single-crystal and amorphous IGZO [9][10][11][12]. Researchers at Cornell University have also investigated CAAC-OS [13][14][15][16][17][18]. In CAAC-IGZO films, atomic arrangement parallel to the substrate surface was observed using cross-sectional transmission electron microscopy (TEM). X-ray diffraction analysis revealed a peak at approximately 31q, indicating c-axis alignment, whereas a-b plane alignment was not observed. Furthermore, no clear grain boundary was observed by plane-view TEM. CAAC-IGZO FETs have extremely low I off values on the order of yoctoamperes per micrometer (10 24 A/Pm) owing to their low carrier densities. These FETs are less likely to suffer from shortchannel effects and are thus suitable for scaling [19]. Consequently, OS is expected to be useful for next-generation high-resolution displays.OS is generally considered unsuitable for high-resolution, small d...