[SID] Sony Develops 11.7-inch Oxide Semiconductor TFT-driven OLED Panel
Sony Corp developed a 11.7-inch OLED panel by using oxide semiconductor TFTs as its driver elements and exhibited it at SID 2010, an international conference on display technologies, in Seattle, the US.
By reducing the property degradation of the oxide semiconductor TFT, the company ensured a lifetime of 10 years or more, which is required for OLED TVs, it said.
The oxide semiconductor TFT used for the OLED panel has an etch stopper structure, and its channel material is amorphous IGZO (In-Ga-Zn-O). The TFT element has a gate width of 22μm and a gate length of 8μm. Its carrier mobility, threshold voltage, subthreshold swing (S) value are 11.5cm2/Vs, 0.27V and 0.3V/decade, respectively.
To reduce the property degradation of the amorphous IGZO TFT, Sony made the following three improvements. First, the company prevents the oxidation of the electrode by changing the structure of the source/drain electrode from the commonly-used Ti/Al/Ti three-layer structure to Ti/Al/Mo structure (Mo is in contact with the amorphous IGZO).
Second, for the passivation film, Sony used Al2O3, which has a high protection performance, instead of SiNx or SiOx. And the passivation film is formed by the DC sputtering method. Third, the company employed a TFT structure in which the amorphous IGZO is completely covered by the passivation film, the etch stopper film and the source/drain electrode.
This time, Sony announced a 11.7-inch OLED panel that was made at the company's research laboratory located Atsugi City, Kanagawa Prefecture, Japan. The panel has a resolution of 960 x 540 pixels, a peak brightness of 600cd/m2 or more, a contrast ratio of 1,000,000: 1 or more, and 100% or higher color gamut vs NTSC. The maximum screen luminance with an all-white signal is 200cd/m2.
The device structure of the organic EL element is a top emission type, which extracts light from the side opposite to the TFT substrate. Compared with the "XEL-1," an OLED TV released by Sony in December 2007, the light emitting device of the new OLED panel is made with materials that have better properties such as color gamut and lifetime, the company said.