Bulk-accumulation oxide-TFT backplane technology for flexible and rollable AMOLED displays: Part II

IN the first of our two-part series on a new backplane technology for flexible and rollable AMOLED displays, we reviewed a bulkaccumulation (BA) amorphous indiumgallium- zinc-oxide (a-IGZO) thin-film transistor (TFT) with 3-5 times the drain current of a comparable conventional single-gate TFT. The advantages of BA TFTs include excellent performance from circuits such as ring oscillators and gate drivers, and also higher robustness under mechanical bending. The TFT backplane necessary for flexible OLEDs can be realized with low-temperature polycrystalline silicon (LTPS) or oxide semiconductors because of the high performance of these materials. Currently, all AMOLED products manufactured on polyimide substrates use LTPS with excimer-laser annealing. Another material with promise for use as TFTs on flexible substrates is amorphous oxide. For the last 10 years, a huge number of research groups have been working on amorphous-oxide-semiconductor (AOS) TFTs both on glass and plastic substrates. The first AOS TFT product was introduced in 2003, and since then many LCD and AMOLED-display products with a-IGZO TFT backplanes have been launched. However, a-IGZO TFTs also have challenges. The low yield, non-uniformity, and bias instability of oxide TFTs limit their wide applications to commercial products. In part one of this article, we explained how our university's research teams have made significant progress in overcoming some of those limitations by developing a bulk-accumulation TFT, which employs an n-type a-IGZO as its active material, a silicon-dioxide layer as both gate-insulator and passivation layer, and molybdenum as its metal electrodes. In the remainder of this article, part two, we will describe a flexible AMOLED display with integrated gate drivers using BA oxide TFTs that is demonstrated with a carbon-nanotube/ graphene-oxide (CNT/GO) buffer embedded in a plastic substrate.