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Thin film transistor (TFTs) is a fundamental building block in electronic circuits. Since the specification for TFTs to drive the pixel of flat panel displays rather differs from that for CPUs and memories because of their large dimension.
Amorphous semiconductors have distinct advantages in the formation of large-sized homogeneous thin films at low temperatures but their Fermi level control by impurity doping or field effect is generally impossible due to high density carrier traps arising from structural randomness of amorphous structure. Amorphous hydrogenated silicon (a-Si:H) is an exceptional semiconductor in which Fermi level is controllable but the mobility remains 0-5-1 cm2/Vs, which is lower by 2 orders of magnitude than that of polycrystalline Si. Their performance was, however, enough to drive liquid panel displays which are driven by voltage. This is the main reason why a-Si:H has been exclusively applied as the channel material of TFTs for driving LCDs, and the display market has now grown to >$100 billion.
Next generation displays such as large-sized high definition LCDs and organic light emitting diodes (OLEDs) needs higher mobility TFTs. In 1995, I proposed a material design concept for transparent amorphous oxide semiconductors (TAOS) with high electron mobility on the basis of a simple consideration of chemical bonding along with several concrete materials. After the validity of this concept was verified by a combined research of experiment and computation, we reported transparent TFTs using In-Ga-Zn-O (IGZO) which is a member of TAOS materials in 2003 and 2004.
The mobility of a-IGZO TFTs fabricated by conventional sputtering method is higher by an order of magnitude than that of a-Si:H. Demonstrations of IGZO TFT-based displays began to emerge in 2007, when Samsung Electronics first reported a-IGZO TFT-based high-definition LCDs. Demonstrations of IGZO TFT-based backplane LCDs in smart phones and tablet computers then appeared around 2012. Arguably the most striking application emerged in 2015, when LG Display demonstrated large-sized OLED televisions. You can see now the beautiful images of 55' and 65' OLEDs TVs in electrical stores and so on. IGZO-TFTs are going to be implemented to large-sized and 8K LCD-TVs. In this lecture, I will talk a research story on TAOS from fundamental idea for materials to technical progress for implementation to displays [1].
Prof. Dr. Hideo Hosono (Tokyo Institute of Technology, Japan)