A morphous semiconductors have created a new area of electronics known as 'giant microelectronics' typifi ed by devices such as solar cells and active-matrix (AM) fl at-panel displays. Single-crystalline semiconductor technology, typifi ed by crystal silicon electronics, is unsuitable for such applications, whereas amorphous or polycrystalline fi lms can be easily formed over large areas of greater than 1 m 2 at low temperature (e.g. < 400 °C) on both glass and plastic substrates, facilitating these new applications. Due to carrier scattering and trapping at defects on grain boundaries in polycrystalline materials (the grain boundary problem), hydrogenated amorphous silicon (a-Si:H) has been used more widely than polycrystalline silicon (p-Si) for practical large-size applications. However, the critical obstacles to be overcome to realize a-Si:H in future applications are low mobility (< 1 cm 2 /V s) and instability against electric stress and photo-illumination.In late 2004, we reported that an amorphous oxide semiconductor (AOS) with the composition a-InGaZnO 4 (a-IGZO) can be applied in the fabrication of fl exible, transparent thin-fi lm transistors (TFTs) having much improved performance compared to conventional TFTs based on a-Si:H and organic materials [1]. AOS materials have superior characteristics to conventional semiconductors, and therefore AOS TFT technology has grown very rapidly toward the realization of TFT backplanes for next-generation flat-panel displays. As summarized in Figure 1, a variety of prototype AM displays have been demonstrated by several companies within the last fi ve years since the fi rst report of AOS TFTs. Herein, we review the unique characteristics of AOS materials in relation to their TFT characteristics, and discuss why AOSs have such attractive electrical properties related to the electronic structures specifi c to transparent oxides.
Active-matrix displays and circuits based on AOS TFT arraysTh e fi rst AOS-TFT was reported in late 2004 (see Figure 1). Just one year later, Toppan Printing quickly followed with the fi rst AM display based on AOS as a fl exible black-and-white electronic paper (e-paper) using a-IGZO TFTs