# Research Progress on Flexible Oxide-Based Thin Film Transistors
## 1 Introduction  
Since the beginning of the 21 st century, the display industry has made great progress, flat panel displays (FPD) started coming into public view as liquid crystal displays (LCD) are pulling out [1; 2; 3; 4; 5; 6; 7; 8]. Through several generations of product upgrades, LCDs have been absolutely dominant before active matrix organic light-emitting diodes (AMOLED) appeared [9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19]. OLEDs are considered as one of the most promising next generation display technologies due to their superior performance such as high efficiency, wide viewing angle, low power and small thickness [20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33]. Recently, the oxide-based TFTs driving OLED have become the most promising technology to realize really flexible displays [34; 35; 36; 37; 38; 39; 40; 41]. The technological advance in TFTs is a significant factor for promoting the development of such displays [42; 43; 44; 45; 46; 47; 48; 49; 50]. Researchers around [PERSON] et al [42], the earliest engaged in this field, have successfully fabricated the first transparent and flexible oxide-based TFTs, which have rapidly attracted the interest of numerous researchers all over the world [51; 52; 53; 54; 55; 56; 57; 58; 59; 60; 61]. Compared with hydrogenated amorphous silicon (a-Si:H) TFTs and low-temperature polysilicon (LTPS) TFTs [62; 63; 64; 65; 66; 67; 68; 69; 70; 71; 72; 73], oxide TFTs are reputed as the next-generation displays TFT technology due to their high performance, such as relatively high carrier mobility, good uniformity, low process temperature, optical transparency, low cost and flexibility [74; 75; 76; 77; 78; 79]. Besides, integrated circuits containing oxide-based TFTs are gradually getting larger and replacing the silicon-based CMOS circuits. Examples are digital logic circuits, oscillation rings, and operational amplifiers [80; 81; 82; 83; 84; 85; 86; 87; 88; 89; 90; 91; 92; 93; 94; 95]. In 2004, ZnO-channel TFT test structures were fabricated by [PERSON] et al. using a bottom-gate structure on thermally oxidized Si, while ZnO has been deposited via radiofrequency (RF) sputtering from an oxide target with an unheated substrate [96]. In these devices, the ZnO-channel TFT mobility was as high as 25 cm\({}^{2}\) V\({}^{-1}\) s\({}^{-1}\), which proved that oxide TFTs may become desirable alternatives for high-resolution display applications. Then, [PERSON] et al. reported a bottom gate configuration ZnO-TFT with a mobility value of 20 cm\({}^{2}\) V\({}^{-1}\) s\({}^{-1}\) and an optical transmittance above 80%, including a glass substrate, via RF magnetron sputtering at room temperature in 2005 [97]. It has been shown that ZnO-TFT can potentially be fabricated on thermal-sensitive substrates, such as plastics and metal foils. With the development of TFTs, the feature size of devices is becoming smaller. Hence the short channel effect on TFTs should be considered. [PERSON] et al., in 2006, fabricated a series of miniaturized ZnO TFTs with various channel widths and lengths [98]. Well-behaved TFTs with channels of 5 \(\upmu\)m were achieved with a saturated mobility of \(>\)8 cm\({}^{2}\) V\({}^{-1}\) s\({}^{-1}\) and on/off ratios of up to 10\({}^{7}\). At the same time, channel layers with amorphous indium gallium zinc oxide (a-IGZO) were employed in TFTs by [PERSON] and his colleagues, with RF magnetron sputtering at room temperature [99]. These devices operated in n-type enhancement mode with a field-effect mobility of 12 cm\({}^{2}\) V\({}^{-1}\) s\({}^{-1}\), an on-off ratio of \(\sim\)10\({}^{8}\), and a subthreshold gate voltage swing of 0.2 V decade\({}^{-1}\) and opened the possibility for high-resolution and integrated circuit applications. A higher mobility of 100 cm\({}^{2}\) V\({}^{-1}\) s\({}^{-1}\) was reported by [PERSON] et al. [100]. However, it is still necessary to understand the origin of the hysteresis to achieve proper device stability under encapsulating conditions.