# Research Progress on Flexible Oxide-Based Thin Film Transistors
## 1 Introduction
An etch stop (ES) structure was employed in TFTs, where a 100-nm-thick SiO\({}_{\text{x}}\) layer was deposited by plasma enhanced chemical vapor deposition to prevent plasma damage to the active layer [101]. Such ES structure showed good stability under the condition of bias and illumination stress. After that, to improve the performance of oxide-based TFT, many workers reported to the use of different electrode materials. In 2018, [PERSON] et al. focused on the high-frequency properties of a-IGZO TFT, and fabricated devices on high resistivity silicon substrates with a Ta\({}_{2}\)O\({}_{5}\) gate dielectric. Such devices, high performance and fully oxide-based complementary logic gates for a range of electronics applications could become reality [59]. Additionally, dual gate structures of a-IGZO TFTs were presented [102; 103; 104; 105; 106; 107]. With such structures, top gates were served as the device properties controlling terminals, the properties of TFTs could be improved by changing the top gate voltage. However, some challenges could still be considered while using top gate devices. In the past ten years, with the progress of flexible electronic devices, the performance of oxide-based TFTs has been enhanced step-by-step and nowadays can satisfy the demand for real commercialization for flexible displays, wearable devices and curved televisions. Besides, with the improved understanding of TFT technology, related techniques, particularly for flexible TFTs, have been developed. Therefore, how to further enhance the performance of TFTs is also significant to other related fields. Several methods of low-temperature processes have been studied such as solution processing of oxide TFTs, which has attracted both industrial and academic interest [108; 109; 110; 111; 112; 113; 114; 115; 116; 117; 118; 119]. Compared to the sputtering processing, employing solution processing in fabrication of TFTs could not only avoid using the expensive equipments, but also be compatible with ink-jet printing and roll-to-roll processes, which can significantly reduce the cost [120; 121; 122; 123; 124; 125; 126; 127; 128; 129; 130; 131]. Hence, research work engaging in the oxide-based TFTs' technology is focusing on all aspects of TFTs to obtain better-performance devices, including materials, structures, processes, stabilities and mechanical properties. In this review, the basic fundamental concept of the carrier transport mechanism of oxide semiconductor materials is introduced in Section 2.1. Secondly, five kinds of conventional configurations of oxide-based TFTs are briefly presented in Section 2.2, including ES, a back channel etch (BCE), top gate (TG), dual gate (DG), and co-planar (CP) structures. Next, the primary strategies to develop flexible oxide-based TFT on different kinds of flexible substrates with different process temperatures are presented in Section 3.1. After that, the mechanical stress effects of common bending behaviors on flexible TFTs were investigated and the effective methods to decrease these mechanical stress effects are given in Section 3.2. In the end, an outlook for the future development of flexible oxide-based TFT is presented.