# Research Progress on Flexible Oxide-Based Thin Film Transistors
## 3 Approaches to Flexible Oxide-Based TFTs
### Flexible TFTs on Different Kinds of Substrates  
#### 3.1.1 PET Flexible Substrate  
PET is currently widely used as a protection layer in the application, due to its good optical transparency and short cut-off wavelength. However, PET will lose its optical and mechanical properties at high processing temperatures, which may bring challenges to device fabrication, integration and operation. During the past ten years, research about oxide-based TFTs fabricated on PET substrates has been reported, which aims to suppress defect formation in PET through evolutionary process steps and optimized device structures. In 2010, [PERSON] et al. first achieved all-amorphous-oxide transparent, flexible TFTs with bilayer gate dielectrics on PET substrates at room temperature [71]. This TFT had combined gate dielectrics (Ta\({}_{2}\)O\({}_{5}\)/SiOx bilayer structure), the active layer consisted of amorphous zinc-indium-tin oxide (a-ZITO) and S/D electrodes of an oxide conductor (a-ZITO), and exhibited a saturated mobility of 20 cm\({}^{2}\) V\({}^{-1}\) s\({}^{-1}\) and on/off ratios >10\({}^{5}\). As shown in Figure 3a, the bilayer gate dielectric with a 70 nm a-Ta\({}_{2}\)O\({}_{5}\) film and 5 nm S\({}_{1}\)O\({}_{x}\) layers exhibited excellent insulating properties. With thin S\({}_{1}\)O\({}_{x}\) layers, the dielectric-semiconductor interfacial compatibility could be enhanced and the hysteresis could be suppressed. However, there was no report on its stability. Another approach to reduce the process temperature is by using solution-based processes. [PERSON] et al. utilized an organic-inorganic heterostructure to prepare high-performance ZnO-TFTs on a PET substrate using a low-temperature process (<150 \({}^{\circ}\)C), as shown in Figure 3b [69]. In this device, PMMA was adopted as a dielectric layer, and a ZnO film formed the active layer. However, the compatibility of the interface between PMMA and ZnO (organic-inorganic) needed to be enhanced through optimized processing or by using other materials. High mobility oxide-based TFTs on flexible substrates are urgently needed, along with the development of high-resolution flexible displays and circuits integrated TFTs. [PERSON] et al. fabricated a flexible BCE structure TFT at room temperature by employing aluminum-doped ZnO (AZO) as an active channel layer [150]. In this way, a higher mobility of 21.3 cm\({}^{2}\) V\({}^{-1}\) s\({}^{-1}\), a drain current on/off ratio of 2.7 \(\times\) 10\({}^{7}\), a low off current value on the order of 10\({}^{-12}\)A, a subthreshold swing of 0.23 V dec\({}^{-1}\) and a threshold voltage of 1.3 V could be obtained, but no study on the stability of the device was provided. Figure 2: Classification of polymer optical films [149]. It is known that the substrates of TFTs should have a large mechanical deformation range, while employed in flexible displays, wearable electronics and in sensors. Thus, the bending stability in flexible applications is one of the most critical issues in the evaluation of device performance. [PERSON] and his co-workers fabricated a bottom gate structure for flexible transparent IGZO TFTs, as shown in Figure 4[151]. Figure 4a shows an un-passivated device, while Figure 4b presents a passivated one. The experimental results show that the 15-nm-thick SiO\({}_{\text{x}}\)-passivated IGZO flexible TFTs reveal a much better performance including a saturation drain-source current of 10.6 \(\mu\)A, a mobility of 8.0 cm\({}^{2}\) V\({}^{-1}\) s\({}^{-1}\), a threshold voltage of 2.85 V, a transconductance of 4.64 \(\times\) 10\({}^{6}\) S, and a subthreshold swing of 0.23 V dec\({}^{-1}\). Tests on the bending stability of the un-passivated and passivated IGZO flexible TFTs show that the variation of mobility and the threshold voltage is within 10% and 8% when bending for 1500 times with a bending radius of 1.17 cm.