# Research Progress on Flexible Oxide-Based Thin Film Transistors
## 3 Approaches to Flexible Oxide-Based TFTs
### Flexible TFTs under Mechanical Stress
#### 3.2.2 Degradation of TFTs under Mechanical Stress  
As shown in Figure 11c, displays bent in one direction are the most common in the market. When the substrates are bent inwards or outwards randomly, TFTs suffer tensile or compressive strain. Depending on the direction of the strain, either along the channel width or along the channel length, different device performances can be observed. In this section, the bending behaviors of strained TFTs are discussed.  
Figure 12 shows two kinds of bending behaviors of IGZO TFTs [160]. In Figure 12a, the TFT was bent along with the channel width, while the TFT in Figure 12b was bent along with the channel length. For both cases, the values of the mobility and the threshold voltage were evaluated from the transfer curves measured at the various bending radii. As one can observe from Figure 12c, there was almost no degradation in the transistor performance until the bending radius was decreased to about 3 mm. When the bending radius dropped below 3 mm, TFTs bent along the channel width were more stable than those bent along the channel length. Meanwhile, the threshold voltage increased because of the stretching and thinning of the polymer dielectric layer.  
The results of the repeatability tests with four forms are shown in Figure 13a-d [160]. In Figure 13a,b the TFTs were bent along the channel width at a bending radius of 3 mm and 1.5 mm, respectively. In Figure 13c,d, the TFTs were bent along the channel length, al  
Figure 12: Two kinds of bending behaviors of IGZO TFTs and their transfer curves; (**a**) bending along channel width and (**b**) along the channel length at various bending radius. (**c**) The extracted, mobility and threshold voltage at various bending radius [160].  
3 mm and 1.5 mm. The variations in mobility and threshold voltage values derived from the transfer curves in Figure 13a-d are shown in Figure 13e. In general, the TFTs bent along the channel width show better performance than those bent along the channel length.  
Figure 14a shows pictures of TFTs in a flexible display under compressive (\(\varepsilon<0\)) and tensile (\(\varepsilon>0\)) strain as the display is being subjected to repeated uni-directional bending [162]. [PERSON] and [PERSON] et al. reported that the threshold voltage in Figure 14b and the TFT mobility in Figure 14c shifted to opposite directions under compressive and tensile strain [81].  
Figure 14: _Cont._  
Figure 13: Transfer characteristics of TFTs repeatedly bent along the channel width and along the channel length with different radius: (**a**) 3 mm and (**b**) 1.5 mm as along the channel width, (**c**) 3 mm and (**d**) 1.5 mm along the channel length. (**e**) the mobility and threshold voltages as a function of the number of bending cycles [160].  
With the help of such bending tests, it was found that the degradation of TFTs is dependent on the direction of the strain. This behavior can be accounted for by considering the impact of the piezoelectric effect on the transport behavior in the ZnO channel/gate insulator/gate region [81]. Figure 15a shows the effect of compressive strain on the TFTs, considering that a negative piezoelectric field affects the interface in a way that shifts V\({}_{\text{TH}}\) towards the negative and the mobility toward the positive values. In Figure 15b, the tensile stain generates a positive piezoelectric field between the ZnO channel and the gate insulator interface. The V\({}_{\text{TH}}\) and the mobility are then shifted into the opposite direction compared to the compressive-strain case. With this mechanism in mind, the changes in electrical transport behavior reported in Figure 14b, c can be reasonably explained.