(Invited) Channel Length Dependence of Mechanical Strain for Flexible (a-IGZO) TFT
Authors: Rahaman Abidur , Hasan Md Mehedi , Billah Mohammad Masum , Jang Jin 
Published: February 2020

We investigated the dependence of mechanical strain upon channel length, in flexible amorphous-indium-gallium-zinc-oxide (a-IGZO) thin-film transistors (TFTs) on 20 μm thick Polyimide substrate by using different radii cylinder. The negative shift of the transfer characteristic associated with strain decreases for increasing channel length (L) and is practically suppressed in devices with L=100 μm.
Amorphous oxide semiconductor (AOS) based devices are the future of transparent and flexible displays because of many advantages such as low deposition temperature and low manufacturing [1]-[2]. Recently, flexible a-IGZO TFTs on plastic substrates have been enthusiastically studied because they have the advantage of being thinner, lighter, bendable, rollable, and unbreakable [3]-[4].
Fig. 1 (a)-(c) shows the effects of vertical mechanical bending on the electrical performance of the flexible TFTs up to 0.80% strain having the TFT channel width (W) = 50 μm and length (L) = 10, 50, and 100 μm, respectively. A distinct robust TFT performance is observed by long channel TFT (L = 100 μm) as compared to short channel TFT (L = 10 μm) under the same strain condition. All the TFTs exhibited a negative shift of VTH (V) after mechanical strain. Negative VTH (V) shift under mechanical strain is thought to generation of donor-like defects (ΔNGD) at the bending part of the TFT channel [5], which might be at the center part of the channel. This highly strained channel region might generate more defects, resulting in an increase of carrier concentration. It has been reported that long channel length TFTs are less sensitive against the generation of deep donor-like defects [6] which also support our experimental results. Therefore, the TFT with different channel lengths can have a different change in transfer curve.
We also performed TCAD simulation to fit the data shown in Fig.1. Circles represent measurement data and solid lines TCAD fitting. In the TCAD simulation, we adopt density of states (DOS) parameters from Mehedi et al.[5]. We also found that the increment of defect states after different strain. These defects are mainly oxygen vacancies [5]-[8]. Therefore, it is confirmed by experimental results and TCAD simulation that the long channel TFTs are more robust and generation of oxygen vacancy might be the origin of the instability under mechanical bending strain.

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Figure 1