Effects of electro-mechanical stressing on the electrical characterization of on-plastic a-Si:H thin film transistors
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Effects of electro-mechanical stressing on the electrical characterization of on-plastic aSi:H thin film transistors Jian Z. Chen1,*, Chih-Yong Yeh2, I-Chung Chiu2, I-Chun Cheng2,3, Jung-Jie Huang4, Yung-Pei Chen4 1 Institute of Applied Mechanics, National Taiwan University, Taipei 10617, Taiwan 2 Graduate Institute of Photonics and Optoelectronics, National Taiwan University, Taipei 10617, Taiwan 3 Dept. of Electrical Engineering, National Taiwan University, Taipei 10617, Taiwan 4 Display Technology Center, Industrial Technology Research Institute, Chutung, HsinChu County 310, Taiwan ABSTRACT We analyzed the effect of electromechanical stressing on the electrical characteristics of hydrogenated amorphous silicon thin-film transistors. It had been shown that the TFTs, fabricated at 150 oC, respond to tension/compression by a rise/fall in electron mobility. In TFTs fabricated using the same process, a slight shift of threshold voltage was observed under prolonged high compressive strain and the gate leakage current slightly increases after ~2% compressive strain. In general, the change of TFT performance due to pure mechanical straining is small in comparison to electrical gate-bias stressing. From the comparison among Maxwell stress (induced by electrical gate-bias stressing), mechanical stress (applied by bending), and drifting electrical force for passivated hydrogen atom, the most significant cause for the change of electrical characterization of a-Si:H TFTs should be the trapping charges inside the dielectric, under combined electrical and mechanical stressing. The mechanical stress does not act on Si-H bonds to drift hydrogen atoms, while it is mainly balanced by the rigid Si-Si networks in a-Si:H or a-SiNx. Therefore, mechanical stress has very little effect on the instability of low temperature processed a-Si:H TFTs. INTRODUCTION Flexible electronics requires devices working during and after mechanical straining. aSi:H TFTs, one of the most promising flexible electronic components, are more defective when fabricated at low process temperature (100-200oC) than those made in the standard range of process temperatures of 250-350oC [1]. The stability of a-Si:H TFTs under mechanical stress has been studied by several groups [2-6]. Gleskova et al. studied the effect of short-term mechanical strain on TFTs processed on Kapton at 150oC [2-4]. They observed a rise of the electron field effect mobility in tension and a drop in compression, and a very small change in subthreshold slope in the opposite direction [2-4,6]. In our previous study, we found small but measurable changes in the electrical characteristics when a very large compressive mechanical strain was applied for a long duration. The gate leakage current slightly rises when the applied compressive strain exceeds 2% [5]. Won et al. experimented on the performance of a-Si:H TFTs under the influence of combined electrical and mechanical stresses. Their results agree with the findings from other research groups: (1) the shift of threshold voltage (
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