Electrical response of polycrystalline silicon thin film transistor on steel foil under mechanical strain
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1116-I01-05
Electrical response of polycrystalline silicon thin film transistor on steel foil under mechanical strain Po-Chin Kuo, Abbas Jamshidi-Roudbari and Miltiadis K. Hatalis Electrical and Computer Engineering, Lehigh University, Bethlehem, PA, 18015 ABSTRACT This work investigates the effects of mechanical strain on electrical characteristics of polycrystalline thin film transistors (poly-Si TFTs). Poly-Si TFTs were fabricated on steel foil substrate and characterized under the strain ranging from -1.2% to 1.1% induced by bending. The electron mobility increased under tensile and decreased under compressive strain while that of the hole exhibited an opposite trend. For p-channel TFTs the normalized threshold voltage and subthreshold slope were a function of strain. In both n- and p-channel TFTs the off current decreased under tensile while it increased under compressive strain. The observed mobility and off current trends in poly-Si TFTs under strain are similar to those reported in single crystalline silicon devices. INTRODUCTION Mechanically flexible electronic systems have the potential to realize novel applications and thus have recently attracted a lot of research interest. Most of the reported flexible electronics are based on amorphous silicon thin film transistor (a-Si:H TFT) technology because this technology is deemed more readily compatible with flexible polymer substrates. However, the low carrier mobility in amorphous silicon and the poor reliability of amorphous silicon TFTs may limit the applications of this technology. Polycrystalline silicon thin-film transistor (poly-Si TFT) technology on steel foils has been successfully demonstrated as a platform for high performance flexible electronics [1] because of the high electron and hole mobility that enables CMOS circuit architectures and the compatibility of this technology with high processing temperatures. High resolution flexible displays have also been realized by this technology [2]. One of the most important features of any flexible electronic system is its ability to be deformed. However, any form of deformation induces mechanical strain to the electronic thin film devices and circuits fabricated on the flexible substrate. Therefore it is necessary to understand the effects of mechanical strain on the electrical characteristics of thin film devices. There are extensive studies over a-Si:H TFTs [3, 4] and also organic TFTs [5] on flexible substrate under mechanical strain. The relevant work on poly-Si TFTs was lacking thus in our earlier effort we investigated the effect of tensile strain on mobility of poly-Si TFTs [6]. To make the work more complete in this paper we report the mobilities and characteristics of both nchannel and p-channel poly-Si TFTs under both uniaxial tensile and compressive strain induced by bending. EXPERIMENT We fabricated arrays of poly-Si TFTs on a 100 µm thick, type 304, 150 mm in diameter stainless steel substrate. The polished steel wafers had a surface roughness (Ra) of only 10 nm. The metal foil wafers were c
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