Influence of flexible substrate in low temperature polycrystalline silicon thin-film transistors: temperature dependent characteristics and low frequency noise analysis

Abstract

The carrier transport of p-type low temperature polycrystalline silicon (LTPS) thin-film transistors (TFTs) on flexible substrate has been intensively studied and compared to that on glass substrate in order to improve device performance. To investigate the origin of carrier transport on different substrates, temperature dependent characterizations are carried out for electrical device parameters such as threshold voltage (VTH), subthreshold swing (SS), on-current (Ion) and effective carrier mobility (μeff). The poly-Si grain size Lgrain and the barrier height EB between grain boundaries are well known to be the main parameters to determine transport in polycrystalline silicon and can be extracted based on the polycrystalline mobility model. However, our systemic studies show that it is not grain size but EB that has more influence on the degradation of LTPS TFT on flexible substrates. The EB of flexible substrate is roughly 18 times higher than glass substrate whereas grain size is similar for both devices on different substrates. Compared to the LTPS TFT on glass substrate, higher EB degrades approximately 24 % of Ion, 30 % of SS and 21 % of μeff on the flexible substrate at room temperature. From low frequency noise (LFN) analysis, it is observed that the total trap density (Nt) for flexible substrate is up to four times higher than that of glass substrate, which also supports the high value of EB in the device fabricated on the flexible substrate.