Development of low-temperature polycrystalline silicon process and novel 2T2C driving circuits for electric paper

Abstract

In this work, we systematically investigate low-temperature polycrystalline silicon (LTPS)-based driving circuits of electronic paper for the aim of adopting small width/length ratio (W/L) of LTPS-based thin film transistors (TFTs) to reduce switch error and thus improve image sticking. Firstly, LTPS-TFTs with extremely low off-state leakage current (I_OFF) even at a large source-drain voltage (V_DS) of 30 V were obtained through detailed explorations of LTPS process technology. Meanwhile, the high on-state current (I_ON) of LTPS-TFTs also meet the requirements of fast signal writing to the storage capacitor due to their extremely high field-effect mobility (approximately 100 cm²/V⋅s), making it possible to fabricate TFTs with relatively small W/L, thereby minimizing switch error. The I_D-V_D test results reveal that the produced LTPS-TFTs can effectively withstand the maximum voltage difference of 30 V during product operation. Subsequently, the optimal W/L of the LTPS-TFT was determined through experimental results. Then, reliability test was conducted on the obtained LTPS-TFTs, revealing that the threshold voltage (V_TH) of the LTPS-TFTs shifted by 0.08 V after 7200 s under negative bias temperature stress (NBTS), and only by 0.19 V under positive bias temperature stress (PBTS). The aging test results of the aforementioned LTPS-TFTs exhibits a new physical phenomenon, that is, the I_OFF of the LTPS-TFTs has a strict matching characteristic with the aging direction. Next, we proposed a novel 2T2C driving circuit for the e-paper, which can effectively avoid the adverse effects of I_OFF on the frame holding period, and plotted it into an array layout. Finally, we combined the optimal fabricating process of the LTPS-TFTs with the 2T2C driving circuit design scheme to produce an e-paper with outstanding image sticking performance.