Investigation on LTPS-TFTs With High-κ Gate Dielectrics

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In this thesis, p-channel LTPS-TFTs with different thickness of the HfO2 gate dielectrics were fabricated and investigated. We compared the electrical characteristics and the NBTI reliability issue of the p-channel LTPS-TFTs with the different thickness of HfO2 gate dielectrics. As for the electrical characteristics, we found that the peak transconductance (Gm_max) increases with the HfO2-thickness. This phenomenon violates the well known knowledge that the gate control ability should be enhanced with a higher gate capacitance. Therefore, it is worthwhile to investigate the mechanism of this phenomenon. We found that the phase change of the HfO2 layers contributes to the Gm_max degradation of the TFTs with the thin HfO2 layer. Besides, it was found that the enhanced Gm_max ¬of the p-channel TFTs with the thicker HfO2 layer is related to the intrinsic charged oxygen vacancies in the HfO2 layer. Finally, the degradation mechanism of the NBTI stress was systematically studied. It was found that the behavior of Vth variation, S.S. degradation and the Gm_max degradation are mainly dependent on the thickness of the HfO2 layer, stress temperature and the stress bias.