Transparent ZnO based thin film transistors (TFTs) have received intensive interest due to their potential of replacing hydrogenated amorphous or polycrystalline silicon (a-Si:H or poly-Si) TFTs. Znic oxide (ZnO) is a transparent compound semiconductor with a wide band gap (3.37 eV) which can be grown as a polycrystalline film at low or even room temperature. ZnO is, therefore, considered to be an ideal material for serving as the channel layer in transparent and flexible TFTs. As an important element, gate insulators for ZnO based TFTs have received increasing attention because ZnO based TFTs switching voltage can be reduced by using high-K gate dielectrics which can lead to high capacitance value. In this presentation, we report on the role of Mg doping in conspicuously reducing leakage currents in Ba_0.6Sr_0.4TiO₃ (BST) high-K gate dielectrics utilized in ZnO based TFTs fabricated on plastic substrates (PET). The 3% Mg-doped BST thin films, deposited by rf magnetron sputtering at room temperature on Pt/Ti/SiO₂/Si substrates, showed a relatively high dielectric constants of ~21. The 3% Mg-doped BST films exhibited remarkably improved leakage current densities less than 5x10^-9 A/cm², as compared to that (5x10^-4 A/cm² ) of undoped BST films at an applied electric field of 250 kV/cm,. All room temperature processed ZnO based TFTs using the 3% Mg-doped BST gate insulator demonstrated a high optical transparency (> 80%, for wavelength > 400 nm), a high field effect mobility of 13.2 cm²/Vs and low voltage device performance of less than 7 V. This result indicates that ZnO based TFTs with 3% Mg-doped BST gate insulator will open up a promising route for a wide variety range of applications in transparent, flexible, and portable electronic devices.

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1. Room temperature fabricated ZnO thin film transistors with MgO coated Bi_1.5Zn_1.0Nb_1.5O₇ gate insulator