We report on the fabrication of low-voltage ZnO thin-film transistors using 1% Ni-doped Ba 0.6 Sr 0.4 TiO 3 as the gate insulator. The Ni-doped BST, deposited by RF magnetron sputtering at room temperature, significantly reduced leakage current density to less than 6×10 −9 A/cm, as compared to a current density of 5×10 −4 A/cm for undoped BST films at 0.5 MV/cm. The ZnO thin-film transistor with the Ni-doped BST gate insulator exhibited a very low operating voltage of 4 V. The field-effect mobility, the current on/off ratio and subthreshold swing were 2.2 cm 2 V/s, 1.2×10 6 , and 0.21 V/dec respectively.Keywords Transistor . ZnO . Low voltage operation . Gate insulator Transparent ZnO films are used as active channel materials, which exhibit n-type semiconductor characteristics with high optical transmittance in the visible spectrum and a wide band gap of 3.3 eV [1, 2]. ZnO thin-film transistors (TFTs) are of particular interest because of their potential to replace hydrogenated amorphous or polycrystalline silicon (a-Si/H or poly-Si) TFTs. This potential exists because good quality ZnO polycrystalline films, showing high fieldeffect mobility can be grown at room temperature. Thus, ZnO-based electronic circuits offer the possibility of low processing costs and good compatibility with plastic substrates [3,4]. Furthermore, reports have been published on the high performance of ZnO (or doped ZnO) TFTs with moderate field-effect mobility and high on/off ratios in active matrix organic light emitting diode (AMOLED) applications [5,6].However, high operating voltages are still a major limitation in portable and battery-powered applications [7]. Therefore, it is important to incorporate a suitable gate insulator to allow for a higher operating current at lower bias voltages. In general, a high permittivity gate dielectric or reduced dielectric thickness is needed to increase the capacitive coupling of the gate electric field to the ZnO channel layer. However, ZnO-TFTs with thin gate dielectrics show poor performance on flexible polymer substrates, which are often characterized by rough surfaces, making these TFTs susceptible to pinhole formation and low manufacturing yields [7]. In order to ensure pinhole-free coverage, the film should be significantly thicker than the roughness of the substrate. Therefore, the use of high-k gate dielectrics with thicknesses over 200 nm is optimal for stable operation of low voltage ZnO-TFTs. While there have been some promising early results for near room temperature grown high-K gate insulators, including barium zirconium titanate (BZT) [8], Bi 1.5 Zn 1.0 Nb 1.5 O 7 (BZN) [2, 3, 4], Al 2 O 3 [9], HfO 2 [9, 10], and TiO 2 [11], they generally suffer from poor leakage current characteristics at voltages above 5 V. The authors recently developed Mn-doped J Electroceram (2009) 23:76-79