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High-κ dielectric is regarded as an effective material to reduce the operating voltage of the amorphous indium gallium zinc oxide (a-IGZO) thin film transistors (TFTs). However, the dielectric with high permittivity often has the drawbacks of inducing small conduction band offset energy and high interface trap density. Here a bilayer HfO2/SiO2 gate dielectric for thin-film transistors (TFTs) is employed to address the issues. Compare to the a-IGZO TFT with solely 15 nm-thick HfO2 gate dielectric, the TFT with the bilayer HfO2/SiO2 (10 nm/5 nm) gate dielectric improves the subthreshold swing (SS) from 0.22 to 0.12 V/decade, the mobility from 1.4 to 7 cm2/V·s and current on-off ratio from 9×106 to 1.3×10 9. Finally, Hooge's parameters (extracted from the low-frequency noise measurement) of a-IGZO TFTs were investigated to understand the defects near the channel/dielectrics interface so that the role of the thin SiO2 layer can be verified. The device with bilayer HfO2/SiO2 structure exhibits a value of 2×10-3, which is an order of magnitude lower than the one with a single HfO2 layer. The Hooge's parameter of our bilayer dielectric is the lowest among the reported metal-oxide based TFTs on the glass substrate.