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A comparative study is made of the low-frequency noise (LFN) in amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistors (TFTs) with Al2O3 and Al2O3/SiNx gate dielectrics. The LFN is proportional to 1/fgamma, with gamma ~ 1 for both devices, but the normalized noise for the Al2O3/SiNx device is two to three orders of magnitude lower than that for the Al2O3 device. The mobility fluctuation is the dominant LFN mechanism in both devices, but the noise from the source/drain contacts becomes comparable to the intrinsic channel noise as the gate overdrive voltage increases in Al2O3/SiNx devices. The SiNx interfacial layer is considered to be very effective in reducing LFN by suppressing the remote phonon scattering from the Al2O3 dielectric. Hooge's parameter is extracted to ~6.0 times 10-3 in Al2O3/SiNx devices.