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To verify the interface state properties of amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film transistors (TFTs) with SiNx and SiO2 gate dielectrics, the low-frequency noise (LFN) of various gate-length devices from 5 to 50 μm was measured and evaluated. Before LFN measurements, the dc characteristics, such as the subthreshold slope (S), were measured for comparison. The interface state density (Nit) extracted by S for an a-IGZO TFT with SiNx gate dielectrics is only 1.3 times higher than that for an a-IGZO TFT with SiO2 gate dielectrics. However, the trap density (Nt) extracted by LFN for an a-IGZO TFT with SiNx gate dielectrics is almost 80 times higher than that for one with SiO2 gate dielectrics. Moreover, carrier number fluctuations are the dominant mechanism for LFNs in an a-IGZO TFT with SiNx gate dielectrics. This large difference between SiNx and SiO2 gate dielectrics in LFN measurement is related to the fast degradation of a-IGZO TFTs with SiNx gate dielectrics by the bias temperature instability or light illumination.