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We compare the mutual interactions between the top-and bottom-gate fields in a dual-gate structure for amorphous InGaZnO4 (a-IGZO) and hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs). We find that a-IGZO TFTs show more significant mutual interactions than a-Si:H TFTs. By using a wide variety of a-IGZO TFTs with varying thicknesses of a-IGZO and gate-insulator layers, we have investigated, on the basis of a conventional fully depleted n-channel silicon-on-insulator model, the mechanism behind parallel shifts in transfer characteristics caused by negative top-gate voltages. Experimental results, even for relatively thick (over 150 nm) a-IGZO layers, are in good agreement with the fully depleted model. This fact may be related to the nonexistence of hole accumulation in a-IGZO, which is an intrinsic property of oxide semiconductors. In contrast, for the a-Si:H TFTs, there is a considerable discrepancy between the experimental results and the model. This discrepancy probably results from the limited penetration of band bending into the a-Si:H layer, as well as from the hole accumulation at the top interface. Such analysis of a-IGZO TFTs is of practical importance in device applications because many issues related to the fabrication and structure of a-IGZO TFTs may be resolved with a better understanding of dual-gate performance.