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Thinning the gate insulator in an hydrogenated amorphous silicon thin-film transistor (a-Si:H TFT) has been studied in a coplanar structure. The threshold voltage decreases with decreasing gate insulator thickness without changing the field effect mobility significantly. The reduction in the threshold voltage is due to the decrease in the charge traps in the SiN/sub x/ and in its film thickness. The coplanar a-Si:H TFT with a gate insulator thickness of 35 nm exhibited a field effect mobility of 0.45 cm/sup 2//Vs and a threshold voltage of 1.5 V. The thickness of the gate insulator can be decreased in the coplanar a-Si:H TFTs because of the planarized gate insulator.