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We discuss the influence of oxide semiconductor thickness on thin-film transistor (TFT) characteristics by measuring transfer characteristics of amorphous InGaZnO (IGZO) TFTs with various IGZO thicknesses and using a simple calculation of depletion width in a semiconductor film. The ON current was nearly constant with respect to IGZO thickness because its value depended on high-density electrons in an accumulation region sufficiently thinner than IGZO thickness. The threshold voltage shifted negatively with increasing IGZO thickness, which indicates that a thicker IGZO film requires a higher negative gate voltage for it to be fully depleted. Calculation results suggest that the threshold voltage variation due to oxide semiconductor thickness variation increases with increasing donor density and oxide semiconductor thickness.