We describe how the off-state current (I-off) property of amorphous InGaZnO (a-IGZO) thin-film transistors is caused by trap-assisted tunneling (TAT) by using a two-dimensional device simulation software application (Atlas 2 D, Silvaco). We found thatI(off)can be increased by controlling the bandgap energy (E-G) and the effective mass of electron (m(e)) of a-IGZO transistors. Whenm(e)was increased from 0.32 to 0.38 m(o)(mass of a free electron), the point at whichI(off)started to increase in the region of negative gate voltage (V-GS) shifted from -4.7 to -7.4 V. In addition, whenE(G)was changed from 3.05 to 3.2 eV, the average value ofI(off)changed from 3.13 x 10(-13)to 2.4 x 10(-14)A. This implies thatE(G)andm(e)influence the increase inI(off)in a-IGZO TFTs because of the difficulty associated with TAT.