We report the effect of the cation composition on the electrical performance of amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors (TFTs) which was deposited by atomic layer deposition (ALD). The In/In+Ga ratio of a-IGZO TFT was increased, enhancement of the field-effect mobility (FE) value was observed. The device with a higher In/In+Ga ratio: the In0.45Ga0.15Zn0.40O transistor showed a higher FE value of 48.3 cm 2 /Vs, VTH of -4.06 V, SS of 0.45 V/decade, and ION/OFF ratio of > 10 7 . Simultaneously, Density of States (DOSs) profile in a forbidden band gap of the a-IGZO semiconductor were extracted based on the Meyer-Neldel rule (MN rule) to obtain an insight into the cation composition dependent performance of a-IGZO TFTs.