The demand for amorphous InGaZnO (a-IGZO) thin film transistors (TFTs) has increased due to their transparent properties. In this paper, we report on the relationship between the subgap density of states (DOS), field-effect mobility (mu(FE)), and unit channel length resistance (r(ch)) on the electrical properties of a-IGZO TFT's. The three tested structures had the same channel width/length and gate insulator thickness with different gate insulator materials, SiNx. SiOx, and SiOx/SiNx. Compared to TFTs with low subgap DOS levels, TFTs with high subgap DOS levels have low mu(FE) values due to the relatively large r(ch) values. (C) 2012 Elsevier Ltd. All rights reserved.