Conduction band offset-dependent induced threshold voltage shifts in a-InGaZnO TFTs under positive bias illumination stress.

In this paper, the dependence of threshold voltage (V_th) changes to amorphous-indium gallium zinc oxide (a-IGZO) thin film transistors (TFTs) under positive bias light illumination (PBIS) on the height of the conduction band offset was studied. Using SiO₂, HfO₂, and Al₂O₃ as gate dielectrics, three different types of a-IGZO TFTs were used in the experiment. Electrical parameters and the density of states of each TFT were measured. Of the devices studied, the interface characteristics between the a-IGZO and the gate dielectric were the best using SiO₂ and the worst using Al₂O₃. HfO₂ had the smallest conduction band offsets (CBO) of 2.26 eV. We also performed PBS and PBIS evaluation to confirm the stability of TFTs. The V_th shift in the three samples was insignificant under PBS, but the V_th shifts occurred under PBIS in the order of HfO₂, Al₂O₃, and SiO₂. The interfacial characteristics of a-IGZO and the dielectric did not change after PBIS in all three devices; the lower the CBO height, the greater the V_th shift after PBIS. The predominant cause of the V_th shift under PBIS is the accumulation of injected photoelectrons that have sufficient energy to tunnel the CBO barrier into the gate dielectric by positive gate bias. [ABSTRACT FROM AUTHOR]