Turn-around of threshold voltage shift in amorphous InGaZnO TFT under positive bias illumination stress

Abstract

Amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) have attracted significant attention in display devices and 3D-stacked dynamic random-access memory (DRAM) devices. They have superior advantages, while they have much remained to be studied such as threshold voltage (VT) instability. We investigated the transfer characteristics of a-IGZO TFTs under positive bias stress (PBS) and positive bias illumination stress (PBIS). The behaviors of VT shift under PBS and PBIS differed due to illumination. The direction of the VT shift depended on the illumination condition (on or off), including the turn-around of the VT shift under PBIS. The two mechanisms of ionized oxygen vacancies and trapped electrons competed, and one mechanism became dominant under PBS and PBIS. The dominant mechanism determined the direction of the VT shift; turn-around occurred if the dominant mechanism changed. In addition, the subthreshold swing and field-effect mobility consistently described the mechanisms using the corresponding energy band diagrams. Understanding the behavior of the VT shift under various stress conditions provides insights into the instability of a-IGZO TFTs.