Bias and light-induced instabilities in a-IGZO thin film transistors

Instabilities due to various causes have been a major topic of investigation for all thin-film transistors (TFTs). Permanent or metastable changes in the current voltage characteristics are induced by combinations of gate and drain voltage, aging, and environmental effects, as well as exposure to light. Amorphous indium gallium zinc oxide (a-IGZO) TFTs are no exception and these phenomena have been reported at an early stage of the technology development. These devices are relatively stable under moderate positive (negative) gate bias at room temperature (VGS < 15 V), but show increasing positive (negative) shifts of the transfer characteristics under higher positive (negative) VGS. The effects are referred to as positive bias stress (PBS) and negative bias stress (NBS). The latter causes negligible changes under operating conditions. More importantly, large (ΔV > 2 V) negative shifts of the transfer characteristics are observed when the devices are exposed to near or above bandgap radiation. The shifts dramatically increase when a negative bias is simultaneously applied. The effect is referred to as negative bias under illumination stress or NBIS and is obviously important since in display applications the devices operate in the presence of light. PBS and NBIS have received great attention in the scientific literature, which has led to an improved, but by no means complete level of knowledge of the mechanisms involved. The main experiments, the characteristics of these effects, and their interpretation through the models that have been developed are reviewed in this chapter. Unanswered questions and possible future directions of research in this area are identified.