Effects of the defect creation on the bidirectional shift of threshold voltage with hump characteristics of InGaZnO TFTs under bias and thermal stress

We have investigated the hump characteristics of amorphous indium gallium zinc oxide (a-IGZO) thin film transistors (TFTs). Under positive gate bias stress, the threshold voltage (V_th) of a-IGZO TFTs showed bidirectional shift with hump; the positive shift in above V_th region and negative shift in subthreshold region. The amount of bidirectional shift depended on the temperature or drain voltage of stress condition. It was concluded that the origins of the bidirectional shift with hump were the shallow donor-like states and deep-level states creation in the semiconductor bulk or at the semiconductor/dielectric interface. Two-dimensional device simulation was also performed to further investigate this phenomenon.