Amorphous InGaZnO (a-IGZO) TFTs have attracted more demands due to the transparency and high mobility. Since TFTs are always exposed to both positive and negative gate bias, the alternating DC bias stress tests are required to ensure stable TFT characteristics. Moreover, the analysis about scaling-down behavior under bias stress condition is required. In this paper, the effects of alternating pulse bias stress on the threshold voltage shift (δVth) of a-IGZO TFTs with respect to the stress time duration (td) are analyzed by the variation of interface trap charges (Qit) and the channel electric field (E-field). In addition, the channel lengthdependent tendency is verified using the concepts of the subgap density of states (DOS) and density of total trap states (NT).