A new type of degradation phenomena featured with increased subthreshold swing and threshold voltage after negative gate bias stress (NBS) is observed for amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs),which can recover in a short time.After comparing with the degradation phenomena under negative bias illumination stress (NBIS),positive bias stress (PBS),and positive bias illumination stress (PBIS),degradation mechanisms under NBS is proposed to be the generation of singly charged oxygen vacancies (Vo+) in addition to the commonly reported doubly charged oxygen vacancies (Vo2+).Furthermore,the NBS degradation phenomena can only be observed when the transfer curves after NBS are measured from the negative gate bias to the positive gate bias direction due to the fast recovery of Vo+ under positive gate bias.The proposed degradation mechanisms are verified by TCAD simulation.