An Investigation Of The Instabilities Of A-IGZO TFTs Under Illumination And Gate Bias Stresses

A unified model is proposed to consistently explain the degradation behaviors of a-IGZO TFTs under different light illumination and gate bias stress conditions. In the proposed model, photo-excited double ionized oxygen vacancies(Vo2+) traps and their transportation under the electric field are two key factors that cause the threshold voltage(Vth) shift of TFTs. Additional traps generated during the formation of Vo2+ also deteriorate the subthreshold characteristics.Then, we investigate the instabilities of the a-IGZO TFTs under blue light illumination and gate bias stresses, involving gate bias stress, light illumination, positive bias illumination stress(PBIS), negative bias illumination stress(NBIS). Under positive bias stress(PBS), the transfer curve of a-IGZO TFT has a positive shift, while no degradation can be observed under negative bias stress(NBS). Under blue light illumination, the transfer curve has a negative shift and the subthreshold swing(SS) also deteriorated. Under PBIS, the transfer curve has a positive shift and accompany with the degradation of SS. Under NBIS, both the threshold voltage and SS have significant degradation. But all the degradation phenomena can be explained by the consistent model.Besides, we also investigate the instabilities of a-IGZO TFTs under different wavelength and different light intensity. When we do the blue light NBIS experiment, we changed the light intensity and observed that it is the light exposure rather than the stress time or light intensity is found to be the key stress factor for negative bias illumination stress(NBIS) induced degradation.