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Persistent photoconductivity(PPC)effect and its light-intensity dependence of both enhancement and depletion(E-/D-)mode amorphous InGaZnO(a-IGZO)thin-film transistors(TFTs)are systematically investigated.Density of oxygen vacancy(Vo)defects of E-mode TFTs is relatively small,in which formation of the photo-induced metastable defects is thermally activated,and the activation energy(Ea)decreases continuously with increasing light-intensity.Density of Vo defects of D-mode TFTs is much larger,in which the formation of photo-induced metastable defects is found to be spontaneous instead of thermally activated.Furthermore,for the first time it is found that a threshold dose of light-exposure is required to form fully developed photo-induced metastable defects.Under low light-exposure below the threshold,only a low PPC barrier is formed and the PPC recovery is fast.With increasing the light-exposure to the threshold,the lattice relaxation of metal cations adjacent to the doubly ionized oxygen vacancies(VO+)is fully developed,and the PPC barrier increases to ～ 0.25 eV,which remains basically unchanged under higher light-exposure.Based on the density of Vo defects in the channel and the condition of light illumination,a unified model of formation of photo-induced metastable defects in a-IGZO TFTs is proposed to explain the experimental observations.