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The theory of oxygen vacancy related deep energy defects and valence band offset (VBO) between gate insulator and channel codetermining the threshold voltage shift (∆𝑽 𝐓𝐇 ) of ZnO thin film transistor under negative gate bias and illumination stress (NBIS) is proposed and investigated systematically. Two kinds of ZnO thin film transistors are fabricated by atomic layer deposition with different gate oxide structures, a control sample with Al2O3 gate oxide and an improved sample with SiO2/Al2O3 gate oxide structures. Among two kinds of devices, the device with SiO2/Al2O3 gate oxide achieves a smaller ∆𝑽 𝐓𝐇 under the NBIS with SiO2 thin film acting as a holes-blocking layer, despite the existence of more defects than control device. The improvement in stability is attributed to large VBO up to 3.08 eV. Moreover, the device with SiO2/Al2O3 gate oxide is evaluated on a 500-nit LCD back light to simulate the practical working environment in displays, which exhibits good stability of ∆𝑽 𝐓𝐇 = -0.3 V for 3600 seconds.