•Deep ultra-violate (DUV) photodetectors (PDs) fabricated by ZnGa2O4 thin film have been studied.•Accelerating life test (ALT) has been examined for UVC PDs.•The reliability issue is caused by the surface water adsorption and ozone compensation.•Al2O3 passivation layer was used to protect the device surface from damage by the atmosphere. Deep ultra-violate (DUV) photodetectors (PDs) fabricated by ZnGa2O4 thin film with back-to-back Schottky type metal–semiconductor-metal (MSM) structure have been studied and presented a superior performance. To further discuss the reliability of DUV device should be the pivot for the DUV application. We were using an accelerating life test (ALT) for UVC PDs by applied in monitoring sterilization light sources during this research. After ALT for one week, the device showed low reliability with a varying current level, and the change of response time and rejection ratio are enormous. Following the XRD and XPS results, we find out that the reliability issue is caused by the surface water adsorption and ozone compensation. Furthermore, the 45-nm-thick Al2O3 passivation layer was deposited by atomic layer deposition (ALD) for isolating the effect of the atmosphere. It resulted in increasing the stability of the current level and reducing the response time for the DUV PDs.