Two types of charge-coupled devices (CCDs) have been irradiated by 60Co gamma-ray and dark signal is characterized before and after irradiation. After exposure to gamma-ray, the evolution of mean dark signal exhibits an inflection point specific to a certain total ionizing dose (TID) value, beyond which dark signal increase rate rapidly reaches a level higher than the ones measured at other TIDs. The cause of inflection point occurrence is analyzed through the use of annealing tool and technology computer-aided design (TCAD) simulation in the custom CCDs. The experimental results demonstrate that when the cumulative oxide charge density achieves a specific value which depends on the fabrication process of the field oxide of CCDs, a sudden increase in dark signal occurs due to a merge between the space charge region and the additional depletion region induced by the oxide trapped charges.