Ion-selective field-effect transistors (ISFETs) are a type of potentiometric sensor with a wide range of potential applications. However, the application of ISFETs is hindered by the instability of the threshold voltage (VT), known as the drift. In this study, we hypothesized that the charge trapping at the interface between the oxide layer and the Si substrate of ISFETs affects the drift, and we examined the effects of the irradiation of UV light on the drift. To investigate the drift more simply, we fabricated a Si wafer sample with a 30 nm Al2O3 layer, which is regarded as the sensing layer of an ISFET, and we measured the drift by capacitance–voltage measurements using an electrochemical cell. Results showed that the amount of drift after light irradiation decreased to 6.6% (37 mV/h) relative to the original drift (558 mV/h) in the best case. It was also suggested that the drift is caused by multiple charge trapping mechanisms. These results indicate that charge trapping is a cause of drift, and light irradiation can be used to suppress drift.