In this study, the thickness effects of $Al_2O_3$ layer on the sensing properties of $SiO_2/Al_2O_3$ (OA) stacked membrane were investigated using electrolyte-insulator-semiconductor (EIS) structure for high quality pH sensor. The $Al_2O_3$ layers with a respective thickness of 5 nm, 15 nm, 23 nm, 50 nm, and 100 nm were deposited on the 5-nm-thick $SiO_2$ layers. The electrical characteristics and sensing properties of each OA membranes were investigated using metal-insulator-semiconductor (MIS) and EIS devices, respectively. As a result, the OA stacked membrane with 23-nm-thick $Al_2O_3$ layer shows the excellent characteristics as a sensing membrane of EIS sensor, which can enhance the signal to noise ratio.