In this work, an all-solid-state electrolyteinsulator-semiconductor (EIS) device is developed for pH sensing performance evaluation of insulating materials. The EIS capacitor incorporates scandium oxide (Sc2O3) sensing film deposited on undoped gallium nitride (u-GaN) by thermal evaporation. The structural and morphological features of the thin films annealed at different temperature (650-850°C), were investigated through X-ray diffraction and AFM analysis. A gold wire with diameter of 25 pm was bonded on the device and served as the quasi reference electrode for the C-V measurements and pH sensitivity characterizations. After correction of the measured capacitance regarding the pH dependent variation of the standard potential E0(pH) of gold, the Sc2O3 EIS capacitor prepared with an annealing at 650°C exhibited a linear response with a sensitivity of 40 mV/pH while the device with the as-deposited Sc2O3 film showed nonlinear behavior and, those annealed at temperature higher than 650°C were shown to be insensitive to pH variation.