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Miniaturized electrolyte–insulator–semiconductor capacitors (EISCAPs) with ultrathin gate insulators have been studied in terms of their pH‐sensitive sensor characteristics: three different EISCAP systems consisting of Al–p‐Si–Ta2O5(5 nm), Al–p‐Si–Si3N4(1 or 2 nm)–Ta2O5(5 nm), and Al–p‐Si–SiO2(3.6 nm)–Ta2O5(5 nm) layer structures are characterized in buffer solution with different pH values by means of capacitance–voltage and constant capacitance method. The SiO2and Si3N4gate insulators are deposited by rapid thermal oxidation and rapid thermal nitridation, respectively, whereas the Ta2O5film is prepared by atomic layer deposition. All EISCAP systems have a clear pH response, favoring the stacked gate insulators SiO2–Ta2O5when considering the overall sensor characteristics, while the Si3N4(1 nm)–Ta2O5stack delivers the largest accumulation capacitance (due to the lower equivalent oxide thickness) and a higher steepness in the slope of the capacitance–voltage curve among the studied stacked gate insulator systems. pH‐sensitive characteristics of miniaturized electrolyte–insulator–semiconductor capacitors (EISCAPs) of Al–p‐Si–Ta2O5, Al–p‐Si–Si3N4–Ta2O5, and Al–p‐Si–SiO2–Ta2O5with ultrathin gate‐insulators are studied. The pH‐sensitive Ta2O5is prepared by atomic layer deposition. EISCAPs with SiO2–Ta2O5stack exhibit better sensor performance, while the Si3N4(1 nm)–Ta2O5stack provides a higher steepness of the capacitance–voltage curve.