This letter addresses the characteristics of biosensor made of ion-sensitive field-effect-transistors (ISFETs) with stacked 3-aminopropyltriethoxysilane/SiO2 sensing membranes. These devices exhibit higher sensitivity, lower hysteresis, and lower drift characteristics as compared with the conventional ones made with a simple layer of oxide. Much improved performance makes ISFETs a favorable choice for future bio-sensing applications.