Electrolyte–insulator–semiconductor (EIS) sensors, used in applications such as pH sensing and sodium ion sensing, are the most basic type of ion-sensitive field-effect transistor (ISFET) membranes. Currently, some of the most popular techniques for synthesizing such sensors are chemical vapor deposition, reactive sputtering and sol-gel deposition. However, there are certain limitations on such techniques, such as reliability concerns and isolation problems. In this research, a novel design of an EIS membrane consisting of an optical material of indium gallium oxide (IGO) was demonstrated. Compared with conventional treatment such as annealing, Ti doping and CF4 plasma treatment were incorporated in the fabrication of the film. Because of the effective treatment of doping and plasma treatment, the defects were mitigated and the membrane capacitance was boosted. Therefore, the pH sensitivity can be increased up to 60.8 mV/pH. In addition, the hysteresis voltage can be improved down to 2.1 mV, and the drift voltage can be suppressed to as low as 0.23 mV/h. IGO-based membranes are promising for future high-sensitivity and -stability devices integrated with optical applications.
High sensitivity pH sensing by using a ring resonator laser integrated into a microfluidic chip