Electrolyte-Insulator-Semiconductor field effect device for pH detecting

This work presents a device for pH measurements called Electrolyte-Insulator-Semiconductor (EIS), which operates as a Metal-Oxide-Semiconductor capacitor but instead of having the metal contact electrode, an electrolyte solution and a reference electrode are used to apply voltage. It was used TiO₂ thin film as insulator and sensitive membrane. These films were obtained after rapid thermal oxidation of Ti thin films deposited by sputtering. They were structurally characterized by Raman and ellipsometry which reveal that the films present rutile crystal structure. We use capacitors to make the electrical characterization of TiO₂ films in order to determine the annealing time that leads to the best thin film properties, defined by high dielectric constant value (high-k), lower charge density (Q₀/q) and flat-band voltage (V_FB) around -0.9V. EIS devices are tested through capacitance x voltage (CxV) measurements using different pH solutions. When in contact to the dielectric film, each pH value adds a contribution to the flat band voltage, resulting in a displacement of the CxV curve. It was possible to determine from the flat band voltage (V_FB), the sensitivity of 41 mV/pH.