Characterization of a silicon-on-insulator based thin film resistor in electrolyte solutions for sensor applications

We characterize the recently introduced silicon-on-insulator based thin film resistor in electrolyte solutions and demonstrate its use as a pH sensing device. The sensor’s response function can be tuned by a back gate potential, which is demonstrated by employing known changes of the pH of the solution. The highest sensitivity to pH changes is obtained when the charge carrier concentration at the back interface of the thin Si-film is low compared to the front interface. Calibration measurements with a reference electrode are used to relate the obtained resistance to the surface potential. Applying the site binding model to fit the measured data for variations of the pH gives excellent agreement. The sensors response can be related to a surface potential change of −50 mV/pH and from the obtained signal–to–noise ratio, the detection limit can be estimated to be 0.03 pH. For a (bio-)molecular use of the sensor element, a passivation of the silicon oxide surface against this pH response can be achieved by depositing an organic layer of poly- methyl-methacrylate (PMMA) onto the devices by spin coating. As expected, the pH response of the surface disappears after the deposition of PMMA. This passivation technique provides an easy and reliable way to obtain a biocompatible interface, which can be further functionalized for the detection of specific molecular recognition events.