Polycrystalline fluorine-doped tin oxide as sensoring thin film in EGFET pH sensor

The Extended Gate Field Effect Transistor (EGFET) is a device composed of a conventional ion-sensitive electrode and a MOSFET device, which can be applied for the measurement of ion content in a solution. The EGFET is fabricated connecting the sensitive membrane to a commercial MOSFET. We investigated the use of fluorine-doped tin oxide films (FTO) as sensitive membrane to EGFET. The commercial FTO shows a low resistivity and a crystalline structure, both determined using conductivity set-up and X-ray diffraction experiments, respectively. So far, it has been known that an amorphous structure is desirable to obtain high sensitivity. Despite of the a crystalline structure, we have fabricated the FTO as EGFET for pH sensor and carried out experiments in order to obtain the response of the device inserted into solutions with pH values from 2 up to 12. In this range, we have quantified a sensitivity of 50 mV/pH, which may have large potential applications as pH and biosensors. In addition, both the film and the structure of the sensor are cheaper and easier to make than in common techniques.