Highly sensitive suspended-gate ion sensitive transistor for the detection of pH

This paper presents a new device for the pH detection. It is based on a suspended polysilicon gate field effect transistor (SGFET). The sensitive layer is made of silicon nitride as for ISFET technology. The suspended bridge, used as gate electrode, is formed with doped polysilicon covered with silicon nitride layers for electrical insulation. The layers are deposited by Low Pressure Chemical Vapor Deposition (LPCVD). Surface micro-technology allows to obtain a small height (0.5μm) suspended-bridge. In this case, the solution penetrates under the gate. The high field effect in the gap between the gate and the channel is enough to change the charges distribution. Very high pH sensitivity, greater than 200 mV/pH, is found with this new structure and it is much higher than the usual Nernstian sensitivity of ISFETs. The device concept, electrical characteristics, and the effect of the thickness of the gap between the bridge and the sensitive layer on the pH sensitivity are discussed in this study.