Novel method to operate piezo-FET-based stress sensor offers tenfold increase in sensitivity

This paper presents a novel method to operate Wheatstone bridges of piezoresistive field effect transistors (FETs) as stress sensors. Such structures consist of a square arrangement of four FETs connected by the source/drain diffusion in each corner. When the FETs are on and the bridge is operated with an input voltage between a pair of opposite contacts, the bridge output voltage appearing between the perpendicular contact pair is proportional to the difference of in-plane normal stress components. In the new method the resistivity of one of the four FETs is individually tuned by varying its gate voltage by ΔV from the common gate voltage of the other three gates, in order to rebalance the bridge. We find that the corresponding sensitivity normalized to the input voltage reaches 4.5 mV/(V MPa). It is thus a factor of about 10 higher than the conventional sensitivity based on the bridge output voltage, which reaches 490 μV (V MPa).