The present invention relates to a Sub-threshold Field Effect Transistor (SF-FET). The invention integrates a MEMS mechanical transducer along with the sensing mechanism in a single device. Forced mass is capacitively coupled onto the FET structure. Dielectric SiO2 forms good interface with underlying silicon substrate. Air dielectric forms second dielectric wherein effective gate capacitance is the series combination of the second dielectric capacitance and fixed dielectric. Inertial displacements are sensed by observing change in drain current (ID) of the sensor due to change in gap height (TGap) of the second dielectric of the sensor caused by forced mass.