The effect of applied stress is to change the number and the mobility of the charge
carriers within a material, thus causing large changes in resistivity. This resultant change
in resistivity is called the piezoresistive effect. The electron charge and the # of charge
carriers can be controlled during the manufacturing process by changing the amount and
type of trace impurity added to the material. By controlling the manufacturing process,
the material’s characteristics can be easily reproduced.
Piezoresistive sensors can be manufactured in similar processes to electronic
integrated circuits and can be made extremely small with micromachining. They have
been used in medical research to implant into tissue to measure bodily stresses (bed-sore
studies) and can be made small enough that they can be inserted into the brain with
minimal cell damage. They can also be used to make strain gauges (gages?) that can
measure stresses of mN. They have also been used to build micromachined
accelerometers.
Compared to piezoelectric materials, piezoresistive materials have very high
sensitivity and better low frequency response.