3: Review of Piezoresistive Materials as Smart Sensors

Chapter 3 reviews piezoresistive materials as smart sensors. Piezoresistivity is defined as a property of certain materials, such as metals and semiconductors, for which the materials electrical resistance changes purely due to mechanical pressure, stress, force, acceleration, strain, and stress. It is the physical property of certain materials which has been widely used to convert a mechanical signal into an electrical signal in smart sensors, accelerometers, tactile sensors, strain gauges, and flow meters and similar devices and microdevices. Metals do not exhibit piezoresistivity as they do not have a bandgap. The resistance of strained metal samples changes due to dimensional changes – this may not be considered as piezo-resistivity. The unit of piezoresistivity is ohm-meter or symbolically Ω–m. Metals and semiconducting materials exhibit such a property. The piezoresistive effect in semiconductors is generally several orders of magnitudes larger than the geometrical effect. This effect is present in semiconductors such as germanium, amorphous silicon, polycrystalline silicon, and silicon carbide, among others. Hence, semiconductor strain gauges with a very high coefficient of sensitivity can be designed, built and operated and utilized in various smart sensor applications and as microelectromechanical (MEMs) or nanoelectromechanical (NEMs) devices and systems.