What Is the Piezoresistive Effect?
The piezoresistive effect describes the change in electrical resistance that occurs when an external force is applied to a semiconductor. This change only affects the material's electrical resistivity. Unlike the piezoelectric effect, it cannot be used to generate a voltage across the device.
The strain from the applied force impacts the material's band structure, which makes it easier or more difficult for electrons to be excited into the conduction band. Consequently, the density of current carriers is altered and the material's resistance changes.
Areas of Application
Piezoresistive Pressure Sensors
Valued for their high sensitivity and linearity, piezoresistive pressure sensors were some of the first MEMS devices to come to market. Various industries implement these devices in their products as a means to measure pressure. For example, the biomedical field uses piezoresistive sensors as tools to measure blood pressure, while the automotive industry uses them to gauge oil and gas levels in car engines.
In a piezoresistive pressure sensor, a piezoresistor is usually implanted in the surface of a thin silicon diaphragm. As pressure is applied, the diaphragm deforms and the resulting strain impacts the carrier mobility and number density.
Many types of accelerometers also make use of the piezoresistive effect. Designed for high-frequency shock measurements, these devices have an advantage over their piezoelectric counterparts as they are able to measure accelerations down to reaching 0 Hz. This capability to measure very low frequencies means that the device can provide an accurate static measurement of acceleration. Piezoresistive accelerometers are often used to analyze shock and vibrations in automotive safety testing, including safety air bag and anti-lock brake systems.