Piezoresistive sensors use changes in semiconductor resistivity under mechanical stress to produce electronic readings. They are used in a range of industries and applications, from automotive to medical, and can be constructed with various semiconductor substrates.
A piezoresistive sensor is a device that uses changes in the resistivity of certain semiconductor materials when subjected to mechanical stress to effect an electronic action. This piezoresistive phenomenon is based on the tendency of these materials to experience changes in their latent resistive characteristics when flexed by exposure to pressure or stress. This causes a corresponding change in any electrical current passing through the device which results in a measurement or reading. The semiconductor materials commonly used in piezoresistive devices are generally the same base metal and silicone families used in most electronic components. These components are available with a wide range of sensitivity characteristics to meet the needs of different industries.
When some semiconductor materials are exposed to mechanical stress, their resistivity or basic ability to oppose the flow of electric current changes. This change in the resistive character of the material obviously changes its specific resistance value. This results in an increase or decrease in any current passed through the device which is then used to indicate or measure the stress involved. The piezoresistive effect is exploited to create a range of deflection-sensitive semiconductor devices used to record and measure stress-inducing forces such as acceleration and pressure.
This type of sensor is typically constructed with semiconductor substrates such as germanium, polycrystalline silicon, amorphous silicon, and single crystal silicon. A typical sensor consists of a pressure sensitive semiconductor diaphragm with several n+ and p+ contacts connected to it. Electric current is passed through the wafer at a rate that depends on its resistance. This current transfer and the resulting reading differ based on changes in resistance in the component as the membrane is exposed to pressure. These sensors range in complexity from simple piezoresistors with limited range and temperature stability to highly stable and accurate Piezo-FETs.
The piezoresistive sensor is used in a wide variety of applications involving the measurement of mechanical stresses. The automotive industry uses them as vacuum and pressure sensors or to provide indications of oil and gas levels. They are also used in the medical field in devices such as blood pressure measuring equipment. The depth gauges used by deep-sea divers also employ piezoresistive sensor technology to produce accurate depth readings. These devices are also used in aircraft altimeters and barometric pressure instruments.
Pressure-dependent sensors of this type are also often used in electronic measuring instruments using circuits and components such as the Wheatstone bridge and bipolar transistors. The accelerometers used to measure positional orientation, acceleration, and vibration forces also use piezoresistive sensor technology to produce their readings. This technology has also found its way into the home environment with many manufacturers of dishwashers, vacuum cleaners and washing machines using them in their products.
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