What is a Piezoresistive Sensor?

A piezoresistive sensor is a device which makes use of changes in the resistivity of certain semiconductor materials when subjected to mechanical stress to effect an electronic action. This piezoresistive phenomenon is based on these materials tendency to undergo 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 translates into a measurement or readout. Semiconductor materials commonly used in piezoresistive devices are generally the same basic metallic and silicone families used in most electronic components. These components are available with a wide range of sensitivity characteristics to suit the requirements of diverse industries.

When certain semiconductor materials are exposed to mechanical stress, their resistivity, or basic ability to oppose electrical current flow, changes. This change in the materials resistive character obviously changes its specific resistance value. This results in a rise or fall in any current passed through the device which is then used to indicate or measure the stress involved. The piezoresistive effect is harnessed to create a range of deflection sensitive semiconductor devices used to record and measure stress inducing forces such as acceleration and pressure.

This kind of sensor is typically constructed of semiconductor substrates such as germanium, polycrystalline silicone, amorphous silicone, and single crystal silicone. A typical sensor consists of a pressure sensitive semiconductor diaphragm with several n+ and p+ contacts attached to it. Electrical current is passed through the wafer at a rate depending on its resistance. This current transfer and resultant readout differs according to the resistance changes in the component when the diaphragm 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 mechanical stress measurement. The automotive industry employs them as vacuum and pressure sensors or to give indication of oil and gas levels. They are also used in the medical field in devices such as blood pressure measurement equipment. 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 measurement instruments which use circuits and components such as the Wheatstone bridge and bipolar transistors. Accelerometers used to measure positional orientation, acceleration, and vibration forces also utilize piezoresistive sensor technology to produce their readouts. This technology has also found its way into the domestic environment with many dishwashers, vacuum cleaners, and washing machine manufacturers using them in their products.