The Piezoresistive Effect in Transducers and Sensors
The change in electrical resistance under mechanical strain is called the piezoresistive effect.
Transducers and sensors working on the principle of the piezoresistive effect are used in automobiles and commercial devices to convert electrical energy to mechanical energy and vice versa.
The piezoresistive effect differs from piezoelectric in that piezoelectric is the property of a material to generate voltage when force is applied, whereas the piezoresistive effect refers to a material's resistivity to change.
Auto manufacturers use piezoresistive accelerometers in automotive safety systems
Sensors and transducers are used in commercial devices to measure or convert one form of energy, such as force, acceleration, and pressure, to another form of energy. Piezoresistive sensors and transducers use the piezoresistive principle to sense or convert mechanical inputs. The change in the electrical resistance of a metal or a semiconductor material when experiencing mechanical strain is the principal governing the piezoelectric effect.
Let’s take a closer look at the piezoresistive effect and how mechanical strain influences electrical resistance in metals and semiconductors.
The Electrical Resistance of Metals and Semiconductors
The electrical resistance, R, of a metal is directly proportional to its resistivity,, and length, l, of the material. It shares an inverse relationship with the cross-sectional area, a, of the metal. Whenever there is any change in the geometrical dimensions, the electrical resistance will vary.
For example, electrical resistance increases when a metal's length increases, when all of the other parameters remain constant. When the cross-sectional area increases, resistance decreases. Resistivity is a material property, and variation of resistivity increases the electrical resistance.
In semiconductors, the redistribution of the holes in the valence band under mechanical strain will cause a change in resistance. When a semiconductor is subjected to mechanical strain, the holes jump from higher energy to lower energy, and the energy bands get deformed. The mobility of the holes and deformation of energy bands increases the resistivity, which increases resistance.
The Piezoresistive Effect
When an external force or strain is applied to a metal or semiconductor, its electrical resistance changes, the change in electrical resistance under mechanical strain is called the piezoresistive effect. The change in resistance due to external input is more dominant in semiconductors when compared to metals.
The piezoresistive effect is different from the piezoelectric effect. In the piezoelectric effect, the mechanical strain or external force applied generates a voltage across the device. This differs from the piezoresistive effect in that the effect only causes a change in resistance.
Piezoelectric transducers use the piezoresistive effect to convert physical quantities to electrical quantities. A piezoresistive transducer is used when a change in mechanical input in a structure needs to be measured. The change in mechanical quantity is the piezoresistive transducer's input. The piezoresistive transducer uses piezo resistors to convert the input to an electrical output.
Usually, the piezo resistors are connected using a Wheatstone bridge with a constant dc power supply. When the mechanical input to the piezoresistive transducer is nil, the Wheatstone bridge is balanced, and the transducer output is zero.
When mechanical input is applied to the piezoresistive transducer, the electrical resistance of the piezoresistor also changes. This change makes the Wheatstone bridge unbalanced. The unbalanced bridge generates an electrical voltage proportional to the mechanical input to the piezoresistive transducer.
Applications of the Piezoresistive Effect
The piezoresistive effect has applications in both transducers and sensors. Some of these applications include piezoresistive pressure sensors, piezoresistive silicon chip transducers, and piezoresistive accelerometers.
Piezoresistive Pressure Sensors
Piezoresistive pressure sensors are beneficial due to their high sensitivity and linearity. They have applications in biomedical devices, automobiles, consumer electronics, and the aerospace industry.
Piezoresistive Silicon Chip Transducers
Piezoresistive silicon chips use the piezoresistive effect in silicon semiconductors. A silicon chip transducer converts applied pressure to proportional electrical voltage with the help of a Wheatstone bridge. The advantages of piezoresistive silicon chip transducers include:
- Linearity due to the absence of hysteresis
- Frequency response up to 100kHz
Compared to piezoelectric accelerometers, piezoresistive accelerometers are capable of measuring acceleration down to 0Hz. This low-frequency capability makes piezoresistive accelerometers suitable for the static measurement of acceleration. As a result, auto manufactures use piezoresistive accelerometers in automotive safety systems such as airbags and anti-lock brakes.
Transducers and sensors working on the principle of the piezoresistive effect are used in automobiles and commercial devices to convert one form of energy to another and are influential in many different industries. Cadence’s software offers design tools and extensive component libraries that help design boards with transducers and sensors.
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