Piezoresistive pressure sensors

But what makes them so special, and why should you choose this technology?

In the following, we will explain how they work and show you the advantages over other pressure sensors to make it easier for you to choose the right sensor.

The piezoresistive effect

The piezoresistive effect describes how the electrical resistance of a material changes under mechanical stress. In simple terms, the resistance of the material changes when it is subjected to pressure or tension. Percy W. Bridgman discovered this effect back in the 1920s.

The effect is as follows:

  • Phase 1: Initially, mechanical deformation occurs when an external force (tension or pressure) acts on the material. This either compresses or stretches the crystal structure of the material.
  • Phase 2: In semiconductor materials such as silicon, the deformation changes the distance between the atoms. This in turn has an effect on the mobility and number of charge carriers within the material.
  • Phase 3: Finally, the change in charge carrier density leads to a change in the electrical resistance of the material. Depending on how the material is deformed, the resistance can either increase or decrease.

This effect is used in pressure sensors to convert pressure changes into electrical signals.

Functionality of piezoresistive pressure sensors

Piezoresistive pressure sensors use the effect described above to convert pressure into a measurable electrical signal. The core element is often a silicon chip with integrated resistors. Due to its compact design, the technology is also suitable for the construction of very space-saving miniature pressure sensors.

Components and mode of operation:

    1. Pressure absorption: The pressure to be measured acts on a diaphragm. This diaphragm can be made of different materials, but stainless steel is often used.
    2. Force transmission: The diaphragm transmits the force generated by the pressure to a piezoresistive resistor.
    3. Change in resistance: The application of force changes the resistance of the semiconductor material. Silicon shows a particularly strong effect here. Its crystal structure changes minimally under pressure, which leads to a significant change in resistance. Silicon is much more sensitive than metallic strain gauges, which enables a higher resolution and accuracy.
    4. Wheatstone bridge: In order to precisely record the small changes in resistance, the resistors are arranged in a Wheatstone bridge. This consists of four resistors, two of which are located on the membrane in areas with tensile stress and two in areas with compressive stress. The voltage at the nodal points of the bridge changes in proportion to the change in resistance and thus provides an electrical output signal that correlates with the applied pressure.
    5. Signal conditioning: The output signal of the Wheatstone bridge is very small and must be amplified. Integrated electronics ensure signal amplification, linearization and temperature compensation, as resistances can be temperature-dependent. The conditioned measurement signal is then converted into an industry-standard, standardized output signal (voltage, current or digital).
    6. Measuring cell: The silicon chip with the resistor bridge is sensitive to external influences. In order to make the piezoresistive measuring method universally applicable, the silicon chip with the bridge resistors is packaged in a so-called measuring cell. The measuring cell consists of a stainless steel body with a thin separating diaphragm. All components are welded together without a seal. The cavity between the silicon chip and the separating membrane is filled with a fluid in order to transfer the process pressure. The measuring cell encapsulated in this way can be used universally, largely independent of the medium. It forms the heart of every piezoresistive pressure sensor.

Piezoresistive pressure measurement vs. resistive pressure measurement

Both piezoresistive and resistive pressure measurement use the principle of changing resistance to measure pressure, but there are significant differences in the way they work and their properties.

  • Resistive pressure measurement (strain gauge): In resistive pressure measurement, a strain gauge is used which is applied to a carrier material and connected to the object to be measured. If the pressure changes, the object deforms, causing the strain gauge to stretch or compress. This mechanical strain leads to a change in the electrical resistance of the strain gauge.
  • Piezoresistive pressure measurement: In contrast, piezoresistive pressure measurement uses the piezoresistive effect in semiconductor materials such as silicon. Here, the resistance of the material changes directly due to the force of the pressure, not due to mechanical deformation of the object.

The following table shows a comparison of the properties of both technologies:

CriterionPiezoresistive pressure sensorResistive pressure sensor (DMS)
Measuring principleResistance change in semiconductors due to the piezoresistive effectChange in resistance due to mechanical stretching of a metal or semiconductor strip
SensitivityVery high Up to 300 mV10 to 100 times lower than piezoresistive sensors
Typical measuring ranges100 mbar to 200 bar10 bar to 1000 bar
Overload resistance4-fold of the measuring range1.5 times the measuring range
AccuracyTypically 0.05%Typically 0.1
Long-term stabilityVery good, hardly any drift or material fatigue, as silicon works hysteresis-free like glass.Poorer, as mechanical expansion leads to material fatigue

What types of pressure can be detected by piezoresistive pressure sensors?

Piezoresistive pressure sensors are versatile and can detect different types of pressure. The following overview shows the most common types of pressure and how piezoresistive sensors are used for them:

  • Relative pressure: Relative pressure sensors measure the pressure in relation to the current atmospheric pressure. They have a ventilation opening that connects the sensor to the environment. These sensors are labeled with “gauge” or “g”. Examples of applications include measuring tire pressure or the fill level in open containers. Several pressure sensors from DDM, such as the PV15, PV15H, PV22, PV22 TEDS, PV22H, PV22H², PV25H, PV25L, PX25H and PX25L, can measure relative pressure.
  • Absolute pressure: Absolute pressure sensors measure the pressure in relation to the absolute vacuum. They are independent of fluctuations in atmospheric pressure. These sensors are referred to as “absolute” or “a”. Application examples are the measurement of air pressure in airplanes or pressure control in vacuum applications. Several pressure sensors from DDM, such as the PV15, PV15H, PV22, PV22 TEDS, PV22H, PV22H², PV25H, PV25L, PX25H and PX25L, can measure absolute pressure.
  • Differential pressure: Differential pressure sensors measure the pressure difference between two points. They are often used for monitoring filters, flow measurements or level measurement in closed containers. The differential pressure sensors PV2722, PV2722H and PV2722L from DDM are suitable for measuring differential pressures.

DDM Sensors offers a wide range of piezoresistive pressure sensors for various pressure types and applications.

Advantages of piezoresistive pressure sensors

Piezoresistive pressure sensors offer a number of advantages over other pressure measurement technologies, particularly resistive pressure measurement with strain gages. These advantages make them attractive for a wide range of applications:

  • Higher precision: Piezoresistive sensors are characterized by higher sensitivity, which leads to better resolution and accuracy in pressure measurement.
  • Compact design: Due to their design, piezoresistive sensors can be more compact than strain gauge-based sensors.
  • Larger measuring range: Piezoresistive pressure sensors can cover a larger measuring range, which makes them more versatile for different applications. The typical measuring ranges are between 100 mbar and 200 bar.
  • Higher long-term stability: By using silicon as the sensor material, piezoresistive sensors have a high long-term stability, as silicon hardly shows any material fatigue. This leads to less drift and more reliable measurements over longer periods of time.

To summarize, piezoresistive pressure sensors are an excellent choice for demanding pressure measurement applications due to their precision, compact design, wide measuring range and high long-term stability.

Areas of application for piezoresistive pressure sensors

Piezoresistive pressure sensors are used in numerous applications due to their high precision, reliability and compact design. Here are a few examples:

  • Automotive industry:
    • Brake pressure measurement: Piezoresistive sensors are used to precisely measure the brake pressure in vehicles to ensure the safety and efficiency of the braking system. The PV-15B brake pressure sensor from DDM has been specially developed for this application.
    • Engine control: In combustion engines, hybrid and electric drives, pressure sensors help to measure the pressure of air/fuel mixtures.
  • Industrial automation:
    • Process control: Piezoresistive pressure sensors are used in the process industry to monitor and control pressure in various processes.
    • Hydraulic systems: In hydraulic systems, they are used to measure and control the pressure to ensure optimum performance.
    • Pneumatic systems: In pneumatics, they are used for precise pressure control.
    • Filter monitoring: Pressure sensors monitor the condition of filters by measuring the pressure drop and thus signaling the need for replacement in good time.
  • Further areas of application:
    • Food and beverage industry: In food and beverage production, they are used for pressure control in various processes.
    • Aerospace: Pressure sensors are used in the aerospace industry, for example in turbine test benches.
    • Racing: Pressure sensors are also used in racing to monitor various parameters.
    • Water management:
      • Well level: Piezoresistive sensors are used to measure the water level in wells.
      • Elevated tank monitoring: They are used to monitor the fill level in elevated tanks.
    • Leak test: Piezoresistive pressure measurement is used for leak testing using the pressure drop method.

Piezoresistive pressure sensors from DDM Sensors

As the DDM Sensors team, we offer you high-quality and precise piezoresistive pressure sensors that are tailored to your requirements. Our sensors are characterized by high quality and precision and are suitable for a wide range of applications. Our range includes miniaturized sensors (PV10, PV15), brake pressure sensors (PV15B), sensors with integrated parameter memory (PV22 TEDS), sensors for hydrogen applications (PV-22H2) and differential pressure sensors (PV2722). Visit our website for more information or contact us for an individual consultation.