What’s The Difference Between Piezoelectric And Piezoresistive Components?

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Piezo comes from the Greek word “piezein,” which means “squeeze” or “apply some pressure.” Whether they take form as a transducer or sensor, piezo components all operate as the result of some degree of physical pressure placed upon them. Most piezo devices are piezoelectric or piezoresistive, and each has its appropriate applications.

Piezoelectric Effect

Under pressure, vibration, or other forms of stress, piezoelectricity forms in certain materials, particularly crystals. Essentially, the piezoelectric effect is merely the result of stressing a piezo element—crystal, ceramic, or biological matter—to generate a charge or voltage.

The piezoelectric effect is linear. The amount of charge generation is proportional to the amount of stress placed upon the piezo material. Interestingly, this effect is reversible (Fig. 1). Applying a charge to the piezo material generates a mechanical response or a pulse. As a result, piezoelectric components find employment in sound production and detection apps, voltage and frequency generation, and a plethora of measurement systems.

1. The piezo tweeter, a.k.a., horn, on the left provides a cost alternative for standard magnetic-coil speakers on the right. On its own, the piezo horn responds only to high frequencies, eliminating the need for a frequency-compensating crossover network.

One example of this reverse deployment is the highly cost-effective and efficient piezo tweeter or horn found in inexpensive consumer-audio speaker systems in place of the somewhat more expensive magnetic-coil high-frequency driver with a paper cone. A piezo tweeter uses a piezoelectric crystal, which generates a small voltage when subjected to vibration or pressure.

When functioning as a high-frequency reproduction device (tweeter), however, the crystal is subjected to a voltage—the high-voltage audio signal coming from the receiver or amplifier. The crystal deforms in variation with its input signal and audibly reproduces the input.

The inexpensive piezo crystal responds only to high frequencies, around 4 kHz and above, making this arrangement cost-effective (Fig. 2). Therefore, it requires no crossover with numerous passive components to operate as a tweeter in a two- or three-way speaker system. Sometimes, depending on the design, placing an inexpensive electrolytic capacitor in series with the piezo horn provides protection by blocking low frequencies that might blow the horn.

2. On the left, the piezoelectric material generates a voltage under pressure or vibration. On the right, the piezoelectric effect is reversible by applying voltage to the piezoceramic for a variety of sound- or pulse-generation purposes.

On a historical note, Pierre and Jacques Curie are credited with the discovery and demonstration of the piezoelectric effect in 1880. Notably, the brothers did not realize the converse piezoelectric effect, which Gabriel Lippmann demonstrated in mathematical form around 1881.

Piezoresistive Effect

The piezoresistive effect also involves pressure or stress. However, changes in resistance across the piezo material are the product, not a charge or voltage. It is a change in electrical resistance of a semiconductor material due to mechanical stress.

Probably the most basic piezoresistive devices are, obviously, piezo resistors (Fig. 3). Form factors include integrated resistor networks, potentiometers, and accelerometers. Made from semiconductor materials, piezoresistive devices most commonly are used in pressure measurement.

3. When pressure is applied to a piezo resistor, depending on the material, its resistance increases.

In 1856, Lord Kelvin noted the change of resistance in mechanically loaded metal devices. Almost 100 years later, C.S. Smith described the piezoresistive effect in silicon and germanium in 1954.

The most common components that rely on the piezoelectric and piezoresistive effects include, but are not restricted to, transducers and sensors. And as you might have guessed, most applications are in detection and measurement.

Transducers

Transducers convert energy from one form to another. Stated earlier, piezoelectric transducers work both ways. They can convert mechanical energy such as pressure and vibration to electrical energy like voltage or current. They also can operate in reverse, converting electrical energy into mechanical energy such as sound or vibration.

Piezoresistive transducers convert mechanical energy into proportionate levels of resistance. They do not convert any form of stimulus to a voltage or current, nor are they reverse active like their piezoelectric cousins, meaning they cannot convert resistance levels to some other form of energy. Both piezoelectric and piezoresistive transducers come in a wide variety of shapes and packages.

Probably the most common piezoelectric component is the disk-shaped variety (Fig. 4). The ultra-thin metal disk comes in a range of diameters. Construction is the same for all sizes. Piezo crystals occupy the center portion of the disk. The outer metal circle and body back is the ground for the component.

4. The thin metal disk is the most common package for piezoelectric transducers. The piezo crystal or material is housed in the center portion, and the rest of the metal package is the ground.

Hookup is a breeze, requiring just two wires: a hot and a ground. This format also is popular since the thin disk fits into the tightest of quarters. It can be sandwiched between two flat surfaces or simply attached to any flat surface.

Piezoresistive transducers tend to be somewhat larger due to their use of semiconductor materials (Fig. 5). However, depending on the application, many housings, sizes, and shapes are available to accommodate compact or larger designs. Since these resistive components only work one way, converting mechanical stimuli into resistance, they find regular employment in pressure-measurement applications.

5. Using silicon semiconductor material for the piezoresistive effect, piezoresistive transducers tend to be slighter larger than their piezoelectric counterparts.

Sensors

Sensors detect or measure (or sometimes detect and measure) physical quantities such as distance, pressure, motion, and temperature. They perform their job via conversion. For example, a thermocouple converts a temperature into a readable voltage.

Piezoelectric sensors rely on the piezoelectric effect to measure a plethora of parameters such as pressure, strain, or force by, once again, converting them to voltages. Technically, one can justifiably say that piezoelectric sensors and transducers are one and the same. But piezoelectric sensors, more often than not, operate purely as sensors and not in the aforementioned reverse mode, i.e., applying voltage to generate an effect.

Additionally, piezoelectric sensors are electromechanical components exhibiting near zero deflection. As a result, they respond across a fairly high-frequency bandwidth and exhibit consistent linearity over a wide amplitude range. They are also available in a wide selection of sizes and lengths (Fig. 6).

6. Maintaining a fairly low profile and available in numerous sizes and lengths, piezoelectric sensors exhibit near zero deflection, making them both rugged and responsive to high frequencies.

Piezoresistive sensors are a bit more sophisticated in their design and the piezo they employ. For instance, they can employ thin metal-film resistors, single-crystal silicon, and other variations. It stands to reason that both the application and the budget will most likely determine which material to choose.

The piezoresistive sensor is a mainstay in pressure-measurement applications. According to Maxim Integrated Products, mono-crystalline silicon pressure sensors have come into wide use lately. Built on semiconductor technology, the resistance change (piezoelectric effect) is notably higher than exhibited in standard strain gauges. Therefore, the sensitivity of mono-crystalline sensors is higher than the sensitivity of most other types.

Available in a wider array of packaging options, piezoresistive sensors offer sensitivities beyond 10 mV/V and stable linearity at constant temperature. They also reliably track pressure changes without hysteresis (Fig. 7). Disadvantages include significant nonlinear dependence of the full-scale signal on temperature up to 1%/Kelvin, initial offsets up to 100% of full scale or more, and offset drift with temperature.

7. Piezoresistive sensors are available in a wider array of packaging options and specify sensitivities greater than 10 mV/V.

Summary

Piezo components are highly functional devices operating in measurement, safety, and test apps, to name a few, in markets as diverse as medical, musical (pickups in acoustic guitars and fingerboard sensitizers in electric guitars), military, and automotive.

Piezoelectric components convert mechanical energy to electrical energy and vice versa, while piezoresistive devices convert mechanical energy to resistance values and that’s it. They do not work in reverse like their piezoelectric counterparts.

The resistive components are a bit more sophisticated in design and therefore tend to be a bit more expensive. Both components have one important thing in common, though. With a bit of imagination, their efficient and creative applications are virtually endless.