Bench Talk

As in many areas of electronics, chemistry is the fundamental driver for how piezo materials work. In piezo materials, induced changes in their crystal structures at the atomic level govern their function. An applied stress and/or pressure to piezo materials causes a deformation in the crystal structure, which invokes a change in the electrical current across the material. When piezo materials classified as either piezoelectric or piezoresistive are subjected to a stress and/or strain, the electrical charge or the resistivity (respectively) changes.

Although piezoelectric materials serve a wider range of applications than piezoresistors, both types of piezo materials are extremely useful in sensor applications, namely stress/strain sensors that can measure if a material is becoming mechanically deformed under an applied load. These sensors are crucial in construction-based applications to measure if parts of a structure are deforming from too much stress. Designers also use piezoelectric materials as power transducers, actuators, conductive adhesives and sealants, high-voltage power sources, and piezoelectric motors. In addition to sensor applications, the other main application of piezoresistive materials is piezoresistive resistors.

Piezoelectric materials work by using the principles of the piezoelectric effect. The piezoelectric effect is the generation of an electrical charge under an applied stress. One of the main features of piezoelectricity is that it is reversible. Therefore, when the stress is released from the material, the electrical charge stops. However, this can work the other way around. In addition to stress causing an electrical charge, if an electrical charge is applied to the piezoelectric material, then its atomic structure will deform and induce stress on the material.

Rearranging the ions within the solid-state lattice at the atomic level produces piezoelectricity. Because most of the materials are solid-state inorganic materials, where the atomic crystal lattice is a regular and repeating array of well-ordered cations and anions, it is the deformation of this regular atomic pattern that generates an electrical charge. Note that the overall charge of the material is neutral, so it will contain the same number of cations and anions within the lattice—not accounting for natural defects, which can occur in solid-state lattices.

Piezoelectricity works in many insulating materials, especially those that have a unit cell—i.e., the fundamental building block of a crystal lattice—with a specific symmetry. Examples of these materials include:

• Natural and synthetic crystals
• Synthetic ceramics
• Group III-V and II-VI semiconductors
• Various other metal oxide complexes

Polymers are a type of material that deviate from the highly ordered solid-state lattice structure. Some polymers are more crystalline—rather than amorphous—in nature. This means that some polymers can generate a piezoelectric charge; however, the intensity of the electrical charge is significantly lower than their inorganic counterparts.

The specific symmetry in piezoelectric materials is a key driver to how their mechanism works. There are 32 different crystal geometries—otherwise known as point groups—a crystal can exhibit. Piezoelectric materials are noncentrosymmetric in nature, which means that they lack an inversion center within the lattice. Thus, piezoelectric materials have only a certain number of applicable lattice types. Given the symmetry requirements, there are 20 viable noncentrosymmetric lattices, which means that only certain materials can generate a piezoelectric current.

The lattice symmetry of the material is important because inducing macroscopic polarization within the lattice creates the electrical charge, and this can only occur under these specific lattice conditions. However, this is often not enough to create a large piezoelectric effect in itself and requires a material to also possess ions with a large effective charge that can move under lattice strains. The mechanism for generating an electrical charge incorporates these different crystallographic aspects. When a stress is applied to the material, the oppositely charged ions move from their normal orientation so that they lie closer to each other within the lattice. This alters the charge balance within the lattice and induces an external electrical field. While the effects occur within the lattice, the effects of the charge imbalance spread throughout the material. A net charge—either positive or negative—appears on the outer face of the crystal as a result. Then, this creates a voltage across the oppositely charged crystal faces which is piezoelectricity. When the pressure stimulus is removed, the lattice returns to its natural state, and the voltage diminishes.

Piezoresistive materials are similar, yet different, to piezoelectric materials. Piezoresistive materials work by using the principles of the piezoresistive effect. Like the piezoelectric effect, the piezoresistive effect is a change under an applied stress; however, the lattice deformations in the piezoresistive effect lead to a change in the resistivity of the material. The piezoresistive effect occurs only in materials that are conductive in some way, be it highly conducting materials such as metals or semiconducting materials.

Materials that conduct electricity are fundamental to the realization of a piezoresistive material. Piezoresistivity partly relies on changing the bandgap of a material to alter its electrical resistivity/insulating properties. Insulating materials have a wide bandgap between the conduction and valence bands in their electronic band structure, so a large energy input is required to mobilize the electrons. By comparison, the valence and conduction bands in metals overlap and is why metals are electrically conductive—as it enables the electrons to flow to the conduction band with minimal energetic obstructions. While the bandgap of semiconductors does not overlap, the energy levels of the valence and conduction bands are very close together. It only takes a little bit of energy input—usually heat—to facilitate the movement of electrons from the valence to the conduction band. There are instances where the resistivity decreases, but the reduction of the bandgap is not by a significant amount. While the change in bandgap in these instances is enough to make many semiconductors and metals more conductive, the effects are negligible for insulating materials.

Because piezoresistivity relates to changing the effects of the electronic bandgap, materials with a zero or a very small bandgap are required. When the ions in a piezoresistive material are subjected to strain, the interatomic distance between the ions changes, and this alters the resistivity of the material. The change in piezoresistivity can go both ways and make the material adopt a greater or lesser resistance. Whether the change in piezoresistivity makes the material adopt a greater or lesser resistance is determined by what happens to the atoms under applied mechanical forces.

The specific change in resistivity is dependent on the type of stress applied to the material. If the material is strained/elongated, most of the atoms will spread further apart from each other. Because the bandgap is controlled by the spacing between ions in a lattice, this extra distance widens the bandgap and reduces the resistivity. On the other hand, if the material is subjected to a compressive force, then the ions in the lattice will move closer together. This reduces the resistivity as the energy required for electrons to pass between ions is reduced. Because the change relates to the distance between ions in the lattice, the geometry of the lattice can also play a role in how the resistivity changes under applied mechanical forces.

The electronic properties of piezoelectric and piezoresistive materials change when there is an applied mechanical stress. Even though the stimulus is the same for both types of materials, the internal mechanisms and property changes are different. For piezoelectric materials, the stress on the crystal lattice causes an electrical charge to pass through what is usually an insulating material by causing a charge imbalance within the material; whereas, the resistivity of a piezoresistive material is altered by deforming the lattice and changing the bandgap of conducting/semiconducting materials.