Page 326 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

306 INTRODUCTION TO SAW DEVICES

models can be run with commercial SPICE software, which helps in the design of better
acoustic microtransducers.

9.3 THE PIEZOELECTRIC EFFECT

Piezoelectric crystals play a dominant role in the communications and electronics industry
where they are commonly used as filters, precision timers, and for frequency control in
oscillator circuits (Mason 1942). The piezoelectric effect can be demonstrated by applying
either a compressive or tensile stress to the opposite faces of a piezoelectric crystal.
Figure 9.1 shows that when the equal and opposite forces, F1 and F2, (generating tensile
stresses) are applied, the resulting deformation of the crystal lattice produces a separation
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of the centres of gravity of positive and negative charges .
This effect results in electrical charge appearing on the surface of the electrodes.
When the force (tensile or compressive) is removed, the strain within the crystal lattice is
released causing charge (and hence current) to flow, thus reestablishing a zero potential
difference between the electrodes. If a sinusoidal stress alternating between the tensile
and compressive forces is applied to opposite crystal faces, a sinusoidal piezoelectric
voltage will appear across the electrodes. In this case, electrical energy is produced from
mechanical energy (generator action). This process of crystal deformation can be reversed.
In other words, when an external voltage is applied to the electrodes, the crystal lattice
will deform by an amount proportional to the applied voltage. In this case, electrical
energy is transformed into mechanical energy (motor action) (Campbell 1998). Therefore,
it is effectively a piezoelectric microactuator.
There are essentially three important properties of piezoelectric transducers that justify
their use for sensing applications (Morgan 1978):

1. An ideal coupling mechanism between the electric circuit and the mechanical properties
of the crystal, ensuring that the frequency of the mechanical acoustic wave is identically
equal to the electrical frequency, that is, a distortion-free interface having extremely
low dissipation.

2. The anisotropic nature of piezoelectric crystals allows for different angles of cut with
respect to the crystallographic axis, which, therefore, allows the use of crystals with a
range of frequencies.

Electrode

F2

Resistor

Figure 9.1 Transformation of mechanical energy into electrical energy

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Note that this effect does not occur in crystalline silicon because its lattice structure is centro-symmetric.