Piezoelectronic Transistor - Piezoelectrics
Piezoelectricity is the property of converting an electrical stimulus into a mechanical output, and also the reverse. It has found many applications, from common devices such as push-start propane grills to sophisticated uses like SONAR, MEMS resonators and atomic force microscopes. Numerous crystalline compounds are used as piezoelectrics, with commonly used options including barium titanate and lead zirconium titanate. All of these piezoelectrics are highly polar crystals. Their atomic structure is deeply linked to their piezoelectric properties; the application of an electric field attracts (or repels) ions, distorting bonds and creating strain.
The use of a piezoelectric integrated with a piezoresistor is a new concept that lies at the core of the PET. For best performance, it is important to use a piezoelectric with a large response. This is why we are interested in using a class of materials known as relaxor piezoelectrics. An example of this is lead magnesium niobate-lead titanate (PMN-PT).
When a voltage is applied to a piezoelectric, it expands. Consequently, there is a contraction in the perpendicular directions. This contraction is clamped by bonds to the substrate, limiting the extent of the motion. The clamping can be reduced by patterning the piezoelectric into mesas or narrow lines. When making a PET, the favored device geometry is a pillar, which improves the piezoelectric response. Scaling the device to smaller dimensions reduces clamping, improving performance.