Piezoresistive transducers

Semiconductors, such as silicon crystals, show a strong change of resistivity following a deflection (piezoresistivity), so for them the term dp/ep of Equation (1.12) is much greater than for metals: a strain gage made of a semiconductor has a gage factor that can vary from 50 to 200. This high gage factor makes semiconductor strain gages very attractive, as with them the problem of obtaining a useful signal from a very rigid transducer with a high natural frequency can be solved.

The dizzying development of semiconductor technology and the realization of integrated circuits in the 1970s strongly influenced the market for transducers. The rationale for this influence is significant even in the mechanical properties of silicon crystals: the elastic modulus is of the same order of magnitude as that of steel, the tensile strength is greater, and the relationship between the modulus of elasticity and the density is three times that of steel.

It must be emphasized that crystal silicon is both the flexural element which deforms and the transducer. Transducers are commercially
available in which the four arms of the Wheatstone bridge are diffused into a silicon diaphragm. The diameter of the transducers ranges from 1 to 30 mm, the resonance frequency varies from 55 kHz to 1 MHz, and the accuracy is 0.1 – r- 0.25% of the full scale.

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