Measurement of Mechanical Strain based on Piezo-Avalanche Effect

We are reporting on the use of the breakdown voltage of a pn junction to measure mechanical strain in micro-structures. The working principle relies on the dependence of silicon band gap to the mechanical stress which affects the current-voltage characteristics of the pn junction. An analytic model is developed and verified experimentally for the phenomenon. A micromechanical device with integrated junctions was designed and fabricated. Mechanical stress was applied onto the structure by subjecting it to mechanical vibrations. It is shown that the breakdown voltage of the device exhibited a high stress sensitivity of about 240ߤ/ܸܯ .ܽܲ. The mechanical stress can also be measured by monitoring the device current while biased at a constant current. In this mode, the steep changes of the junction current in breakdown region led to nearly a tenfold higher stress sensitivity compared to a piezoresistive sensor. The high sensitivity, simple measurement, and potential for miniaturization for piezo-avalanche sensing make it a promising technique for measurement of stress in micro- and nano-mechanical devices.