Measurement of mechanical strain based on piezo-avalanche effect

We report on the use of the breakdown voltage of a pn junction to measure mechanical strain in microstructures. The working principle relies on the dependence of the silicon bandgap on the mechanical stress which affects the current-voltage characteristics of the pn junction. An analytical 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 240 μV/MPa. The mechanical stress can also be measured by monitoring the device current while biased at a constant voltage. In this mode, the steep changes of the junction current in the 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 the measurement of stress in micro- and nanomechanical devices.