While the diaphragm of PMUTs (piezoelectric micromachined ultrasonic transducers) is driven with large displacement in nonlinear regime, they exhibit unstable vibration displacement due to hysteresis response. In this study, the nonlinear behavior of PMUTs related to initial deflections of the diaphragm is analyzed for stable driving with large displacement. The PMUT with initial deflection of 0.7 µm showed an increase in resonant frequency with displacement of the diaphragm, called the hard spring effect. In the case of 6.2 µm initial deflection, the PMUT has the soft spring effect, resulting in larger maximµm displacement than that of the hard spring effect. On the other hand, we have found that the PMUT having mediµm initial deflection of 4.0 µm exhibited mixed frequency response of hard spring and soft spring, which demonstrated the largest maximµm displacement. In addition, the frequency region without hysteresis become wider by increasing the drive voltage. From these results, we conclude that stable driving in large displacements is achieved with optimized initial diaphragm deflection and drive voltage.
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