Table 1.

A summary of PMUT structures reported by researchers.

Reference	Piezoelectric Material	PMUT Structure	Center Frequency	Benefits
Eovino et al. [34]	AlN	Ring-shaped PMUT	1.5 MHz	Improved acoustic pressure and directivity compared with circular-shaped PMUT
Thao et al. [35]	PZT	Island-shaped PMUT	1.8 MHz	Improved mechanical robustness
Liu et al. [36]	PZT	Annular-shaped PMUT	11.9 MHz	High effective electromechanical coupling factor and large static displacement sensitivity
Akhbari et al. [38]	AlN	Self-curved PMUT	647 kHz	High effective electromechanical coupling factor and large fill-factor
Wang et al. [39]	AlN	Piston-like PMUT.	2.3 MHz	Higher transmitting and receiving sensitivities than circular PMUT
Wang and Zhou [40]	AlN	PMUT with a fully free edge structure	6 MHz	Increased pressure output compared with classical PMUT
Akhbari et al. [15]	AlN	Bimorph PMUT with two active piezoelectric layers and the dual-electrode	200 kHz–1 MHz	Large acoustic intensity and sensitivity