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The characteristics of aluminum nitride (AlN) Lamb wave resonators utilizing the lowest symmetric (S0) mode with grounded, floating, and open bottom surface configurations are theoretically and experimentally investigated. The Lamb wave resonator without the bottom electrode exhibits a quality factor (Q) as high as 2,573 but a low effective coupling of 0.18% at 949.7 MHz. On the contrary, the Lamb wave resonator with a floating bottom electrode shows an effective coupling of 1.05% but a low Q of 850 at 850.3 MHz because the imperfect interface between the AlN plate and bottom electrode usually degrades the Q. Limited by the larger static capacitance, in contrast to the floating bottom electrode, the Lamb wave resonator with a grounded bottom electrode shows a smaller effective coupling of 0.78% and a low Q of 800 at 850.5 MHz. These results suggest that the resonator with a floating bottom surface is suitable for filter applications, whereas that with an open bottom surface is preferred for sensor and oscillator applications.