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We report the development of aluminum nitride (AlN) thin film bulk acoustic resonator (FBAR). This resonator has an air gap beneath the resonator to obtain high Q factor and low spurious response. Germanium (Ge) was used as a sacrificial layer for the air gap. This technique gives very simple process and high CMOS compatibility. The FBAR was evaluated about the effect of the air gap and the electrode size. The FBAR achieved a resonant frequency of 2 GHz, a Q factor of 780 and an effective electro-mechanical coupling constant (k/sub eff//sup 2/) of 5.36%.