This paper describes the development of a miniaturized antenna duplexer for the W-CDMA application. As the required insertion loss in the W-CDMA system is severe even for the film bulk acoustic resonator (FBAR) technology, we propose a modified FBAR structure, which we called an AlN "just-etched" FBAR, to reduce acoustic losses. The characteristics of the proposed FBAR are verified by both simulation and experimental data and it is confirmed that the lateral-leakage of acoustic waves is successfully decreased. As results, the Q-factor at the anti-resonance and the effective k 2 are improved from 700 to 900, and 7.4% to 7.5%. The W-CDMA duplexer is designed using the proposed FBAR and integrated lumped elements for the compact and low-loss phase shifters. The insertion losses of the fabricated 3 x 3 mm 2 duplexers are as small as 1.5 dB and 1.6 dB in the Tx and the Rx band with a high isolation.

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