We present a novel device structure applicable to various RF MEMS devices that consist of a dielectric membrane and metallized air cavity. RF MEMS device components are formed on the dielectric membrane, which is suspended above the metallized air cavity. The metallization of the bottom of the air cavity allows us to realize the ground plane. Moreover, by applying the air cavity, the influence of the substrate on the loss characteristics of the device can be minimized. In this paper, we also present a novel RF MEMS process module to realize our proposed device structure. By means of our RF MEMS process module, four RF passives could be successfully developed. The implementation of these passives could be achieved simultaneously on the same silicon substrate. Our fabricated passives are a grounded coplanar waveguide (GCPW), a 90° hybrid, an elliptic low pass filter (LPF) and an inductor. All of them are fabricated on a silicon nitride membrane suspended above the 30 µm deep metallized air cavity. They have been developed as components of a wireless communication transceiver module at 12 GHz. We performed characteristics measurement and obtained 2.03 dB and 2.18 dB as insertion losses, and 21.63 dB as the return loss for the 90° hybrid at 12 GHz. For the elliptic LPF and GCPW, we obtained the insertion losses at 12 GHz as 1.86 dB and 0.1 dB mm⁻¹, respectively. The measurement results are also in agreement with the simulation results. Our proposed device structure and RF MEMS process module have the advantages in terms of loss characteristics, manufacturing cost and applicability.