An RF SOI device combines a triple-layer stressing stack and patterned low-k features (i.e., low-k polymer structures and/or air gap regions) disposed in pre-metal dielectric over the gate structures of NMOS transistors. The triple-layer stressing stack includes a thick SiN or oxynitride lower stressor layer that applies tensile stress in the channel regions of the NMOS transistors, a thin intermediate buffer layer, an upper etch-stop layer. After Metal-1 processing is completed, a special etching process is performed to define air gaps in the pre-metal dielectric over the NMOS gate structures using upper layer(s) of the triple-layer stressing stack as an etch stop to prevent damage to the stressor layer. A non-conformal dielectric material or an optional low-k dielectric material is then deposited in or over the air gaps to complete formation of the low-k features, and an optional capping or sealing layer is formed over the completed low-k features.