A three dimensional (3D) micro-electro-mechanical system (MEMS) device is provided. The device comprises a central MEMS wafer, and top and bottom cap wafers. The MEMS wafer includes a MEMS structure, such as an inertial sensor. The 5 top cap wafer, the bottom cap wafer and the MEMS wafers are stacked along a stacking axis and together form at least one hermetic cavity enclosing the MEMS structure. At least one of the top cap wafer and the bottom cap wafer is a silicon-on- insulator (SOI) cap wafer comprising a cap device layer, a cap handle layer and a cap insulating layer interposed between the cap device layer and the cap handle layer. At 10 least one electrically conductive path extends through the SOI cap wafer, establishing an electrical convection between an outer electrical contact provided on the SOI cap wafer and the MEMS structure.