A ferroelectric loaded waveguide resonator capable of operation at microwave, millimeter-wave and higher frequencies and suitable for integration into a three-dimensional monolithic microwave integrated circuit (3D MMIC) is disclosed. The resonator includes a resonator cavity, which, in one form of the invention, is formed by two parallel metal layers and a metallized wall structure extending between the metal layers. The cavity is filled with dielectric material and includes a layer of ferroelectric material, which is used to control the resonant frequency by varying a voltage bias applied to the ferroelectric layer. The cavity includes a slot in one of the metal layers and a coupling strip formed adjacent to the slot to provide electromagnetic coupling to other components, such as a voltage controlled oscillator (VCO). The invention can also be applied to other multi-metal semiconductor or wafer level packaging technologies.