Integrated circuit structures configured with low loss transmission lines are disclosed. The structures are implemented with group III-nitride (III-N) semiconductor materials, and are well-suited for use in radio frequency (RF) applications where high frequency signal loss is a concern. The III-N materials are effectively used as a conductive ground shield between a transmission line and the underlying substrate, so as to significantly suppress electromagnetic field penetration at the substrate. In an embodiment, a group III-N polarization layer is provided over a gallium nitride layer, and an n-type doped layer of indium gallium nitride (InzGa1-zN) is provided over or adjacent to the polarization layer, wherein z is in the range of 0.0 to 1.0. In addition to providing transmission line ground shielding in some locations, the III-N materials can also be used to form one or more active and/or passive components (e.g., power amplifier, RF switch, RF filter, RF diode, etc).