IC structures that include transmission line structures to be integrated with III-N devices are disclosed. An example transmission line structure includes a transmission line of an electrically conductive material provided above a stack of a III-N semiconductor material and a polarization material. The transmission line structure further includes means for reducing electromagnetic coupling between the line and charge carriers present below the interface of the polarization material and the III-N semiconductor material. In some embodiments, said means include a shield material of a metal or a doped semiconductor provided over portions of the polarization material that are under the transmission line. In other embodiments, said means include dopant atoms implanted into the portions of the polarization material that are under the transmission line, and into at least an upper portion of the III-N semiconductor material under such portions of the polarization material.