Disclosed is semiconductor structure with an insulator layer on a semiconductor substrate and a device layer is on the insulator layer. The substrate is doped with a relatively low dose of a dopant having a given conductivity type such that it has a relatively high resistivity. Additionally, a portion of the semiconductor substrate immediately adjacent to the insulator layer can be doped with a slightly higher dose of the same dopant, a different dopant having the same conductivity type or a combination thereof. Optionally, micro-cavities are created within this same portion so as to balance out any increase in conductivity due to increased doping with a corresponding increase in resistivity. Increasing the dopant concentration at the semiconductor substrate-insulator layer interface raises the threshold voltage (Vt) of any resulting parasitic capacitors and, thereby reduces harmonic behavior. Also disclosed herein are embodiments of a method for forming such a semiconductor structure.