Semi-insulating silicon substrates are very attractive as handle wafers in Silicon On Insulator (SOI) technologies because they would provide very low-absorption substrates for radio frequency (RF) and monolithic microwave integrated circuits. We have shown both theoretically and experimentally that deep level doping with transition metals is an excellent method to create this semi-insulating Si. This work aims to study the science and engineering of high resistivity silicon substrates for high frequency integrated circuits such that it can be applied industrially. The project aims to better understand the diffusion and doping versus resistivity relations of appropriate deep level impurities (including Mn), and hence to maximise the resistivity of the silicon handle wafer. Contamination issues arising from the deep level impurities will be addressed by investigating diffusion barriers and also by developing a back-end processing approach that takes advantage of the high diffusivity of some deep level impurities. SOI wafers will be fabricated on semi-insulating silicon substrates and detailed high frequency characterisation will be carried out.