The concept of fully encapsulated, semi-insulating silicon (SI-Si) Czochralski silicon on insulator substrates for silicon microwave devices is presented. The experimental results show that, using gold as a compensating deep level impurity in silicon, a resistivity of 180kΩcm is achieved in this work. Sufficiently high resistivity substrates for microwave applications are obtained by depositing gold on silicon and annealing for 1 h at 1000°C . Silicon dioxide and silicon nitride are considered as diffusion barriers for use in the new technology. At 1000°C , the diffusivity of gold in plasma-enhanced chemical vapor deposited silicon nitride is ∼3.7×10−13cm2normals−1 , whereas for silicon dioxide, it is less than 6×10−14cm2normals−1 . These results imply that silicon nitride would be ineffective as a diffusion barrier in the SI-Si technology but that a 600 nm thick buried silicon dioxide might allow devices to be processed with a thermal budget of up to 15 h at 1000°C without impurity contamination.