High frequency properties of substrates such as substrate losses and cross-talk were investigated for silicon-on-insulator (SOI) materials and for a novel depleted silicon material. Simulations were made to reveal high frequency properties of SOI MOSFETs made on fully depleted SOI materials of different substrate resistivities. The substrate resistivity was varied from 0.01 Omega cm to 10 k Omega cm, It was found that the high frequency properties of intrinsic transistors were independent of the substrate resistivity, but a system consisting of both active and passive components showed reduced cut-off frequency, f(T),. and maximum oscillation frequency, f(max), for low substrate resistivities. The obtained information on the importance of the silicon substrate resistivity has been used to manufacture a depleted silicon material by use of wafer bonding. The novel material is intended as substrate for high frequency applications. The space charge regions surrounding a bonded silicon/silicon interface deplete the silicon thereby reducing substrate losses at high frequencies. The novel material has been characterised electrically for frequencies up to 40 GHz using metal transmission lines and cross-talk structures on its surface. The measurements have been compared to similar measurements on bulk silicon, SIMOX and quartz substrates. The depleted region of the novel material shows up in its electrical characteristics.