Silicon addition to a-C:H (hydrogenated amorphous carbon) thin films offers many advantages and a full understanding of the electrical properties of a-C:H:Si thin films is necessary for applications like biomedical implants and biosensor devices. The thin films were deposited with 13.56 MHz RF-PECVD (plasma enhanced chemical vapour deposition) using acetylene, argon and tetramethylsilane (TMS) vapour on p-type and n-type Si (100) wafers, and some of the films were further modified by thermal annealing. Silicon incorporation into a-C:H improves the thermal stability against graphitisation but decreases the film resistivity and breakdown strength. The breakdown strength variation is likely to be determined by inclusion of conductive clusters but the source of these clusters is unclear, since Raman indicates an increase in sp(3) content. The contact potentialdifference (CPD) of a-C:H thin films decreased by (approx. 0.21-0.43 eV) with increasing annealing temperatures, due to reduction in bandgap towards graphite, and increasing amounts of silicon, suggesting a more p-type structure due to higher trap densities, although change in bandgap or surface charge may also play a role. (C) 2003 Elsevier B.V. All rights reserved.