A series of Si-based thin films, including amorphous Si, SiC, as well as the conventional SiOx and SiNx, was investigated in terms of the electrical characteristics of GaAs/Al0.3Ga0.7 As heterostructure diodes and heterojunction bipolar transistors (HBTs). All the films were found effective in reducing the leakage current and long term degradation. Less size-dependence of the current gain was found for the HBTs passivated by amorphous Si and SiC. In addition, the devices passivated by amorphous Si and SiC films exhibited better performance during high power operation. This is attributed to the high thermal conductivity of these two materials.