The de characteristics of GaAs/Al0.3Ga0.7As heterostructure diodes and heterojunction bipolar transistors (HBTs) passivated by various Si-based thin films, i.e. amorphous Si, SiC, as well as the conventional SiOx and SiNx, were investigated and compared. All these films were found effective in reducing the leakage current and long term degradation. Less size-dependent on the current gain for the passivated HBTs by amorphous Si and SiC was observed. In addition, the devices passivated by amorphous Si and SiC films exhibited more stable characteristics during high power operation, which were indicated by the unchanged junction voltage in the diodes and the stable microwave performance in the HBTs. These de and thermal properties of the passivated devices demonstrate the potential of SiC and amorphous Si as excellent passivation materials for power devices.