Effects of thermal annealing in the properties of PECVD amorphous-Si0.8C0.2:H layers were studied. In order to reduce the density defects and increase the electrical conductivity, some samples were annealed: at 500 °C during 1 hour followed by 15 minutes at 800 °C. The results show that in the course of the thermal process, the hydrogen diffuse outside the film and the annealed Si0.8C0.2:H films tend to segregate in silicon clusters and, as a result, partially crystallize. Electrical dark conductivity shows an increase in more than six orders of magnitude, while the optical bandgap decreases from 1.9 eV to 1.4 eV. Annealed- Si0.8C0.2 films have been used as emitter in silicon bipolar transistors. Results indicate promising features such as low base currents and good emitter Gummel numbers.