A common SciFi concept, but does not seem reasonable in reality. While silicon has the same valence properties as carbon, many of its compounds are solids while the corresponding carbon compounds are gases or liquids except at high temperatures. Solids do not provide the mobility for life forms capable of evolving to development of of a technological culture. I also suspect that a brain requires more rapid chemical reactions than provided by most silicon compounds. Solids cannot be the basis for a flexible hand (or other type of appendage) capable of picking up a stick or stone for use as a weapon or tool is a necessary precursor to the evolution of a creature which can develop a technological culture.