Silicon (or Raw Silicon) is a crafting ingredient added in Galacticraft 2. It is obtained by mining Silicon Ore. When mined, the Silicon Ore will drop the Silicon and does not need to be smelted (much like Diamonds). It is a required ingredient when crafting Basic Wafers, Advanced Wafers, or Blue Solar Wafers using the Circuit Fabricator.
From Galacticraft 3.0.10, other mods' silicon can also be used in the Circuit Fabricator, if set that way in Galacticraft's config.