It's not biome specific, and it is there. Just now in the latest GC build I made a new world dug down and found Silicon Ore in less than 2 minutes.
It's rather rare compared with metal ores - but it is more common than diamond or lapis lazuli. On average, there should be around 15 blocks of Silicon Ore per chunk, in two or three 'veins' of up to 7 ore blocks each.
You were correct to make your search between heights y = 1 and y = 25. I recommend search between y = 15 and y = 25 - the probability of finding it will be a little higher there. It's because deeper down, below y = 15, there is more other stuff (bedrock / lava / redstone / diamonds / etc) so less places where silicon ore could be.