Where is Silicon Valley located?

In Northern California, the Silicon Valley lies within the lower part of the San Francisco Bay Area.

To be more precise, Silicon Valley runs along the peninsula from Palo Alto, CA in the north through San Jose, CA in the south. But electronics and computer companies have long since expanded to encompass the entire Bay Area, not just the original Silicon Valley area.

BTW, please don't call it Silicone Valley, that is an entirely different subject.