Silicon Valley is a nickname for the southern portion of the San Francisco Bay Area, which is located in part of the U.S. state of California known as Northern California. It is home to many of the world's largest high-tech corporations, as well as thousands of startup companies. Geographically, it encompasses all of the Santa Clara Valley, the southern half of the San Francisco Peninsula, and southern portions of the East Bay. It includes parts or most of Santa Clara County, San Mateo County, and Alameda County.