Silicon Valley is a geographical area.
Silicon Valley . The region has expanded to include the southern half of the San Francisco Peninsula in San Mateo County , and southern portions of the East Bay in Alameda County . The word `` silicon '' originally referred to the large number of silicon chip innovators and manufacturers in the region , but the area is now the home to many of the world 's largest high-tech corporations , including the headquarters of 39 businesses in the Fortune 1000 , and thousands of startup companies . As more high-tech companies were established across the Santa Clara Valley , and then north towards the Bay Area 's two other major cities , San Francisco and Oakland , the `` Silicon Valley '' name eventually came to refer to all high-tech businesses in the region .