The southern tip is Bernal Rd. which is the southern most part of San Jose. There are several start-ups in that area as well as a few incubators. South of Bernal is farms and small towns. The northern most part is an area close to San Francisco Airport (SFO) called Brisbane. It is just north of SFO and is the last town before one gets to the “skyscrapers” (SF). There are a lot of biotech companies in that area but also walmart.com and other tech companies.
And the unofficial capital of Silicon Valley is really Palo Alto, where Stanford is located. San Jose tends to claim themselves as the capital of Silicon Valley, but that’s not really the case. I was at a panel discussion at Stanford a while back where a VC mentioned that 75% of the ROI to their LPs (folks that give VCs money) come from a 25 mile radius around Stanford University. The largest concentration of start-ups are in Palo Alto, Mt. View, Sunnyvale, etc., all of which are within a 25 mile radius of Stanford. I myself went to San Jose State University so I have no bias here.
San Francisco tends to be it’s own location within the entrepreneurial community. There are tons of start-ups there and many of the “Sand Hill VCs” invest in companies there, but there seems to be a separation between Silicon Valley and San Francisco. Within SF, the section of the city that is getting the most traction is SoMa (which stands for South of Market). I think it has to do with it’s proximity to the highways and also it tends to be sunnier there than other parts of the city. The demographic of the start-ups in SF tend to be internet focused and under 35 yrs. old with most of the employees in their mid 20’s. SF is certainly the hottest spot within the Bay Area to both live and work so it doesn’t surprise me that is where most of the action happens.