CALIFORNIA's Silicon Valley, the 50-mile stretch between San Francisco and San Jose, is perhaps the most productive and innovative land mass in the world. The Economist has identified 99 listed technology companies with market values of over $1 billion that call the Valley home. Together, these 99 companies are worth some $2.8 trillion (an increase of 75% over the past 30 months), and account for around 6% of all corporate America's corporate profits.

Silicon rally: How the US technology sector has changed since 1980

The Valley has experienced dizzying heights before of course. During the dot-com bubble, the market capitalization of just 64 companies peaked at around $2.4 trillion in September 2000. Once the dot-com bubble burst that same group of firms was worth just $630m 14 months later. The Valley is not immune to failure: fallen giants such as Sun Microsystems, which was worth nearly $200 billion at its peak, plus another 17 big tech firms have since been acquired or gone bust. As our briefing discusses, today may well be different: whereas in 2000 the trailing 12-month price-earnings ratio of Silicon Valley firms was 120, this time around it is just 20 thanks to stellar earnings from Apple and Google. And among those companies with extrordinary private-market valuations and measly revenues such as Uber, valued at $41 billion, and Airbnb, valued at $25.5 billion, the risk is concentrated within a small group of cash-rich venture capitalists.