At the heart of any microprocessor are transistorsplenty of them. It’s the sheer number of transistors that make today’s CPUs more powerful then everand give their designers more headaches.
Chemistry gave us transistors nearly 60 years ago and continues to improve them, mainly by letting engineers make them ever smaller. But as transistors, and the chips containing them, have continued to shrink, new problems have arisen, and once again chemistry will be needed to solve them.
In addition to winning its inventorsJohn Bardeen, Walter Brattain, and William Shockley of Bell Labsa Nobel Prize in 1956, the transistor performs two tasks very well. Amplification is one. But often the device simply acts as the electronic equivalent of an on-off switch. When on, it completes a circuit and lets current flow; when off, it breaks the electrical path.
A lone transistor isn’t good for much, but there’s strength in numbers. By grouping vast quantities together and turning them on and off as appropriate (to represent ones and zeros) a microprocessor can manipulate enormous amounts of dataat extraordinary speeds.
Transistors are possible because the semiconductors they’re built fromelements like silicon and germaniumhave one strange and very useful property: They carry electricity better than insulators, such as glass, but not as well as conductorsaluminum for example.
Scientists can push a semiconductor’s conductivity one way or the other by dopingadding small amounts of impurities. (Arsenic and boron are two of the commonly used elements.) “When you dope silicon you make it act differently,” says Shahidi. “It can act like an insulator or as a metal.” Doping specific areas of a semiconductor in different ways is what creates a transistor enables its switching action.
Silicon, the semiconductor used to make transistors today, is one of the most abundant elements on earth. It makes up 25.7 percent of the crust by weight and is the principle ingredient in sand. Yet that first Bell Labs transistor, developed in 1947, was actually made from germanium. Still, there’s a reason California has no prime real estate known as Germanium Valley.
The element proved to be less than ideal for manufacturing transistors, partly because of its limited availability and high cost. There was a larger problem, though: The insulator form, germanium oxide, was soluble in water and would dissolve in the etching process used to fabricate multiple transistors on a single chip. “Germanium was hard to work with, so there was a move to silicon,” says Ken David, director of the components research group at Intel.
Silicon, however, is creating its own brand of trouble. The first microprocessor, the Intel 4004, introduced in 1971, contained 2,300 transistors. Today’s processors have hundreds of millions. We want that to increase, though, and for good reason: The more transistors you pack on a chip, the faster and more powerful the microprocessor.
Moore’s Lawnamed after its creator and Intel cofounder, Gordon Moorepredicts that the number of transistors on a chip will roughly double every 2 years. His prediction remains remarkably accuratebut transistors are getting so small that their size is creating significant power and heat problems. So far, advances in chemistry have been able to mitigate these issues and keep his law going strong. Continued…