PHOTO: Intel

IN THE FAB: By the end of 2007, two fabs at Intel will be churning out the first commercial microprocessors made up of transistors fundamentally redesigned using new materials.

The problem, ultimately, is one of power. At five atoms, that sliver of SiO₂ insulation was so thin that it had begun to lose its insulating properties. Starting with the generation of chips fabricated in 2001, electrons had begun to trickle through it. In the processors made just two years later, that trickle became some 100 times as intense.

All that current was a drain on power and a source of unwanted heat. Laptops were heating up too much and draining their batteries too quickly. Servers were driving up their owners' electric bills and taxing their air conditioners. Even before we ran out of atoms, designers had devised some tricks to throttle back on the power without losing speed. But without a way to stanch the unwanted flow of electrons through that sliver of insulation, the battle to make ever more powerful processors would soon be lost.

To understand why, you need a quick lesson (or refresher) in semiconductor basics. The type of transistor that is chained together by the hundreds of millions to make up today's microprocessors, memory, and other chips is called a metal-oxide-semiconductor field effect transistor, or MOSFET. Basically, it is a switch. A voltage on one terminal, known as the gate, turns on or off a flow of current between the two other terminals, the source and the drain [see illustration, “The Transistor”].

MOSFETs come in two varieties: N (for n-type) MOS and P (for p-type) MOS. The difference is in the chemical makeup of the source, drain, and gate. Integrated circuits contain both NMOS and PMOS transistors. The transistors are formed on single-crystal silicon wafers; the source and drain are built by doping the silicon with impurities such as arsenic, phosphorus, or boron. Doping with boron adds positive charge ­carriers, called holes, to the silicon crystal, making it p-type, while doping with arsenic or phosphorus adds electrons, making it n-type.

Taking an NMOS transistor as an example, the shallow source and drain regions are made of highly doped n-type silicon. Between them lies a lightly doped p-type region, called the transistor channel—where current flows. On top of the channel lies that thin layer of SiO₂ insulation, usually just called the gate oxide, which is the cause of the chip industry's most recent technological headaches.

Overlying that oxide layer is the gate electrode, which is made of partially ordered, or polycrystalline, silicon. In the case of an NMOS device it is also n-type. (The silicon gates replaced ­aluminum gates—the metal in “metal-oxide semi­conductor”—in work described in the 1969 IEEE Spectrum article. But the “MOS” acronym has nevertheless lived on.)

The NMOS transistor works like this: a positive voltage on the gate sets up an electric field across the oxide layer. The electric field repels the holes and attracts electrons to form an ­electron-­conducting channel between the source and the drain.

A PMOS transistor is just the complement of NMOS. The source and drain are p-type; the channel, n-type; and the gate, p-type. It works in the opposite manner as well: a positive voltage on the gate (as measured between the gate and source) cuts off the flow of current.

In logic devices, PMOS and NMOS transistors are arranged so that their actions complement each other, hence the term CMOS for complementary metal-oxide semiconductor. The arrangement of CMOS circuits is such that they are designed to draw power only when the transistors are switching on or off. That's the idea, anyway.

Although the basic features and materials of the MOS transistor have stayed pretty much the same since the late 1960s, the dimensions have scaled dramatically. The transistor's minimum layout dimensions were about 10 micrometers 40 years ago, and are less than 50 nm now, smaller by a factor of more than 200. Suppose a 1960s transistor was as big as a three-bedroom house and that it shrank by the same factor. You could hold the house in the palm of your hand today.