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Silicon Magic

What sort of huge breakthroughs will allow the semiconductor industry to make these leaps? Actually, none. The experts all pretty much agree: the next three generations of microprocessors, at least, will simply extend the familiar properties of silicon. It’s not that there aren’t plenty of dramatic innovations at the ready, including more-exotic semiconducting materials like germanium and indium phosphide and techniques for stacking layers of transistors into three-dimensional chips. It’s just that the industry can do it with silicon, so it will-because it’s cheaper. “Each time someone develops workable new materials or exotic device structures, silicon researchers keep catching up,” says Berkeley’s Bokor. “There’s a very strong interest in industry in making the least-radical change possible.”

Chip makers will still have to make a few key modifications to today’s methods, starting with the photolithographic process used to chemically etch circuit patterns onto chips. In photolithography machines, lenses focus ultraviolet light through a stencil-like “mask” onto silicon wafers coated with a photosensitive material. The photolithography machines used to produce today’s chips aren’t precise enough to project 65-nanometer features. But new, higher-resolution techniques are being worked out-for example, ultrafine gratings that break up and recombine the light beams so that they reinforce each other at the tiny spots where light is needed and cancel each other out everywhere else. To get to 45 nanometers and below, manufacturers may switch to machines now under development that use either extreme ultraviolet light, which has a shorter wavelength and can therefore be used to etch smaller features, or beams of electrons, which can be finely controlled to etch patterns onto silicon directly, without a mask.

New forms of silicon will also lend a hand. For instance, chips will get a speed boost from silicon that has been deposited over a layer of silicon germanium, whose atoms cause the slightly misaligned atoms of pure silicon to stretch out a bit. This “strained” silicon speeds the journey of electrons through transistors. An additional boost will come from adding a layer of insulating material underneath the semiconducting layers, further enhancing their electrical properties. Microprocessor maker AMD has reported speed jumps of up to 25 and 30 percent, respectively, for the two techniques. IBM and Intel have already begun making chips with strained silicon, and IBM says products combining strained silicon with “silicon-on-insulator” designs could be on the market within several years.

Transistors are also getting a makeover. As the features of transistors shrink, electrons are more likely to stray off their intended course and leak across barriers, even when the transistor is supposed to be off. This leakage wastes power and interferes with transistors’ ability to switch between their 0 and 1 states reliably-and it’s going to get worse. To plug the leak, the industry is turning to a slightly different transistor design, one pioneered by Bokor and his Berkeley colleagues Tsu-Jae King and Chenming Hu in the late 1990s.

In a conventional transistor, the main point of leakage is a channel of material squeezed between the source and the drain, two larger blocks of silicon that define electrons’ principal entry and exit points. A structure called a gate lies atop the channel, like a pontoon bridge across a canal. When a positive voltage is applied to the gate, negatively charged electrons are drawn toward it, opening up a pathway for more electrons to flow through the channel from the source to the drain. The problem, as transistors get smaller, is that electrons can sneak through the thin channel even when the gate isn’t charged. The Berkeley group’s “fin” design ameliorates leakage by raising the whole transistor above the silicon’s surface and reshaping the channel as a narrow, vertical fin that stretches from source to drain like the crossbar of an H. The fin sits on an insulating material, which reduces electron leakage, and the gate drapes over the fin, touching it on both vertical surfaces, which doubles the effect of the positive voltage. Intel is already turning to a variation of this design, which should start showing up in microprocessors by 2007.

As a bonus, higher-performing materials and transistor designs make it possible to run chips at lower voltages. This reduces power consumption and, consequently, the risk of overheating, which rises as chips get denser.

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