Record-setting p-type transistor

Print edition : February 22, 2013

In this micrograph of an experimental transistor, the blue highlighting indicates areas of 'strain', where germanium atoms have been forced closer together than they find comfortable. One of the reasons for the transistor’s record-setting performance is that the strain has been relaxed in the lateral direction. Photo: fasfa

Almost all electronic chips use two types of transistors: p-type, which have positive charge carriers, and n-type, which have negative charge carriers. In an n-type transistor, the charge carriers are electrons, and their flow produces an ordinary electrical current. In a p-type transistor, on the other hand, the charge carriers are positively charged “holes”. Improving the performance of the chip as a whole requires improvements in both types.

Researchers from the Massachusetts Institute of Technology’s (MIT) Microsystems Technology Laboratories (MTL) have developed a p-type transistor with the highest yet measured “carrier mobility”. “Carrier mobility” measures how quickly charge carriers, whether positive or negative, move in the presence of an electric field. The new device is twice as fast as previous experimental p-type transistors and almost four times as fast as the best commercial p-type transistors.

It derives its speed from its use of germanium instead of silicon. It also employs what is called a trigate design, which could solve some of the problems that plague microchips at extremely small sizes (and which Intel has already introduced in its most advanced chip lines). The new device thus could help sustain the rapid increases in computing power in accordance with Moore’s Law.

A transistor is basically a switch: In one position, it allows charged particles to flow through it; in the other position, it does not. Increased mobility can translate into either faster transistor switching speeds, at a fixed voltage, or lower voltage for the same switching speed. Each logic element in an integrated circuit usually consists of complementary n-type and p-type transistors and it is the clever arrangement of these that reduced the chip’s power consumption. In general, it is easier to improve carrier mobility in n-type transistors; the MTL researchers’ new device demonstrates that p-type transistors should be able to keep up.

The scientists led by Judy Hoyt achieved their record-setting hole mobility by “straining” the germanium in their transistor—forcing its atoms closer together than they would ordinarily be. To do that, they grew the germanium on top of several different layers of silicon and a silicon-germanium composite. The germanium atoms naturally try to line up with the atoms of the layers beneath them, which compresses them together.

Another crucial aspect of the new transistor is its trigate design. By demonstrating that they can achieve high hole mobility in trigate transistors, Judy Hoyt and her team have also shown that their approach will remain useful in the chips of the future.

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