When these materials are stacked, their electronic properties interact to form quantum wells--places where charged particles such as electrons can be confined--that act as transistors, says Michael Mayberry, director of components research and vice president of Intel's technology and manufacturing group. To avoid the strain and cracking, the researchers added buffer layers of the two materials. The trick is to make sure that the buffer layers contain concentrations of atoms that are slightly more compatible with silicon. But as more layers are added, the atomic spacing perfectly matches that of the transistor layers. Mayberry says that the buffer is slightly more than one micrometer thick, and it keeps any defects from affecting the transistors. Intel's approach is unique in that the researchers have grown the buffer layers out of the same material that they use for the transistors. In addition, he says, Intel has shown that only a thin buffer layer is necessary to get good quality. If you can bring the layer structure on silicon, then the substrates feel and look like silicon, and all the tools that have been developed for silicon manufacturing can be reused in this new technology.
Compound semiconductors are attractive to engineers because electrons move through them easier than they move through silicon. This means that the compound semiconductors can work as fast as, or faster than, a silicon-based transistor, but without needing as large a voltage. And as devices shrink, it's crucial that they require low voltages: otherwise, they overheat and leak electricity--problems that are beginning to plague silicon. However, compound semiconductors aren't easy to grow directly on silicon. The materials are often incompatible with silicon--the atoms are spaced so that they don't layer well. When layered directly on top of one another, the result is a cracked crystal and defective transistors
Some suspect that carbon nanotubes or another carbon material called graphene could be the answer. (See "Carbon Nanotube Computers" and "New Graphene Transistors Show Promise.") But others are putting money and research into compound semiconductors, a class of semiconductor that is made from a combination of elements from the third and fifth columns of the periodic table.