Beyond Silicon

Intel is exploring different materials for computer chips.

Last week, at the semiannual Intel Developer Forum in San Francisco, chip-maker Intel announced a transistor made from a material called indium antimonide (InSb) that had some impressive stats: it was clocked at 1.5 times the speed of silicon-based transistors and used one-tenth the power.

According to Intel's director of technology strategy, Paolo Gargini, who presented the results, a shift from silicon might be crucial for the chip-making industry, so it can build smaller devices over the next couple of decades. As transistors made of silicon keep shrinking, the material's limitations are becoming more apparent. "Silicon is not the best semiconductor," Gargini says.

But of course silicon is both highly prevalent and relatively inexpensive, and its manufacturing process has been honed for 30 years. What makes so-called "compound semiconductors" -– those made out of more than one element, such as indium antimonide -– so attractive is their special electrical and optical properties.

Electrons can pass through an indium antimonide crystal 50 times faster than through a silicon crystal, Gargini says. As a result, not only are electronic operations significantly faster, but less power is needed to push the electrons.

Compound semiconductors also have optical properties that could help speed up communication between transistors on a chip and multiple chips within a device. These materials easily emit and detect light -– a characteristic that has been studied and improved for decades, says David Hodges, electrical engineer at the University of California, Berkeley. Therefore, he says, light emitters and detectors made of compound materials could potentially replace copper wires, which are a major "impediment of speed."

Compound materials also have their disadvantages, though. Currently, hundreds of billions of transistors are manufactured at a time on top of silicon wafers that can be as large as 12 inches in diameter. The crystals of compound materials, such as indium antimonide (InSb), gallium arsenide (GaAs), indium arsenide (InAs), and indium gallium arsenide (InGaAs), however, tend to break apart easily, and so can't be made into such large wafers, says Gargini. This means that compound materials could never completely replace silicon as the wafer base for electrical devices, he says.

Instead, "islands" of InSb transistors must be deposited on the large-diameter silicon substrate. But depositing indium antimonide transistors onto silicon creates an additional challenge. The atoms in a silicon crystal are spaced 0.543 nanometers apart, while the atoms in indium antimonide are 0.648 nanometers apart. Because of this mismatch, when the two materials are placed next to each other, not all of the atoms at the interface bond together, resulting in ineffective devices.