A cross-section of an experimental compound-semiconductor transistor developed by Jesus del Alamo’s group at MIT’s Microsystems Technology Laboratories. Image: Jesus del Alamo
Broadly speaking, the
two major areas of research at Massachusetts Institute of Technology’s (MIT)
Microsystems Technology Laboratory (MTL) are electronics—transistors in
particular—and microelectromechanical systems, or MEMS—tiny mechanical devices
with moving parts. Both strains of research could have significant implications
for manufacturing in the United
States, but at least for the moment, the
market for transistor innovation is far larger.
MTL’s Judy Hoyt is
proof of the influence that academic research can have on that market. In the
1990s, she helped pioneer the use of “strained silicon”—silicon whose atoms
have been pried apart slightly more than normal—to improve the performance of
microchips. Intel, most of whose chips are produced in the United States,
was the first chipmaker to introduce strained silicon, in 2003. But by now,
Hoyt says, the technology has percolated throughout the industry.
Hoyt, a professor of
electrical engineering at MIT, argues that U.S. chipmakers have a commercial
incentive to keep manufacturing at home. Innovative ideas that afford a
competitive advantage often emerge from the very process of building and
operating a fabrication facility, Hoyt says. “In the course of doing the
manufacturing, additional know-how gets generated,” she says. “If you do design
here and do the actual fabrication and manufacturing elsewhere, it’ll work for
a while, but in the long run, you can lose early access to the most advanced
technologies.” Indeed, Intel—whose domination of the microprocessor market
depends in large part on always being a few steps ahead of its competition
technologically—recently began building a massive new production facility in Arizona.
Hoyt continues to
research chip technologies that are an even more radical departure from the
norm than strained silicon was in the 1990s—and will thus require “additional
know-how” to produce, she says. One approach is building transistors from
strained nanowires of silicon or germanium. In particular, Hoyt says, her group
is investigating techniques for fabricating the nanowires that would make mass
production viable. She’s also collaborating with several other MTL researchers
on what she describes as “new concepts about how a digital switch might
operate”—concepts that, for example, involve quantum tunneling, a
counterintuitive physical phenomenon in which a subatomic particle seems to
magically pass through a physical barrier.
MTL’s Jesus del Alamo, on the other hand, researches ways to keep the existing
transistor design going as long as possible. That design hasn’t changed much in
the last 50 years, but chipmakers are finally bumping up against the
fundamental physical limits of silicon. Del Alamo studies compound
semiconductors—so called because they combine multiple elements, such as
gallium, indium and arsenic—whose electrical properties offer advantages over
silicon. If chipmakers can successfully introduce these exotic materials into
their fabrication processes, they might be able to continue to improve chip
performance without abandoning existing chip designs.
That’s a big “if,”
however, as the chemicals and conditions required to create
compound-semiconductor circuitry are incompatible with the processes currently
used to produce silicon chips. The first chipmaker to reconcile the two could
enjoy a pronounced competitive advantage.
“It was really Intel’s
leadership about six, seven years ago that focused attention on the
difficulties of traditional silicon scaling,” says del Alamo, the Donner
Professor of Electrical Engineering at MIT. “For years, it was only Intel,
ourselves and slowly, a small trickle of other universities that started
looking at this. To the extent that U.S. industry in the form of,
essentially, Intel is three, four years ahead of the rest of the world, then we
might have an edge in this technology that might have implications for
semiconductors are used in high-speed electronics, such as the devices that
process data in fiber-optic networks. But while compound-semiconductor transistors
offer substantial performance gains over silicon transistors, they’re also
substantially larger. For years, del Alamo’s group has been working to
establish whether compound semiconductors’ advantages will persist even at the
small scales required for digital-logic applications. Recently, del Alamo says,
using MTL’s own fabrication facilities, his group has produced a
compound-semiconductor chip with transistors whose critical dimensions are
roughly the same as those of transistors found in commercial microprocessors.
The transistors’ performance wasn’t up to the standard that commercial
applications would demand, but, del Alamo says, “we think we know why, and we
believe we can address the problems. So the technology looks very promising.”
Sharing an office suite with del Alamo is Tomás Palacios, an MTL researcher
best known for his work with graphene, a material that consists of a single
layer of carbon atoms and has remarkable electronic and mechanical properties.
But Palacios’ group is
also researching another compound semiconductor, gallium nitride, whose
influence on manufacturing could extend well beyond the semiconductor industry.
Gallium nitride is commonly used in light-emitting diodes, but Palacios is
investigating its application in power electronics—the devices in both the
electrical grid and household electronics that change the voltage of electrical
power or switch back and forth between alternating and direct current.
“Intrinsically, in the
U.S., we are going to have higher wages than in many other places in the world,
so if we want to compete with those places, we need to be more efficient,” says
Palacios, the Emanuel E. Landsman (1958) Career Development Associate Professor
of Electronics at MIT. Approximately 30% of the electricity produced in the United States,
Palacios explains, goes to manufacturing.
“Most of that
electricity is used in powering motors and engines of different kinds,” he
says. “Those motors are actually very inefficient. They are being controlled by
old power electronics.” Gallium nitride power electronics, Palacios says, could
cut the electricity consumed by U.S.
manufacturing by more than 30%. “For a given voltage, gallium nitride has three
orders of magnitude less resistance than conventional silicon-based
electronics, so it’s much more efficient,” Palacios says.
The problem is that
gallium nitride is such a good conductor that transistors made from it are hard
to shut off. Palacios’ group has developed—and filed a couple of patents on—a
new transistor design that should address that problem. They continue to refine
the design, in the hopes of making gallium nitride power electronics both more
efficient and easier to mass produce.