MIT researchers and colleagues at the University of Augsburg, in Germany, investigated the curious electrical properties of a material produced by stacking layers of lanthanum aluminate on layers of strontium titanate. Image: MIT |
In
the 1980s and ’90s, competition in the computer industry was all about “clock
speed”—how many megahertz, and ultimately gigahertz, a chip could boast. But
clock speeds stalled out almost 10 years ago: Chips that run faster also run
hotter, and with existing technology, there seems to be no way to increase
clock speed without causing chips to overheat.
In
Science, MIT researchers and their
colleagues at the Univ. of Augsburg in Germany report the discovery of a
new physical phenomenon that could yield transistors with greatly enhanced
capacitance. And that, in turn, could lead to the revival of clock speed as the
measure of a computer’s power.
In
today’s computer chips, transistors are made from semiconductors, such as
silicon. Each transistor includes an electrode called the gate; applying a
voltage to the gate causes electrons to accumulate underneath it. The electrons
constitute a channel through which an electrical current can pass, turning the
semiconductor into a conductor.
Capacitance
measures how much charge accumulates below the gate for a given voltage. The
power that a chip consumes, and the heat it gives off, are roughly proportional
to the square of the gate’s operating voltage. So lowering the voltage could
drastically reduce the heat, creating new room to crank up the clock.
Shoomp!
MIT Professor of Physics Raymond Ashoori and Lu Li, a postdoc and Pappalardo
Fellow in his lab—together with Christoph Richter, Stefan Paetel, Thilo Kopp,
and Jochen Mannhart of the Univ.
of Augsburg—investigated
the unusual physical system that results when lanthanum aluminate is grown on
top of strontium titanate. Lanthanum aluminate consists of alternating layers
of lanthanum oxide and aluminum oxide. The lanthanum-based layers have a slight
positive charge; the aluminum-based layers, a slight negative charge. The
result is a series of electric fields that all add up in the same direction,
creating an electric potential between the top and bottom of the material.
Ordinarily,
both lanthanum aluminate and strontium titanate are excellent insulators,
meaning that they don’t conduct electrical current. But physicists had
speculated that if the lanthanum aluminate gets thick enough, its electrical
potential would increase to the point that some electrons would have to move
from the top of the material to the bottom, to prevent what’s called a “polarization catastrophe.” The result is a conductive channel at the juncture
with the strontium titanate—much like the one that forms when a transistor is
switched on. So Ashoori and his collaborators decided to measure the
capacitance between that channel and a gate electrode on top of the lanthanum
aluminate.
The researchers’ experimental setup consisted of a sample of the lanthanum aluminate-strontium titanate composite, which looks like a slab of thick glass, with thin electrodes deposited on top of it. Photo: MIT |
They
were amazed by what they found: Although their results were somewhat limited by
their experimental apparatus, it may be that an infinitesimal change in voltage
will cause a large amount of charge to enter the channel between the two
materials. “The channel may suck in charge—shoomp! Like a vacuum,” Ashoori
says. “And it operates at room temperature, which is the thing that really
stunned us.”
Indeed,
the material’s capacitance is so high that the researchers don’t believe it can
be explained by existing physics. “We’ve seen the same kind of thing in
semiconductors,” Ashoori says, “but that was a very pure sample, and the effect
was very small. This is a super-dirty sample and a super-big effect.” It’s
still not clear, Ashoori says, just why the effect is so big: “It could be a
new quantum-mechanical effect or some unknown physics of the material.”
“For
capacitance, there is a formula that was assumed to be correct and was used in
the computer industry and is in all the textbooks,” says Jean-Marc Triscone, a
professor of physics at the Univ.
of Geneva whose group has
published several papers on the juncture between lanthanum aluminate and
strontium titanate. “What the MIT team and Mannhart showed is that to describe
their system, this formula has to be modified.”
There
is one drawback to the system that the researchers investigated: While a lot of
charge will move into the channel between materials with a slight change in
voltage, it moves slowly—much too slowly for the type of high-frequency
switching that takes place in computer chips. That could be because the samples
of the material are, as Ashoori says, “super dirty”; purer samples might
exhibit less electrical resistance. But it’s also possible that, if researchers
can understand the physical phenomena underlying the material’s remarkable
capacitance, they may be able to reproduce them in more practical materials.
Triscone
cautions that wholesale changes to the way computer chips are manufactured will
inevitably face resistance. “So much money has been injected into the
semiconductor industry for decades that to do something new, you need a really
disruptive technology,” he says.
“It’s
not going to revolutionize electronics tomorrow,” Ashoori agrees. “But this
mechanism exists, and once we know it exists, if we can understand what it is,
we can try to engineer it.”
