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Nanoscience and Nanotechnology
Nanotech reality check: Engineers take the temperature of new transistor designs
Nanoscience and nanotechnology excite researchers because to delve into the scale
of molecules and atoms is to explore a strange new world, where materials and
structures take on unusual and potentially useful properties. Different as it
is, this world is nevertheless real. It forces trade-offs and demands compromises.
Mechanical engineering Associate Professor Kenneth Goodson and electrical engineering
Research Associate Eric Pop, can tell you that while the menu has changed, there
are still no free lunches.
Many engineers, for example, are cleverly exploiting nanotechnology to make smaller
and smaller transistors, the basic components of all computer chips. But Goodson
and Pop study how transistors generate heat. They are finding that nanotechnology
often brings new problems of heat dissipation that must be resolved before these
new-fangled transistors can work.
“What I’ve learned as an electrical engineer,” Pop says, “is
that all the tricks electrical engineers pull to carefully design the electrostatics
of these devices, to make them electrically better, often end up hurting their
thermal properties.” Goodson and his former students Pop, and Sanjiv Sinha (PhD 2005 ME) detail
the heat problems raised by nanotechnology in a paper slated for publication in
an upcoming issue of the Proceedings of the IEEE.
Certainly Goodson and his students are not trying to discourage or gainsay their
fellow engineers. By studying problems associated with heat, they can help electrical
engineers account for and avoid them. “The brightest
minds in EE are inventing new generations of nanotransistor technologies, however many of these will be possible only through careful thermal design,” Goodson says.
A matter of scale
The small scale that defines nanotechnology gives rise to a fundamental problem
with heat. Transistors are becoming so small that they are leaving little room
for the physical processes that dissipate heat to operate. Here’s what’s
going on: Temperature quantifies the intensity of atomic motion in a material.
When electrons move through a silicon transistor, they increase the amplitude of vibrations
of the atoms within the crystal structure (like a passing truck can shake nearby
windows), increasing its temperature. At first these vibrations called phonons
oscillate very quickly but move very slowly, making it hard for the hotspot within
the transistor to cool off. Eventually they interact with their environment, transferring
their energy to faster moving “acoustic” phonons (so called because
they behave much like sound waves). The acoustic phonons leave the transistor
quickly and things cool off. The problem with the space crunch is that it impairs
the energy transfer from slow to fast vibrations and makes it harder for the acoustic
phonons to leave. This leads to much higher temperatures in the transistor.
As if decreasing volumes weren’t enough of a problem, another one is that
as dimensions shrink, the ratio of surface area to volume goes up (think of shrinking
the radius of a soda can down to the radius of a spaghetti strand). An increased
role for surface area is a problem because phonons—which carry heat away—typically
have a lot of trouble crossing boundaries from one material into another. Instead
of passing through, they might instead bounce off, as sunlight is sometimes reflected
by the surface of a pond. In transistors, these boundaries exist along material
surfaces, so the more important surface area is, the more potential there is for
heat to have trouble escaping.
Materials and connections
Material surfaces are not only more important, but also more prevalent in the
newest generations of transistors. The reason for the added complexity is that
the tiny scale of nanotechnology presents electrical problems that engineers must
stave off with novel transistor structures and exotic materials. Some of these
schemes, while successful electrically, create new heat dissipation challenges.
One example is an increasingly prominent transistor technology called silicon-on-insulator
(SOI). An SOI transistor has a layer of insulating material underneath that lowers
its electrical capacitance. The result is that the transistor can switch on and
off more quickly. But it turns out that this electrical insulation is about 100
times worse than silicon at conducting heat, Goodson says.
Despite their thermal drawbacks, SOI transistors have begun finding their way
into chips. A more exotic pair of incredibly thin computer chip building blocks
— silicon nanowires and carbon nanotubes — are still under development
in research labs. It is not too early, however, for engineers to consider their
thermal properties, Goodson says. This is because Pop has found that both the
nanowires and nanotubes heat up considerably when carrying significant amounts
of current. In fact, nanotubes exposed to air in the laboratory actually burn,
breaking instantly when a large enough current is passed through. In a computer
chip, nanotubes and nanowires would be surrounded by a layer of insulating material
(like snakes in sweaters), so they wouldn’t burn up. But the insulation
would still trap heat inside. While both nanotubes and nanowires might still prove
extremely useful in future chips, Pop says that electrical engineers should temper
some of the early exuberance about these structures, in light of these heat dissipation
questions.
There is little doubt among engineers that nanoscience and nanotechnology hold
tremendous potential for revolutionizing not only electronics, but also fields
such as environmental science, medicine and energy. At the end of the day, however,
the promise won’t be fulfilled if innovations on paper cannot work in real
products. Goodson and Pop see their work as ensuring that good ideas are fully
feasible. Their investigation of heat in nanoscale transistors is in that spirit.
“We’ve been the first people to start looking at these things —
to kind of look ahead and ask whether these things are really worth it electrically
and thermally,” Pop says. “Because if they’re not, then why
bother with the manufacturing?”
January 2006
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Last Modified: July 21 2006 07:27:40 PM |
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