Heat is a sad fact of life for current generation
electronics. Any Android, iPhone, or BlackBerry user can tell you that
smartphones tend to get pretty hot at times. And by today's standards a
balmy 85 degrees Celsius, while hot enough to cook an egg, is a pretty
"good" operating temperature for
a high-powered PC graphics processing unit.
But that could all soon change, according to the
results of a new study by researchers at the University of Illinois.
Examining graphene transistors, a team led by mechanical science and
engineering professor William King [profile]
and electrical and computer engineering professor Eric Pop [profile] made
a remarkable discovery -- graphene appears to self-cool.
I. What is Graphene?
Graphene is somewhat like a miniature
"fence" of carbon. The material consists of a single-atom thick
layer composed of hexagonal units. At each point of the hexagon sits a
carbon atom that is bonded to its three close neighbors.
The material behaves like a semiconductor, despite
being made of organic atoms. It offers remarkable performance at an incredibly
small scale, thus the electronics industry views it as a potential material
to power electronic devices of the future.
A variety of methods exist for producing graphene.
The earliest method was an exfoliation technique that involved stripping
individual graphene layers off a layer of graphite (the material found in
pencil lead) -- this technique (as of 2008) cost as much as $100M USD to
produce a single cubic centimeter of material. However, rapid advances in
production have allowed manufacturers to begin scaling up production to the
point where tons of exfoliated graphene can now be produced.
techniques promise to drop the price even further. One
method, epitaxial growth on silicon cost $100 per cubic centimeter in 2009.
Its limitation is that, obviously, it requires silicon (eliminating some
desirable properties like flexibility). South Korean researchers have
tested another promising method, nickel metal transfer.
Graphene is fascinating from a physics
perspective. In 2005 physicists at the University of Manchester and the
Philip Kim group from Columbia University demonstrated that quasiparticles
inside graphene were massless Dirac fermions. These unusual particles help
give rise to the material's unique characteristics.
II. Graphene as a Self-Cooling Device
Despite the extreme interest in the material,
deal of mystery still surrounds Graphene. Because it is so
extremely thin, it is difficult to test and measure
accurately certain properties of the material.
Overcoming technical challenges, the University of
Illinois team used an atomic force microscope tip as a temperature probe to
make the first nanometer-scale temperature measurements of a working graphene
What they found was that the resistive heating
("waste heat") effect in graphene was weaker than its thermo-electric
cooling effect at times. This is certainly not the case in silicon or
other semiconductors where resistive heating far surpasses cooling effects.
What this means is that graphene circuits may not
get hot like traditional silicon-based ones. This could open the door to
dense 3D chips and more.
Further, as the heat is converted back into
electricity by the device, graphene transistors may have a two-fold power
efficiency gain, both in ditching energetically expensive fans and by recycling
heat losses into usable electricity.
Professor King describes, "In silicon and
most materials, the electronic heating is much larger than the self-cooling.
However, we found that in these graphene transistors, there are regions where
the thermoelectric cooling can be larger than the resistive heating, which
allows these devices to cool themselves. This self-cooling has not previously
been seen for graphene devices."
Professor Pop adds, "Graphene electronics are
still in their infancy; however, our measurements and simulations project that
thermoelectric effects will become enhanced as graphene transistor technology
and contacts improve."
A paper has been published [full
text] in nanotechnology's most prestigious peer-reviewed journal, Nature
Nanoscience. University of Illinois graduate student Kyle
undergraduate Feifei Lian and postdoctoral researcher Myung-Ho
Bae [profile] are listed as co-authors on the paper.
III. What's Next?
The study should provide even more motivation for
semiconductor manufacturing companies like Intel, GlobalFoundries, and TMSC to
lay down the process work necessary to mass-produce circuits based on graphene
transistors, capacitors, etc.
As for the University of Illinois team, they plan
to next use their new measurement technique to analyze carbon nanotubes and other
novel structures that are of interest to future electronics applications.
Their work is funded via a grant from the Air
Force Office of Scientific Research and the Office of Naval Research.