A three-year research study at the Lawrence Berkeley National Lab, or LBNL, culminated in the discovery of a rare material that makes electronic devices operate faster due to its ability to conduct heat more efficiently.
According to study author and campus materials science and engineering professor Junqiao Wu, this discovery came into fruition when his graduate student Penghong Ci was trying to improve ways to conduct heat more efficiently in silicon chips.
“We found a material that can conduct heat much better than what we are currently using in our computer chips,” Wu said. “If the heat does not get out quickly, it will build up, slow down your device and eventually burn it.”
According to the research paper, each computer chip is composed of tens of billions of transistors — the basic device units in chips made of silicon — where electrical currents run and generate heat. However, when silicon is reduced to small sizes, it becomes inefficient in conducting heat transfer.
Wu said silicon consists of three different types of isotopes: silicon-28, silicon-29 and silicon-30. When silicon is purified into a single piece of isotopically pure silicon-28, it conducts heat more efficiently because silicon-29 and silicon-30 hold slightly heavier atoms that slow down heat conduction.
Wu added that the obstruction of heat by silicon-29 and silicon-30 is akin to the obstruction of a water wave by a physical object when you throw a rock in a pool.
While isotopically pure silicon-28 is a rare material that is not sold in the market, it was supplied to Wu’s research team by LBNL staff scientist Joel Ager. Wu added that the lab obtained silicon-28 in the 1990s for a study from former Soviet Union universities, and Ager had access to some of the leftover material.
“They have not used this material probably in like 20 years until a few years ago when we had this idea to measure the thermal conductivity of nanowires,” Wu said.
The research paper notes that heat transport in silicon nanowires is difficult due to rough surfaces that “scatter” the waves of atomic vibration that carry heat.
As a result, Wu said his team decided to make a nanowire from isotopically pure silicon-28 and discovered that heat conduction improved by 150% from that in natural silicon nanowires with the same size and surface roughness.
“We believe with our current discovery that we could potentially find material that can make the computer faster and run at a much higher frequency,” Wu said.
Besides improved heat conductivity, Wu said that isotopically pure silicon-28 can also do all the “other electronic functionality.”
In the next stages, Wu and his team plan to make a transistor out of silicon-28 and compare its functionality to a transistor made out of natural silicon to see if this change in material makes transistors better.
“If this experiment is approved, then we will make a big breakthrough here and we can make computer chips or integrated circuits that can run at a higher power and higher speed without breaking down,” Wu said.