Study Finds Increasing Risk of Electronics Overheating

As transistors size decreases, electron concentration increases, and phonons become less effective in carrying out heat. (Image courtesy of MIT.)

A new study conducted by MIT engineers has revealed that interactions between electrons and phonons in computer chips may play a larger role in affecting heat dissipation than previously thought. The study suggests that as electron concentrations rise due to the ever-decreasing size of transistors, electronic devices may face an increasingly high risk of overheating.

Three-Pulse Photoacoustic Spectroscopy

Previous experiments have shown that semiconductors with high electron concentrations have a reduced capacity to dissipate heat. However, it was assumed that this reduction was due to material defects arising from doping.

The new study began with calculations that showed an alternative explanation. The researchers showed that in silicon, when the electron concentration is above 10¹⁹ e^-/cm³, electron-phonon interactions would strongly scatter the phonons and reduce the ability of the material to dissipate heat. This reduction would climb as high as 50 percent when the concentration reaches 10²¹ e^-/cm³.

The next step was to verify these findings, which presented something of an experimental challenge. Lead researcher Bolin Liao explained: “…the challenge to verify our idea was, we had to separate the contributions from electrons and defects by somehow controlling the electron concentration inside the material, without introducing any defects.”

The team solved this problem by expanding on a conventional technique called two-pulse photoacoustic spectroscopy. In this technique, you shine two precisely tuned lasers on a material; one which generates a photon pulse and another which measures the pulse as it scatters.

The team added a third laser to the ensemble, which had the effect of increasing the material’s electron concentration without introducing any material defects. This three-pulse photoacoustic spectroscopy allowed the researchers to compare the phonon scattering at different levels of electron concentrations. They discovered that the phonons decayed much faster after introducing the third laser, meaning that the increased electron concentration acted to dampen the phonon activity.

Experimental results showing that phonon scattering increases with increasing electron concentration. The yellow dashed line shows the phonon scattering rate without the third laser beam. (Image courtesy of Nature Communications.)

“Very happily, we found the experimental result agrees very well with our previous calculation, and we can now say this effect can be truly significant and we proved it in experiments,” said Liao. “This is among the first experiments to directly probe electron-phonon interactions’ effect on phonons.”

Their findings pose a problem because phonons are traditionally responsible for carrying heat away in a material. If phonon scattering increases due to high electron concentrations, it means the phonons will be less effective in dissipating heat. And since the increased phonon scattering starts occurring in silicon with electron concentrations of 10¹⁹ e^-/cm³, this problem may need tackling sooner rather than later, since that electron concentration is comparable to or lower than that found in some current transistors.

“From our study, we show that this is going to be a really serious problem when the scale of circuits becomes smaller,” cautions Liao. “Even now, with transistor size being a few nanometers, I think this effect will start to appear, and we really need to seriously consider this effect and think of how to use or avoid it in real devices.”

You can read the team’s study in Nature Communications. To learn about a different direction of computer chip advancement, read Prototype Photonic Chip Shines a Light on Practical Quantum Computing.