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Smartphones and laptops becoming too hot to handle
Posted on May 1st, 2017 by Dr. Sina Ebnesajjad in Chemical R&D
Almost everyone who has ever used a smart phone or laptop computer has experienced those microelectronic devices becoming too hot. Even solid-state devices heat up. The common belief is that the defects in semiconductor chips cause heat generation. The defects arise from the process of “doping,” in which additional elements like phosphorous and boron are added to silicon to increase its electron concentration. It turns out things are not quite so simple.
Researches at Massachusetts Institute of Technology (MIT) have discovered the reasons that smartphones and other electronic devices get hot. The results of their studies were published on October 12, 2016, in the journal Nature Communications (Source: Liao, B. et al. Photo-excited charge carriers suppress sub-terahertz phonon mode in silicon at room temperature. Nat. Commun. 7, 13174, 2016).
First we need to learn about phonons (not a typo and not photons). According to the Encyclopedia Britannica “phonon is a unit of vibrational energy that arises from oscillating atoms within a crystal, for example sodium chloride crystal. They can be viewed as Bosonic particles, which propagate through the crystal and interact with electrons. “Phonons and electrons are the two main types of elementary particles or excitations in solids. Whereas electrons are responsible for the electrical properties of materials, phonons determine such things as the speed of sound within a material and how much heat it takes to change its temperature.”
Jennifer Chu from MIT’s News Office has published an article that describes Liao et al’s work (Source: J. Chu, http://news.mit.edu/2016/electron-phonon-interactions-affect-heat-dissipation-computer-chips-1012, October 12, 2016). She writes: researchers have found that previously underestimated interactions between electrons and heat-carrying particles called phonons generate heat. And these interactions can play a significant role in preventing heat dissipation in microelectronic devices. Liao and colleagues conducted experiments by using precisely timed laser pulses to measure the interactions between electrons and phonons in a very thin silicon wafer. As the concentration of electrons in the silicon increased more of those electrons scattered phonons and prevented them from carrying heat away.
The lead author Dr. Bolin Liao, said, “If phonons are scattered by electrons, they’re not as good as we thought they were in carrying heat out. This will create a problem that we have to solve as chips become smaller.” On the other hand, Liao said, “this same effect may benefit thermoelectric generators, which convert heat directly into electrical energy. In such devices, scattering phonons, and thereby reducing heat leakage, would significantly improve their performance. Now we know this effect can be significant when the concentration of electrons is high. We now have to think of how to engineer the electron-phonon interaction in more sophisticated ways to benefit both thermoelectric and microelectronic devices.”
In transistors made from semiconductor materials such as silicon, and electrical cables made from metals, electrons are the main agents responsible for conducting electricity through a material. A main reason why such materials have a finite electrical resistance is the existence of roadblocks to electrons’ flow – namely, interactions with the heat-carrying phonons, which can collide with electrons, throwing them off their electricity-conducting paths.
The MIT group had previously calculated that in silicon, the most common semiconductor material, when the concentration of electrons reaches above 1019 per cm3, the interactions between electrons and phonons would strongly scatter phonons. And, they would reduce the material’s ability to dissipate heat by as much as 50 percent when the concentration reaches 1021 per cm3. That’s a really significant effect, but people were skeptical, Liao says. In the past experiments on materials with high electron concentrations scientists assumed the reduction of heat dissipation was due not to electron-phonon interaction but to defects in materials.
“So 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,” Liao says. The team developed a technique called three-pulse photo-acoustic spectroscopy to precisely increase the number of electrons in a thin wafer of silicon by optical methods, and measure any effect on the material’s phonons. The technique expands on a conventional two-pulse photo-acoustic spectroscopy technique, in which scientists shine two precisely tuned and timed lasers on a material.
The first laser generates a phonon pulse in the material, while the second measures the activity of the phonon pulse as it scatters, or decays. Liao added a third laser, which when shone on silicon precisely increased the material’s concentration of electrons, without creating defects. When he measured the phonon pulse after introducing the third laser, he found that it decayed much faster, indicating that the increased concentration of electrons acted to scatter phonons and dampen their activity.
“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,” Liao says. “This is among the first experiments to directly probe electron-phonon interactions’ effect on phonons.” Interestingly, the researchers first started seeing this effect in silicon that was loaded with 1019 electrons per cm3– comparable or even lower in concentration than 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,” Liao says. “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.”
These recent discoveries are quite promising for the resolution of some of the barriers to the future miniaturization of semiconductor chips. The importance of basic research in semiconductor chips is well justified considering the forecast growth rate of 31% per year for the next five years (Source: Global Semiconductor Chip Packaging Market 2017-2021: Latest Market Dynamics Influencing the Industry, by Research and Markets, www.prnewswire.com).
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Dr. Sina Ebnesajjad
President at FluoroConsultants Group, LLC