Modern electronics are becoming ever smaller and more powerful, and heat removal is turning into a serious engineering problem. Researchers at Cornell University led by Zhiting Tian have proposed a calculation method that predicts how an external electric field affects the thermal conductivity of semiconductor materials. The work was published in the journal npj Computational Materials.
The focus is on aluminum nitride, a promising material for next-generation electronics. It is a wide-bandgap semiconductor: it can withstand significantly stronger electric fields than conventional materials. Such properties make it possible to create compact and powerful devices, including high-voltage systems and high-power radio-frequency equipment. However, higher power also means more heat, which can reduce the efficiency, reliability and service life of devices.
Heat in a crystal is carried by atomic vibrations, or phonons. As phonons move, they collide and scatter, and this determines how well a material conducts heat. The researchers hypothesized that an electric field could redistribute electrons in the crystal, changing the interaction forces between atoms and, as a result, the behavior of phonons.
To test this, the team combined two computational approaches: density functional theory (DFT), which describes the electronic structure of the material, and the non-equilibrium Green's function (NEGF) method. This combined DFT-NEGF method had previously been used to study electron transport, but not to calculate phonon-driven thermal conductivity in the presence of an electric field.
By modeling an aluminum nitride crystal between electrodes, the scientists applied the field in opposite directions. The calculations showed that, depending on the direction of the field relative to the crystal's own polarity, some atomic bonds strengthen and others weaken, which affects heat transfer. Without a field, the thermal conductivity across the film was about 334 W/(m*K). With the field in one direction it increased, and in the opposite direction it decreased.
This means engineers may have to account for changes in thermal conductivity during device operation rather than treating it as constant. Moreover, the orientation of the semiconductor film can be chosen so that the operating field helps dissipate heat. "We could intentionally grow the film in a direction where the electric field favors heat dissipation," Tian explained.
In addition to the calculations, the group conducted experiments with hexagonal boron nitride, a wide-bandgap material used as a dielectric in promising devices. Thin single-crystal flakes were placed between metal electrodes, strong fields were applied, and thermal resistance was measured using a laser-based frequency-domain thermoreflectance method. In one device, thermal resistance dropped to 25% in repeated tests. The changes grew gradually, over tens of minutes to hours, and partially recovered. Conventional electrical measurements did not detect these changes. The results were published in Nano Letters.
The authors emphasize that these are still laboratory studies, not finished devices. Nevertheless, they provide experimental confirmation that in wide-bandgap materials heat transfer can change noticeably under the influence of an electric field. In the future, this could help create electronics with more efficient cooling.