Electric fields change how semiconductors dissipate heat
Cornell researchers have developed a computational method, backed by the first experimental measurements of its kind, showing how an electric field can change the way a semiconductor conducts heat.
The research, led by Zhiting Tian, at the Sibley School of Mechanical and Aerospace Engineering, predicts that the cross-plane thermal conductivity of the ultrawide-bandgap semiconductor AlN, a promising material for next-generation electronics, can either increase or decrease depending on the direction of an applied electric field.
The findings, described in the paper 'Thermal Conductivity of Wurtzite aluminium Nitride Under External Electric Field' in npj Computational Materials, suggest that engineers could eventually design electronic components that dissipate heat more effectively under an operating electric field by aligning the field direction favourably with the crystal polarity.
“When we think about the thermal conductivity of a material, the way we usually model it is without an applied electric field,” Tian said. “But in real device operations, especially for those of high power density devices, the electric field is actually significant.”
AlN is capable of withstanding substantially higher electric fields than conventional semiconductor materials, enabling smaller, more powerful electronics in high-voltage power systems and high-power RF devices. But greater power density also produces more heat, potentially reducing efficiency, reliability and device lifespan.
Tian and her students, including first author and doctoral student Joyce Christiansen-Salameh, suspected that electric fields might influence the atomic vibrations (phonons) responsible for transporting heat through these materials.
As phonons travel, they interact and scatter, affecting how efficiently heat moves through the material. The researchers reasoned that an electric field could redistribute electrons within the crystal, changing the forces between atoms and, consequently, the behaviour of phonons.
To investigate, the team developed a computational framework combining density functional theory (DFT), which describes the electronic structure of materials, with a technique called nonequilibrium Green’s function (NEGF). The combined approach, known as DFT-NEGF, had previously been used to study electron transport but had not been applied in this way to calculate phonon-based thermal conductivity under an electric field.
The researchers simulated AlN crystals positioned between electrodes and subjected them to electric fields in opposite directions. They calculated how the fields redistributed electrons, changed the forces between atoms and altered the scattering of phonons.
The results showed that the electric field could strengthen or weaken certain atomic bonds, depending on its direction relative to the crystal’s built-in polarity. Those changes, in turn, influenced how readily heat traveled through the material.
With no electric field applied, the calculated thermal conductivity perpendicular to the AlN film was approximately 334 watts per meter-kelvin. Applying an electric field in one direction increased that conductivity, while reversing the field decreased it.
The effect suggests a new consideration for designing high-power electronics. Rather than treating thermal conductivity as a fixed property, engineers may need to account for how it changes during operation. More intriguingly, they might deliberately orient semiconductor films so that operating electric fields enhance heat dissipation.
“We could intentionally grow the film in the direction where the electric field is favourable to help dissipate heat,” Tian said.
Putting theory to the test
A complementary experimental study led by Tian moves beyond predictions to measurements. In research published Nov. 9 in Nano Letters, the Cornell team investigated hexagonal BN. While the AlN study developed a computational framework for predicting changes in heat conduction, the BN study examined the phenomenon experimentally.
The researchers placed thin, single-crystal flakes of BN between metal electrodes, applied strong electric fields, and used frequency-domain thermoreflectance, a laser-based technique, to measure changes in thermal resistance, a measure of how strongly the flake and its interfaces with electrodes impede the flow of heat.
In one device, thermal resistance decreased by as much as 25 percent during repeated tests. The changes developed gradually over periods of tens of minutes to hours, with recovery observed during or between measurement cycles. The researchers also found that conventional electrical measurements did not reveal the changes detected by their thermal measurements.
The findings, reported in 'Reversible Electric-Field-Induced Changes in the Thermal Resistance of Hexagonal Boron Nitride' establish an experimental approach to probing electric-field effects on thermal transport and provide experimental evidence that heat transport in an ultra-wide-bandgap material can change substantially under electrical stress. The first author of the study is doctoral student Amelia Schaeffer.
Both studies were sponsored by the US Office of Naval Research.
