Integration of a phase change material for junction-level cooling in GaN devices

Next generation gallium nitride (GaN) RF power transistors offer higher power, higher efficiency and wider bandwidth than competing Si technologies. However, the high power densities available in GaN power transistors create new challenges for heat dissipation. This paper presents a novel micro-scale thermal storage design that involves phase change material (PCM) filled grooves etched in the substrate to remove the heat generated in the active regions of a pulsed-mode GaN transistor. High electron mobility transistors (HEMTs) were fabricated on a GaN-on-Si wafer. Backside patterning and etching were done to thin the Si substrate under the active channel region of the selected transistors. A phase change material (PCM) with a melting temperature of 118°C was deposited in the etched grooves. Electrical measurements were carried out to compare the performance of transistors with and without PCM filled grooves. It was found that the groove etching did not degrade the transistor performance under low power conditions where the junction level heating is not enough to start the PCM melting process. From the current-voltage characteristics at different temperatures ranging from 25°C to 120°C, it was found that at higher temperatures, the current density in the PCM-enabled device was larger than in the reference device, due to the enhanced thermal management. The role of PCM was confirmed when measurements at temperatures well above the melting temperature of the PCM did not show signs of increase in the current density. The maximum current density in the device with PCM material was found to be much more stable under pulsed conditions than in current state-of-the-art devices.