Novel junction level cooling in pulsed GaN devices

Gallium nitride (GaN) based RF power transistor technology offers the unique combination of higher power, higher efficiency and wider bandwidth. However, the extremely high power densities create new challenges for heat dissipation. In pulsed GaN devices, each duty cycle consists of an active period of heat generation followed by an inactive period. The device temperature oscillates causing thermal stresses leading to device fatigue and life reduction. In this paper, we present a novel junction level cooling technique based on a compact thermal storage design that involves phase change material (PCM) filled micrometer-sized grooves etched in the semiconductor substrate. PCM located close to the junction absorbs waste heat during the active period and dissipates the heat to a heat sink during the inactive period. Computational simulations proved the feasibility of the concept and showed reduction in junction level temperatures. High electron mobility transistors (HEMTs) were fabricated on a GaN-on-silicon wafer with micrometer-sized grooves. The selection of appropriate PCM (critical for concept´s success) to completely fill the grooves was done by performing wetting tests. DC and pulsed characterization of the new PCM-enabled devices showed up to 10% improvement in the electrical device performance (due to enhanced thermal management) compared to baseline GaN transistors.