Understanding the Role of Near-Junction Diamond Heat Spreaders in Packaged 20-Gate GaN HEMT Chips via Thermal Simulation

The study of heat dissipation from GaN high-electron-mobility transistors (HEMT) is important for devising an effective thermal management strategy. In this article, chip-level and package-level solutions using polycrystalline diamond (PCD) heat spreaders are revisited. A model of a 20-gate GaN HEMT is built and packaged into TO-220, and its thermal profile is simulated using Ansys Icepak. For the original Si-GaN-Si3N4 chip, the maximum junction temperature, Tj_max = 175.2 degrees C, is recorded at output power density of 5 W mm-1. The relative magnitude of Tj_max with respect to the remaining non-junction area on the GaN surface Delta Tj_max = 5.5% with the "spike-like" plot indicates the thermal crosstalk effect. Deposition of 10 mu m-thick PCD (thermal conductivity of 500 W m-1 K-1) heat spreader on the Si-GaN-Si3N4 chip results in Tj_max = 173.1 degrees C and Delta Tj_max = 4.2% (-25%). If Si3N4 is replaced with a PCD layer, and another 10 mu m-thick PCD heat spreader is deposited, it results in Tj_max = 171.0 degrees C and Delta Tj_max = 3.1% (-44%). The spike-like plot is blunted by both PCD layers due to the improved spreading of heat from the hotspot area. The drop in Tj_max is very small, as the main function of PCD is simply to spread the heat, not to improve the sinking of the heat from the package to the ambient air. This information is critical so as to avoid a thermal management strategy heavily invested in chip-level heat spreaders without acknowledging the major contribution of package-level heat sinks.