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, T_{j_max} = 175.2°C, is recorded at output power density of 5 W mm⁻¹. The relative magnitude of T_{j_max} with respect to the remaining non-junction area on the GaN surface ΔT_{j_max} = 5.5% with the “spike-like” plot indicates the thermal crosstalk effect. Deposition of 10 μm-thick PCD (thermal conductivity of 500 W m⁻¹ K⁻¹) heat spreader on the Si-GaN-Si₃N₄ chip results in T_{j_max} = 173.1°C and ΔT_{j_max} = 4.2% (−25%). If Si₃N₄ is replaced with a PCD layer, and another 10 μm-thick PCD heat spreader is deposited, it results in T_{j_max} = 171.0°C and ΔT_{j_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 T_{j_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.