Transient and Steady-State Thermal Measurements of GaN-on-SiC HEMT Transistors Under Realistic Microwave Drive

A new thermoreflectance and load-pull measurement set-up is presented, which enables steady-state and transient thermal imaging of RF and microwave transistors and PAs under realistic excitations. The functionality of the system is demonstrated by measuring the steady-state and transient thermal distributions of a commercially available 25-W 20-finger gallium nitride (GaN) on silicon carbide (SiC) transistor operating at 3.5 GHz. Temperature distributions and profiles are presented with the device operating under DC bias, and when it is tuned for maximum efficiency and maximum output power conditions. Transient thermal images reveal different thermal rise times on a per-channel basis.