Thermal Design Rules of AlGaN/GaN-Based Microwave Transistors on Diamond

Reliable operation of high power GaN amplifiers at maximum performance relies on the mutual optimization of several design parameters constrained by a defined thermal budget. On high thermal conductivity, substrates, such as SiC and diamond, undergo small changes within the design that can lead to drastic changes in channel temperature. We utilize finite element simulations to provide design rules for device structures for GaN-on-diamond amplifiers, benchmarked against GaN-on-SiC. At 8 W/mm power density, a 13 μm gate pitch GaN-on-diamond design compared to a commonly employed 40 μm gate pitch GaN-on-SiC design results in ~40 °C and ~20 °C cooler peak channel temperature, i.e., 182 °C for single and 201 °C for polycrystalline diamond (PCD) substrates despite the 3× larger areal power density. The simulations were validated with 1.25 mm wide, 10-finger GaN-on-diamond high-electron-mobility transistors (HEMT) devices of 13 and 40 μm gate pitch, with good electrical performance in pulsed I-V measurements and Raman thermography measurements. The commonly used Au-Sn die-attach is determined as the next limiting factor for GaN-on-Diamond technologies which require the development of new die-attach materials.