Recent advancements in material growth, processing technologies, device architecture, and reliability testing have propelled AlGaN/GaN HEMTs to the forefront of high-power radio frequency (RF) electronics applications including wireless communications, advanced radar systems, and electronic warfare. However, despite the rapid maturation of electrical device performance, thermal management of acute device self-heating is the most prominent developmental bottleneck limiting device performance.
To mitigate this acute self-heating, traditional AlGaN/GaN HEMT device substrate materials (typically Si or SiC) have been replaced with high thermal conductivity (k ≈ 2000 W/mK) chemical vapor deposited (CVD) polycrystalline diamond (PCD). However, the structure of PCD has been demonstrated to severely diminish the advantageous thermal properties of bulk diamond. In addition, achieving a high-quality interface between GaN and diamond is challenging and requires the use of a thermally resistive transition layer or interface material. Furthermore, GaN-on-diamond fabrication processes lead to the development of a residual stress state throughout the AlGaN/GaN heterostructure that can be detrimental to device functionality.
To address these challenges, this work explores the feasibility and thermal limitations of using PCD as a substrate material for the thermal management of GaN-based HEMTs for RF applications. To understand the extent of thermal property degradation present in the initial microns of PCD and to inform CVD growth process optimization, two steady-state thermometry techniques were used to characterize the in-plane thermal conductivity of PCD thin films. To identify the most effective GaN-on-diamond fabrication process among competing alternatives, a spatially resolved optical stress metrology technique was used to characterize the through-thickness residual stress distribution within the GaN layer of a variety of GaN-on-diamond wafer samples.