Advanced solutions for thermal management and reliability of gallium nitride-based high electron mobility transistors on diamond substrates

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Hines, Nicholas J.
Graham, Samuel
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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. Finally, a steady-state thermal finite element model (FEM) was used to demonstrate the thermal advantage gained by optimizing the near-interface PCD thermal conductivity through wafer bonding high quality bulk PCD without the initial NCD nucleation layers and further mitigating the GaN-PCD interface resistance. The comparative thermal modeling results have demonstrated the outstanding peak temperature reduction capable with present GaN-on-PCD technologies with respect to the high-power RF industry standard GaN-on-SiC technologies.
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