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Gallium nitride (GaN)-based high electron mobility transistors (HEMTs) are essential components in modern radio frequency power amplifiers. In order to improve both the device electrical and thermal performance (e.g., higher current density operation and better heat dissipation), researchers are introducing AlN into the GaN HEMT structure. The knowledge of thermal properties of the constituent layers, substrates, and interfaces is crucial for designing and optimizing GaN HEMTs that incorporate AlN into the device structure as the barrier layer, buffer layer, and/or the substrate material. This study employs a multi-frequency/spot-size time-domain thermoreflectance approach to measure the anisotropic thermal conductivity of (i) AlN and GaN epitaxial films, (ii) AlN and SiC substrates, and (iii) the thermal boundary conductance for GaN/AlN, AlN/SiC, and GaN/SiC interfaces (as a function of temperature) by characterizing GaN-on-SiC, GaN-on-AlN, and AlN-on-SiC epitaxial wafers. The thermal conductivity of both AlN and GaN films exhibits an anisotropy ratio of ∼1.3, where the in-plane thermal conductivity of a ∼1.35 μm thick high quality GaN layer (∼223 W m−1 K−1) is comparable to that of bulk GaN. A ∼1 μm thick AlN film grown by metalorganic chemical vapor deposition possesses a higher thermal conductivity than a thicker (∼1.4 μm) GaN film. The thermal boundary conductance values for a GaN/AlN interface (∼490 MW m-2 K−1) and AlN/SiC interface (∼470 MW m−2 K−1) are found to be higher than that of a GaN/SiC interface (∼305 MW m−2 K−1). This work provides thermophysical property data that are essential for optimizing the thermal design of AlN-incorporated GaN HEMT devices.
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The Doping Dependence of the Thermal Conductivity of Bulk Gallium Nitride Substrates
Abstract Gallium nitride (GaN) has emerged as one of the most attractive base materials for next-generation high-power and high-frequency electronic devices. Recent efforts have focused on realizing vertical power device structures such as in situ oxide, GaN interlayer based vertical trench metal–oxide–semiconductor field-effect transistors (OG-FETs). Unfortunately, the higher-power density of GaN electronics inevitably leads to considerable device self-heating which impacts device performance and reliability. Halide vapor-phase epitaxy (HVPE) is currently the most common approach for manufacturing commercial GaN substrates used to build vertical GaN transistors. Vertical device structures consist of GaN layers of diverse doping levels. Hence, it is of crucial importance to measure and understand how the dopant type (Si, Fe, and Mg), doping level, and crystal quality alter the thermal conductivity of HVPE-grown bulk GaN. In this work, a steady-state thermoreflectance (SSTR) technique was used to measure the thermal conductivity of HVPE-grown GaN substrates employing different doping schemes and levels. Structural and electrical characterization methods including X-ray diffraction (XRD), secondary-ion mass spectrometry (SIMS), Raman spectroscopy, and Hall-effect measurements were used to determine and compare the GaN crystal quality, dislocation density, doping level, and carrier concentration. Using this comprehensive suite of characterization methods, the interrelation among structural/electrical parameters and the thermal conductivity of bulk GaN substrates was investigated. While doping is evidenced to reduce the GaN thermal conductivity, the highest thermal conductivity (201 W/mK) is observed in a heavily Si-doped (1–5.00 × 1018 cm−3) substrate with the highest crystalline quality. This suggests that phonon-dislocation scattering dominates over phonon-impurity scattering in the tested HVPE-grown bulk GaN substrates. The results provide useful information for designing thermal management solutions for vertical GaN power electronic devices.
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- Award ID(s):
- 1934482
- PAR ID:
- 10207169
- Date Published:
- Journal Name:
- Journal of Electronic Packaging
- Volume:
- 142
- Issue:
- 4
- ISSN:
- 1043-7398
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1115/1.4047578
