We report polycrystalline diamond epitaxial growth on β-Ga₂O₃ for device-level thermal management. We focused on establishing diamond growth conditions on β-Ga₂O₃ accompanying the study of various nucleation strategies. A growth window was identified, yielding uniform-coalesced films while maintaining interface smoothness. In this first demonstration of diamond growth on β-Ga₂O₃, a diamond thermal conductivity of 110 ± 33 W m⁻¹ K⁻¹ and a diamond/β-Ga₂O₃ thermal boundary resistance of 30.2 ± 1.8 m²K G⁻¹ W⁻¹ were measured. The film stress was managed by growth optimization techniques preventing delamination of the diamond film. This work marks the first significant step towards device-level thermal management of β-Ga₂O₃ electronic devices.

GaN radio frequency (RF) power amplifiers offer many benefits including high power density, reduced device footprint, high operating voltage, and excellent gain and power-added efficiency. Accordingly, these parts are enabling next-generation technologies such as fifth-generation (5G) base transceiver stations and defense/aerospace applications such as high-performance radar and communication systems. However, these benefits can be overshadowed by device overheating that compromises the performance and reliability. In response to this, researchers have focused on GaN-on-diamond integration during the past decade. However, manufacturability, scalability, and long-term reliability remain as critical challenges toward the commercialization of the novel device platform. In this work, a diamond-incorporated flip-chip integration scheme is proposed that takes advantage of existing semiconductor device processing and growth techniques. Using an experimentally validated GaN-on-SiC multifinger device model, the theoretical limit of the cooling effectiveness of the device-level thermal management solution has been evaluated. Simulation results show that by employing a ~ 2-μm diamond passivation overlayer, gold thermal bumps, and a commercial polycrystalline carrier wafer, the power amplifier's dissipated heat can be effectively routed toward the package, which leads to a junction-to-package thermal resistance lower than GaN-on-diamond high electron mobility transistors (HEMTs). Furthermore, simulation results show that this approach is even more promising for lowering the device thermal resistance of emerging ultra-wide bandgap devices based on β-Ga ₂ O ₃ and AlGaN, below that for today's state-of-the-art GaN-on-diamond HEMTs.

Aluminum gallium nitride (AlGaN) high electron mobility transistors (HEMTs) are candidates for next-generation power conversion and radio frequency (RF) applications. AlxGa1-xN channel HEMT devices (x = 0.3, x = 0.7) were investigated using multiple in-situ thermal characterization methods and electro-thermal simulation. The thermal conductivity, contact resistivity, and channel mobility were characterized as a function of temperature to understand and compare the heat generation profile and electro-thermal transport within these devices. In contrast to GaN-based HEMTs, the electrical output characteristics of Al0.70Ga0.30N channel HEMTs exhibit remarkably lower sensitivity to the ambient temperature rise. Also, during 10kHz pulsed operation, the difference in peak temperature between the AlGaN channel HEMTs and GaN HEMTs reduced significantly.

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.

AlN thin films are enabling significant progress in modern optoelectronics, power electronics, and microelectromechanical systems. The various AlN growth methods and conditions lead to different film microstructures. In this report, phonon scattering mechanisms that impact the cross-plane (κz; along the c-axis) and in-plane (κr; parallel to the c-plane) thermal conductivities of AlN thin films prepared by various synthesis techniques are investigated. In contrast to bulk single crystal AlN with an isotropic thermal conductivity of ∼330 W/m K, a strong anisotropy in the thermal conductivity is observed in the thin films. The κz shows a strong film thickness dependence due to phonon-boundary scattering. Electron microscopy reveals the presence of grain boundaries and dislocations that limit the κr. For instance, oriented films prepared by reactive sputtering possess lateral crystalline grain sizes ranging from 20 to 40 nm that significantly lower the κr to ∼30 W/m K. Simulation results suggest that the self-heating in AlN film bulk acoustic resonators can significantly impact the power handling capability of RF filters. A device employing an oriented film as the active piezoelectric layer shows an ∼2.5× higher device peak temperature as compared to a device based on an epitaxial film.