(Invited) Considerations and Strategies for High-Temperature Ultra-Wide Bandgap Gallium Oxide Power Modules

There is a compelling need for high-density power electronic components and systems capable of operation at high ambient temperatures in automotive, aerospace, and down-hole applications. However, these needs are challenging the fundamental limit of silicon-based converters. While wide-bandgap (WBG) power semiconductors, particularly silicon carbide (SiC) and gallium nitride (GaN), have become promising alternatives to silicon (Si), they show diminished performance benefits at high temperatures. In the last several years, an ultra-wide-bandgap (UWBG) semiconductor, gallium oxide (Ga2O3), has emerged as a viable candidate for high-temperature power electronics with capabilities beyond existing technologies due to its large bandgap, controllable doping, and the availability of large-diameter, relatively-inexpensive substrates. The fundamental limit for high-temperature operation of semiconductors is the concentration of intrinsic carriers, which increases with temperature. Thanks to the UWBG of Ga2O3 (4.8 eV, compared to 1.1 eV for Si, 3.2 eV for SiC, and 3.4 eV for GaN), it achieves over 10-times lower intrinsic carrier concentration than Si. Compared to Si, SiC, and GaN devices, unipolar Ga2O3 devices also have superior theoretical limit for the trade-off between on-resistance and breakdown voltage, enabling a higher power conversion efficiency and power density. While Ga2O3 shows promise in these respects, due to its low thermal conductivity, conventional packaging and cooling strategies are not suitable. Simulations reveal that, compared to SiC, Ga2O3 power devices experience greater hot spot effects with higher peak junction temperatures and greater temperature differences across the chip, which could reduce the reliability of the packaged device. Simulations and preliminary experiments show that top-side cooling can counteract these effects and result in comparable thermal performance to SiC devices. This improvement shows the significant impact that the package and cooling strategy have on the performance of the Ga2O3 devices. This presentation will review this critical device-package-thermal relationship with electrothermal simulations and experimental measurements. Moreover, while Ga2O3 power devices have demonstrated superior high-temperature stability compared to SiC devices, package advancements at temperatures above 200°C are limited. The major limitation for high-temperature power modules is the encapsulation. The encapsulation provides essential electrical insulation, as well as corrosion resistance and protection. Conventional encapsulants are polymers, which have high dielectric strength for good electrical insulation, and low elastic modulus for low thermo-mechanical stress. However, typical polymers degrade rapidly at temperatures above 200°C. Accordingly, non-polymeric materials are needed for higher temperature operation. Hydroset ceramics have been evaluated as a high-temperature encapsulant. However, their porous structure reduces their electrical insulation effectiveness, and can cause reduced corrosion resistance. An alternate non-polymeric material is glass. Low-melting-temperature glass composites have emerged as a promising high-temperature encapsulant. It was found that the lead-glass composite has improved thermal stability compared to commercial polymeric materials. While the partial discharge inception voltage (PDIV) of the polymeric materials decreased by more than 80 % after just 100-200 hours of soaking at 250°C, the lead-glass composite samples showed no change in PDIV after 1000 hours. This feature is particularly important for realizing the full potential of Ga2O3, which has a critical breakdown field strength of 8 MV/cm. Accordingly, to operate these devices at high temperature and high-voltage, the package encapsulant must simultaneously have high thermal stability and dielectric strength. This presentation will review advancements in power module packaging that are needed to realize the full potential of Ga2O3 power devices.