【Knowledge Discover】The Next Frontier of Power Electronics: Toward Multi-kV-Class Gallium Oxide Devices

      With the global shift toward electrification, power electronic systems are increasingly required to operate at higher voltages with higher efficiency. Following silicon carbide (SiC) and gallium nitride (GaN), β-phase gallium oxide (Ga₂O₃) is emerging as a promising candidate for multi-kilovolt (multi-kV) devices.

      This article, based on expert insights, outlines the development of gallium oxide technology from the perspectives of device structures, material growth, and circuit-level challenges.

 

Device Structures: Vertical MOSFETs for >10 kV Operation

      Gallium oxide power devices under development include:

      ●Schottky barrier diodes (SBDs)

      ●Lateral MOSFETs

      ●Vertical MOSFETs

      ●RF transistors

      ●Photoconductive semiconductor switches (PCSS)

      For voltage levels above 10 kV, vertical device architectures are considered more suitable.

      Compared with lateral devices, where the electric field is concentrated near the surface, vertical structures distribute the electric field within the bulk material. This helps improve breakdown voltage and reduces sensitivity to surface conditions. In addition, vertical devices utilize the thickness of the material, enabling higher current capability within a smaller chip area, which can reduce cost per unit power.

 

Key Device Approaches

      Two main approaches are being explored for >10 kV operation:

      ●Vertical FinFET
      This structure avoids the need for p-type doping in β-Ga₂O₃, which remains challenging. Demonstrations at the 10 kV level have been reported. However, the structure relies on submicron-scale features, leading to higher fabrication complexity and cost.

      ●Vertical Trench MOSFET
      This approach uses micron-scale features, making it more compatible with conventional fabrication processes and potentially lower in cost. Its performance depends on the design of the current blocking layer (CBL), which must support high voltage in the off-state. Techniques such as in-situ doping and ion implantation are being investigated.

      Both approaches require high-quality, thick drift layers.

 

Material Growth: Thick Drift Layer Challenges

      For vertical devices, the drift layer is a critical component, typically ranging from 25 to 100 μm in thickness. It must simultaneously achieve:

High growth rate

      ●Low and well-controlled doping concentration

      ●Smooth surface morphology

      ●Two main epitaxial growth techniques are used:

      HVPE (Halide Vapor Phase Epitaxy)
      Offers high growth rates suitable for thick layers. However, it often results in rough surfaces that require post-growth polishing, increasing cost and material loss. Achieving uniformity on large-area wafers remains challenging.

      MOCVD (Metal-Organic Chemical Vapor Deposition)
      Provides better uniformity and scalability but currently has lower growth rates. The goal is to increase the growth rate to around 10 μm/h while maintaining low impurity levels and smooth surfaces.

 

Doping Control and Material Quality

      As the thickness of epitaxial layers increases, maintaining material quality and precise doping becomes more challenging:

      ●In MOCVD, background carbon impurities must be suppressed, as they act as compensating centers.

      ●In HVPE, chlorine-related impurities need to be controlled to avoid unintentional n-type doping.

      Substrate orientation also plays a key role. For example, the (011) orientation is considered favorable for growing high-quality thick layers.

      Surface morphology is closely linked to doping uniformity, which directly affects breakdown performance.

 

Circuit- and System-Level Challenges

      Even with high-voltage devices, system integration presents several challenges:

      High switching stress
      Fast switching leads to high dv/dt, placing stress on both devices and insulation systems.

      Electromagnetic interference (EMI)
      High di/dt can generate significant EMI, requiring careful design of both active and passive components.

      Thermal management
      Gallium oxide has relatively low thermal conductivity, making advanced cooling and packaging necessary.

 

System-Level Advantages

      In medium- and high-voltage systems (e.g., MVDC/MVAC and HVDC/HVAC), gallium oxide devices offer potential advantages.

      Currently, high-voltage operation often requires multiple devices connected in series, increasing system complexity. Higher-voltage single devices could reduce the number of components, leading to:

      ●Lower system complexity

      ●Reduced control requirements

      ●Lower conduction losses

      Applications such as solid-state transformers (SST) and modular multilevel converters (MMC) may benefit from high-voltage gallium oxide devices by simplifying system architecture and improving efficiency.