EFFICENCY METRIC Baliga figure of merit for wide-bandgap semiconductors

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The Baliga figure of merit (BFOM) is a material metric that quantifies how effectively a power semiconductor minimizes conduction losses. In reality, BFOM is a formula that shows a material’s intrinsic ability to achieve lower conduction losses at high breakdown voltages.This article discusses the Baliga figure of merit and why engineers should consider it before choosing a semiconductor material for any power electronics application.

The power electronics industry considers BFOM, along with other important metrics such as wide-bandgap energy, thermal conductivity, breakdown field, maximum operating temperature, and more. While these metrics are not found in datasheets, engineers use them in shortlisting candidate materials on paper before committing months to designing a power electronics device.

What does the Baliga figure of merit measure?

The Baliga figure of merit measures the “specific on-state resistance” of the drift region of an FET. But why is the drift region so important? The drift region of a semiconductor device is a lightly doped region placed between two heavily doped layers. Placing a lightly doped layer between two heavily doped layers makes it less conductive and resistive to the current flow.

In short, the drift region carries the device’s blocking voltage. The higher the voltage, the more this layer resists the current flow. At high voltages, it accounts for most of the device’s on-state resistance (Ron). So, keeping the drift region’s resistance low is key to making the power device efficient.

The Baliga figure of merit for silicon is taken as a standard reference of “1” to calculate values for other semiconductor materials. As a material-level formula, the BFOM multiplies three important material properties: permittivity, electron mobility, and critical breakdown field.

Permittivity (ε_r) represents the ability of a semiconductor device to resist an electric field and possibly store charge. A higher permittivity means that a semiconductor material can support conduction.

When calculating, the exact value of material permittivity is not taken. Instead, the value is first divided by the value of the permittivity of silicon.

Electron mobility (μ) is the measure of how fast electrons move under the application of an electric field. High electron mobility means that the current flows with lower resistance.

Similar to permittivity, the exact value of electron mobility is not taken, but is first divided by the electron mobility value of silicon.

Critical breakdown field (E_C) is the maximum field a semiconductor material can withstand before it breaks down—starts acting like an insulator. The value of the breakdown field is first divided by the breakdown field of silicon. It is then cubed in the BFOM formula.

The electric field is cubed in the formula, holding utmost importance. If a power semiconductor withstands a higher electric field before breaking down, engineers can build thinner drift regions for the same blocking voltage. In addition, engineers can also build a heavily doped drift region for the same breakdown voltage.

In addition to the BFOM formula, engineers can consider the specific on-resistance of a semiconductor to evaluate behavior at different breakdown voltages.

BFOM of wideband-gap semiconductors

Various sources document BFOM values for different semiconductor materials at room-temperature of 300 Kelvin.

Although a subject of research, higher values of gallium oxide, diamond, and aluminum nitride indicate a high capability for ultra-low-loss high-voltage switching. It is important to note that BFOM values can vary from source to source because the underlying material constants, including critical electric field and temperatures, change.

Numbers might shift, but what doesn’t change is that wideband-gap materials exhibit higher values. Higher BFOM of wideband-gap materials shows that engineers can obtain the same voltage rating in a smaller device and exhibit lower conduction losses.

Where does the BFOM fall short?

BFOM doesn’t fall short in anything; it is incomplete! Engineers cannot solely consider BFOM when choosing a power semiconductor material. It only covers conduction losses in low-frequency switching. Recent studies suggest that BFOM values fall sharply when temperatures increase from 300 K to 620 K. This is important because power semiconductor devices run hot.

End-to-end design considerations cover other metrics as well. In 1989, Baliga himself published another figure of merit, the Baliga high-frequency figure of merit (BHFFOM) for high-frequency switching, based on power losses.

The story doesn’t end here. Johnson figure of merit (JFOM) for high-frequency RF circuits and Keyes figure of merit (KFOM) for thermal limits on switching in densely packed ICs, and several metrics are evolving as well. It’s just that BFOM is the new headline to market newer semiconductors, such as aluminium nitride and gallium oxide.

References

• https://www.poweramericainstitute.org/wp-content/uploads/2017/04/WBG-semiconductors-and-the-simulation-of-power-devices-Lectures-1-4-Petru-Andrei.pdf

• https://pubs.acs.org/aaembp/article-abstract/4/8/4140/385608/Bandgap-Mobility-Dielectric-Constant-and-Baliga-s?redirectedFrom=fulltext

• https://ieeexplore.ieee.org/document/43098

• https://pmc.ncbi.nlm.nih.gov/articles/PMC8510091/

• https://www.patsnap.com/resources/blog/articles/gallium-oxide-power-semiconductor-landscape-2026/

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