Breakdown voltage and on-resistance limit (unipolar limit) calculator
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The specific on-resistance of a unipolar drift layer is Ron,sp = 4BV²/(ε·μ·Ec³): proportional to the square of breakdown voltage and inversely proportional to the cube of the critical field Ec. SiC and GaN have about ten times silicon's Ec, so for the same voltage the drift layer can be one-tenth as thick and the theoretical on-resistance falls by a factor of several hundred. That is why wide band gap semiconductors suit power devices.
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The theoretical limit of the drift layer alone. Real devices add channel, substrate and contact resistance; in SiC MOSFETs rated 600 V or less, channel resistance dominates. Silicon superjunction MOSFETs and IGBTs exceed this limit through charge compensation and conductivity modulation.
Breakdown voltage and specific on-resistance (unipolar limit)
Materials compared (breakdown voltage 1200 V)
| Material | Eg | Ec (V/cm) | μn (cm²/Vs) | ND (cm⁻³) | Drift thickness | Ron,sp (mΩ·cm²) | BFOM (vs. Si) |
|---|---|---|---|---|---|---|---|
| Silicon (Si) | 1.12 eV | 3.0×10⁵ | 1400 | 2.4×10¹⁴ | 80.0 µm | 147 | 1 |
| Gallium arsenide (GaAs) | 1.42 eV | 4.0×10⁵ | 8500 | 4.8×10¹⁴ | 60.0 µm | 9.27 | 15.9 |
| 4H-SiC | 3.26 eV | 2.8×10⁶ | 950 | 1.8×10¹⁶ | 8.6 µm | 0.322 | 457 |
| Gallium nitride (GaN) | 3.44 eV | 3.3×10⁶ | 1250 | 2.3×10¹⁶ | 7.3 µm | 0.161 | 914 |
| Gallium oxide (β-Ga₂O₃) | 4.85 eV | 8.0×10⁶ | 200 | 1.5×10¹⁷ | 3.0 µm | 0.0635 | 2,320 |
Why wide band gap semiconductors suit power devices
Higher breakdown voltage requires a thick, lightly doped drift layer to hold the voltage, and that raises on-resistance. In unipolar devices, specific on-resistance scales with the square of breakdown voltage and inversely with the cube of the critical field Ec. The Ec of SiC and GaN is about ten times that of silicon, so for the same voltage the drift layer can be one-tenth as thick with a hundred times the doping, cutting on-resistance in theory by a factor of several hundred.
Baliga's figure of merit (BFOM = ε·μ·Ec³) sums up this difference in one number. In silicon, unipolar MOSFETs become impractical around 1 kV and IGBTs have been used instead; in SiC, MOSFETs achieve low losses even at 1.2 to 3.3 kV, combining low conduction loss with high-frequency operation and small switching losses.
FAQ
What is Baliga's figure of merit?
BFOM = ε·μ·Ec³, the material property that sets the minimum specific on-resistance of a unipolar device at a given breakdown voltage. A higher value means lower conduction loss.
Can real devices beat the unipolar limit?
Yes. Silicon superjunction MOSFETs use charge compensation and IGBTs use conductivity modulation to go below it. Real unipolar devices, on the other hand, add channel, substrate and contact resistance above it.
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Last updated: October 23, 2026