Silicon carbide (SiC) is no longer just a niche semiconductor. Its exceptional electrical and thermal properties make it indispensable for next-generation power electronics, EV inverters, RF devices, and high-frequency applications. Among SiC polytypes, 4H-SiC and 6H-SiC dominate the market—but choosing the right one requires more than just “which is cheaper.”
This article provides a multi-dimensional comparison of 4H-SiC and 6H-SiC substrates, covering crystal structure, electrical, thermal, mechanical properties, and typical applications.
1. Crystal Structure and Stacking Sequence
SiC is a polymorphic material, meaning it can exist in multiple crystal structures called polytypes. The stacking sequence of Si–C bilayers along the c-axis defines these polytypes:
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4H-SiC: Four-layer stacking sequence → Higher symmetry along c-axis.
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6H-SiC: Six-layer stacking sequence → Slightly lower symmetry, different band structure.
This difference affects carrier mobility, bandgap, and thermal behavior.
| Feature | 4H-SiC | 6H-SiC | Notes |
|---|---|---|---|
| Layer stacking | ABCB | ABCACB | Determines band structure and carrier dynamics |
| Crystal symmetry | Hexagonal (more uniform) | Hexagonal (slightly elongated) | Affects etching, epitaxial growth |
| Typical wafer sizes | 2–8 inch | 2–8 inch | Availability increasing for 4H, mature for 6H |
2. Electrical Properties
The most critical difference lies in electrical performance. For power and high-frequency devices, electron mobility, bandgap, and resistivity are key factors.
| Property | 4H-SiC | 6H-SiC | Impact on Device |
|---|---|---|---|
| Bandgap | 3.26 eV | 3.02 eV | Wider bandgap in 4H-SiC allows higher breakdown voltage, lower leakage current |
| Electron mobility | ~1000 cm²/V·s | ~450 cm²/V·s | Faster switching for high-voltage devices in 4H-SiC |
| Hole mobility | ~80 cm²/V·s | ~90 cm²/V·s | Less critical for most power devices |
| Resistivity | 10³–10⁶ Ω·cm (semi-insulating) | 10³–10⁶ Ω·cm (semi-insulating) | Important for RF and epitaxial growth uniformity |
| Dielectric constant | ~10 | ~9.7 | Slightly higher in 4H-SiC, affects device capacitance |
Key Takeaway: For power MOSFETs, Schottky diodes, and high-speed switching, 4H-SiC is preferred. 6H-SiC is sufficient for low-power or RF devices.
3. Thermal Properties
Heat dissipation is critical for high-power devices. 4H-SiC generally performs better due to its thermal conductivity.
| Property | 4H-SiC | 6H-SiC | Implications |
|---|---|---|---|
| Thermal conductivity | ~3.7 W/cm·K | ~3.0 W/cm·K | 4H-SiC dissipates heat faster, reducing thermal stress |
| Coefficient of thermal expansion (CTE) | 4.2 ×10⁻⁶ /K | 4.1 ×10⁻⁶ /K | Matching with epitaxial layers is critical to prevent wafer warping |
| Maximum operation temperature | 600–650 °C | 600 °C | Both high, 4H slightly better for prolonged high-power operation |
4. Mechanical Properties
Mechanical stability affects wafer handling, dicing, and long-term reliability.
| Property | 4H-SiC | 6H-SiC | Notes |
|---|---|---|---|
| Hardness (Mohs) | 9 | 9 | Both extremely hard, second only to diamond |
| Fracture toughness | ~2.5–3 MPa·m½ | ~2.5 MPa·m½ | Similar, but 4H slightly more uniform |
| Wafer thickness | 300–800 µm | 300–800 µm | Thinner wafers reduce thermal resistance but increase handling risk |
5. Typical Applications
Understanding where each polytype excels helps in substrate selection.
| Application Category | 4H-SiC | 6H-SiC |
|---|---|---|
| High-voltage MOSFETs | ✔ | ✖ |
| Schottky diodes | ✔ | ✖ |
| Electric vehicle inverters | ✔ | ✖ |
| RF devices / microwave | ✖ | ✔ |
| LEDs and optoelectronics | ✖ | ✔ |
| Low-power high-voltage electronics | ✖ | ✔ |
Rule of Thumb:
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4H-SiC = Power, speed, efficiency
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6H-SiC = RF, low-power, mature supply chain
6. Availability and Cost
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4H-SiC: Historically harder to grow, now increasingly available. Slightly higher cost but justified for high-performance applications.
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6H-SiC: Mature supply, generally lower cost, widely used for RF and low-power electronics.
Choosing the Right Substrate
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High-voltage, high-speed power electronics: 4H-SiC is essential.
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RF devices or LEDs: 6H-SiC is often sufficient.
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Thermal-sensitive applications: 4H-SiC provides better heat dissipation.
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Budget or supply considerations: 6H-SiC may reduce cost without compromising device requirements.
Final Thoughts
Although 4H-SiC and 6H-SiC may appear similar to the untrained eye, their differences span crystal structure, electron mobility, thermal conductivity, and application suitability. Choosing the correct polytype at the beginning of your project ensures optimal performance, reduced rework, and reliable devices.
Post time: Jan-04-2026