Silicon carbide (SiC) has become a cornerstone material for next-generation power electronics, high-temperature systems, and high-frequency devices. What makes SiC unique is that it can crystallize into many polytypes—over 200 have been identified—even though all of them share the same chemical formula. Among these, 4H-SiC and 6H-SiC are by far the most commercially important.
From the outside, they appear similar: both are hexagonal polytypes with high thermal conductivity, strong covalent bonding, and wide bandgaps. However, subtle differences in atomic stacking give them distinct electronic behaviors and determine how they are used in semiconductor devices.
This article provides a clear and original explanation of how 4H-SiC and 6H-SiC differ in crystal structure, physical properties, and practical applications.
1. Why SiC Forms Different Polytypes
SiC is composed of alternating silicon and carbon layers. Although each layer has the same atomic arrangement, their stacking order may change. This stacking sequence is what generates different polytypes.
A simple analogy is stacking identical playing cards in different offset patterns. The cards do not change, but the overall shape does.
In SiC:
Even such small structural changes are enough to alter the band structure, energy levels, and carrier mobility.
2. Crystal Structure Comparison
4H-SiC
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Stacking sequence repeats every four layers
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Crystal symmetry is hexagonal
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C-axis lattice constant is approximately 10.1 Å
Because its stacking sequence is shorter and more uniform, the resulting crystal exhibits less anisotropy and more consistent electronic properties along different directions.
6H-SiC
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Stacking sequence repeats every six layers
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Hexagonal crystal symmetry
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C-axis lattice constant is approximately 15.1 Å
The longer repeat distance creates multiple nonequivalent atomic sites, making the band structure more complex and leading to direction-dependent carrier mobility.
3. Bandgap and Electronic Properties
| Property |
4H-SiC |
6H-SiC |
| Bandgap (Eg) |
~3.26 eV |
~3.02 eV |
| Electron mobility (cm²/V·s) |
~900 (parallel to c-plane) |
~400–500 |
| Breakdown electric field |
~3 MV/cm |
Slightly lower than 4H-SiC |
| Electron saturation velocity |
Higher |
Lower |
4H-SiC offers:
These characteristics make it especially suitable for high-voltage and high-frequency devices.
6H-SiC, while still a wide-bandgap material, shows lower mobility due to the more complex stacking sequence.
4. Thermal and Mechanical Characteristics
Both polytypes share the same strong covalent Si–C bonds, giving them:
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high thermal conductivity
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excellent mechanical strength
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resistance to radiation and chemical corrosion
Thermal conductivity values are similar:
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4H-SiC ≈ 4.9 W/cm·K
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6H-SiC ≈ 4.7 W/cm·K
The differences are too small to significantly influence device selection.
5. Applications: Where Each Polytype Excels
4H-SiC: The Industry Standard for Power Electronics
4H-SiC is dominant in:
Its superior electron mobility and breakdown field directly improve device efficiency, switching speed, and thermal robustness. This is why almost all modern SiC power devices are based on 4H-SiC.
6H-SiC: Niche but Still Valuable
6H-SiC is used in:
Because its electronic properties vary with crystal direction, it sometimes enables material behaviors not achievable with 4H-SiC.
6. Which Polytype Should Engineers Choose?
If the goal is:
then 4H-SiC is the clear choice.
If the application involves:
then 6H-SiC remains useful.
7. Conclusion
Although 4H-SiC and 6H-SiC share the same elemental composition, their different stacking sequences create distinct electronic landscapes. For modern power electronics, 4H-SiC offers superior performance and has become the industry’s dominant polytype. Meanwhile, 6H-SiC continues to play an important role in specialized optoelectronic and RF fields.
Understanding these structural and electronic differences helps engineers choose the most suitable material for next-generation semiconductor devices.
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