Introduction: Why Semi-Insulating SiC Matters

Semi-insulating silicon carbide (SI-SiC) substrates play a critical role in RF, microwave, and high-power electronic applications. With their:

• Wide bandgap
• High thermal conductivity
• High breakdown electric field
• Excellent microwave stability

SI-SiC substrates have become the preferred foundation for GaN HEMT devices and high-frequency power amplifiers.
Among the various SiC polytypes, 4H-SiC and 6H-SiC are two important hexagonal crystal structures. While both have been used historically, 4H semi-insulating SiC has emerged as the industry standard.
This article provides a comprehensive technical comparison between 4H and 6H semi-insulating SiC and explains the industry shift toward 4H.
 

1. Crystal Structure Differences Between 4H and 6H SiC

Silicon carbide is a polytypic material, meaning it exists in multiple crystal structures distinguished by their stacking sequence of Si–C bilayers.

Parameter	4H-SiC	6H-SiC
Crystal system	Hexagonal	Hexagonal
Stacking periodicity	4 layers	6 layers
Bandgap (300K)	~3.26 eV	~3.02 eV
Electron mobility	Higher	Lower
Anisotropy	Lower	More pronounced

Key Implications

4H-SiC has a wider bandgap
4H-SiC exhibits significantly higher electron mobility
6H-SiC shows stronger anisotropic transport behavior
These structural differences directly impact high-frequency and high-power device performance.

2. Semi-Insulating Mechanisms: Deep Level Compensation

Semi-insulating SiC achieves high resistivity (10⁷–10¹¹ Ω·cm) through deep-level compensation mechanisms.
Two main approaches are used:

1. Vanadium (V) Doping

Vanadium introduces deep acceptor levels in the bandgap, trapping free carriers and compensating residual impurities.

2. High-Purity Semi-Insulating (HPSI) Technology

Instead of heavy transition-metal doping, HPSI substrates rely on ultra-low impurity control and intrinsic defect engineering to achieve high resistivity with lower microwave loss.

Characteristics of 4H Semi-Insulating SiC

Resistivity up to 10⁸–10¹¹ Ω·cm
Stable deep-level centers
Low microwave loss
Excellent RF linearity
Industry standard for GaN epitaxy

Characteristics of 6H Semi-Insulating SiC

Typical resistivity: 10⁷–10⁹ Ω·cm
Different deep-level positions
Higher RF loss compared to 4H
Gradually phased out of mainstream RF applications

3. Electrical Performance Comparison

Property	4H SI-SiC	6H SI-SiC
Bandgap	Wider	Narrower
Electron mobility	~900–1000 cm²/V·s	~400–500 cm²/V·s
Breakdown field	Higher	Slightly lower
Microwave loss	Lower	Higher
Thermal conductivity	Similar (~4.5 W/cm·K)	Similar

Why Mobility Matters in RF Devices

Higher electron mobility improves:
High-frequency response
Power efficiency
Linearity
Gain performance
This makes 4H semi-insulating SiC particularly well suited for:
5G and emerging 6G base stations
Radar systems
Satellite communications
High-power microwave modules

4. Defect Density and Material Maturity

Crystal defects such as micropipes, threading dislocations, and basal plane dislocations (BPDs) significantly impact device reliability.

4H-SiC

Substantially reduced micropipe density
Lower dislocation density with modern growth techniques
Mature epitaxial compatibility for GaN
Strong global supply chain

6H-SiC

Earlier industrial maturity in the 1990s
Performance limitations in high-frequency applications
Gradually replaced by 4H in advanced RF markets

5. Industry Transition: Why 4H Became the Standard

Over the past two decades, the semiconductor industry has transitioned from 6H to 4H for several reasons:
Superior RF performance
Lower microwave loss
Better GaN epitaxial compatibility
Higher electron mobility
Scalable manufacturing for 6-inch and 8-inch wafers
Today, 4H semi-insulating SiC dominates the GaN-on-SiC ecosystem for RF and microwave devices.
6H semi-insulating SiC is now primarily of historical significance.

6. Final Comparison Summary

Application Area	Preferred Polytype
RF Power Devices	4H
GaN HEMT Substrates	4H
High-Power Electronics	4H
Future Industry Roadmap	4H

Overall Conclusion

In modern RF and high-power electronics, 4H semi-insulating silicon carbide has become the definitive industry standard, while 6H represents an earlier generation technology that has largely been phased out.

7. Future Outlook for Semi-Insulating SiC

Key development directions include:
Ultra-low defect density substrates
Advanced HPSI optimization
8-inch wafer scale production
Further microwave loss reduction
Material engineering for mmWave and beyond
As RF frequencies continue to increase and power density requirements rise, substrate material quality will remain a critical competitive factor.
If your company is developing:
GaN HEMT devices
RF power amplifiers
Microwave modules
High-power electronic systems
Selecting the right semi-insulating SiC substrate can directly impact device efficiency, reliability, and long-term scalability.
For technical consultation or customized substrate solutions, feel free to contact our engineering team.