In both power and RF semiconductor fields, silicon carbide (SiC) substrates have become a critical material. With the rise of electric vehicles, renewable energy, energy storage, and 5G communications, SiC substrate sizes have gradually expanded from 4-inch and 6-inch to 8-inch. However, while 8-inch conductive (N-type) substrates have been developed, 8-inch semi-insulating (SI) substrates have yet to reach mass production. This article provides a professional analysis from the perspectives of material properties, technical challenges, market demand, and industrial strategy.

1. Key Differences Between N-Type and SI-Type

N-Type (Conductive): Doped with nitrogen (N) to achieve electron conductivity, mainly used for power devices such as MOSFETs, Schottky diodes, and power modules. Its primary goal is to support high current density, high voltage tolerance, and low conduction loss.

Semi-Insulating (SI): Doped with vanadium (V) or controlled defects to achieve high resistivity (>10⁸ Ω·cm), primarily for RF devices like GaN-on-SiC microwave power amplifiers, millimeter-wave radar, and 5G base station RF modules. SI substrates require extremely uniform resistivity, low leakage, and low dielectric loss to ensure high-frequency performance.

Thus, although both are 4H SiC substrates, their application scenarios and performance specifications differ significantly. N-type substrates focus on high power and efficiency, while SI-type emphasizes high frequency, low loss, and minimal leakage.

2. Technical Challenges for 8-Inch SI SiC

Despite rapid development of 8-inch N-type substrates, 8-inch SI-type remains at the laboratory stage due to several technical challenges:

Doping Uniformity: Achieving uniform vanadium (or other compensating dopants) distribution in a 200mm crystal is extremely difficult. Non-uniformity leads to resistivity variations that can degrade RF device performance.

Defect Control: SI substrates are highly sensitive to micropipes, dislocations, and stacking faults. RF devices require minimal leakage and highly uniform dielectric properties, imposing stricter crystal growth standards than power devices.

Crystal Stress and Warpage: Large-diameter crystal growth introduces significant thermal stress, increasing the risk of cracks or warpage. Vanadium doping reduces thermal conductivity, further exacerbating stress distribution.

Inspection and Verification: SI crystals require high-precision characterization of resistivity, defects, and stress. The large volume and cost of 8-inch wafers make inspection time-consuming and technically challenging.

These challenges make 8-inch SI SiC far more difficult to mass-produce than N-type substrates.

3. Market Considerations

High Demand for Power Devices: EVs, PV inverters, energy storage, and fast chargers drive strong demand for 8-inch N-type substrates, prompting aggressive R&D investment.

Smaller RF Market: Applications like 5G base stations, millimeter-wave radar, and satellite communication require SI-type substrates, but the overall market size is significantly smaller than the power electronics market. Currently, 6-inch SI-SiC is sufficient for most RF applications.

Risk vs. Return: Developing 8-inch SI-type substrates requires substantial capital investment, yet the short-term market cannot absorb the volume. Manufacturers prioritize 8-inch N-type substrates for economic reasons.

4. Industrial Strategy and Future Outlook

Short to Medium Term (3–5 years): 8-inch N-type substrates will reach mass production first, becoming the mainstream for power devices. SI-type will remain mostly at 6-inch, with limited R&D and validation.

Long Term (5+ years): As the GaN-on-SiC RF market expands and high-frequency applications increase, coupled with breakthroughs in crystal growth and doping techniques, 8-inch SI-type substrates may eventually enter production. However, technical risk and economic feasibility remain major challenges.

5. Conclusion

The absence of 8-inch semi-insulating SiC substrates in the market is due to two primary factors:

Higher material growth and doping complexity compared to N-type, particularly in achieving uniform resistivity and defect control.

Insufficient RF market size, which does not justify the large-scale investment required for 8-inch SI wafer production.

Consequently, the industry and manufacturers focus on 8-inch N-type substrates for power applications, while 8-inch SI substrates remain in the laboratory and small-scale validation stage. In the future, as high-frequency demand grows and technical hurdles are overcome, 8-inch SI SiC substrates may gradually become viable.