Semi-insulating silicon carbide is an important substrate material for high-frequency and high-power RF devices because it combines high electrical resistivity with high thermal conductivity.

Unlike conductive n-type SiC substrates used mainly for vertical power devices, semi-insulating SiC is designed to suppress parasitic electrical conduction through the substrate. This makes it particularly suitable for RF, microwave and high-frequency device structures where substrate loss can directly affect gain, efficiency and frequency performance.

Among semi-insulating SiC materials, two approaches have historically been used:

• High-Purity Semi-Insulating SiC, or HPSI
• Vanadium-doped semi-insulating SiC

Although both can provide very high resistivity, the mechanism used to achieve semi-insulating behavior is different.

For RF device engineers and wafer buyers, this difference can influence:

• Electrical stability
• RF loss
• Thermal behavior
• Deep-level traps
• Back-gating effects
• Epitaxial compatibility
• Device reliability

For this reason, substrate selection should not be based on resistivity alone.

Why RF Devices Need Semi-Insulating SiC

An RF device operating at microwave or millimeter-wave frequencies must minimize unwanted current flow through the substrate.

If the substrate is electrically conductive, parasitic capacitance and current paths can increase RF losses.

Semi-insulating SiC provides two important characteristics simultaneously:

High electrical resistivity

and

High thermal conductivity

This combination is especially attractive for high-power RF applications.

Research on semi-insulating SiC has long identified microwave and high-frequency electronics as key applications because the material can provide both the resistance required for low-loss operation and sufficient thermal conductivity to dissipate device heat.

Typical applications include:

• RF power amplifiers
• Microwave devices
• Radar electronics
• Satellite communication
• Telecom infrastructure
• High-frequency switches
• RF front-end modules
• High-power HEMT structures
• High-temperature RF electronics

For these applications, substrate quality affects not only wafer processing but also the electrical environment underneath the active device.

1. What Is HPSI 4H-SiC?

HPSI means:

High-Purity Semi-Insulating Silicon Carbide

Unlike intentionally vanadium-doped SiC, HPSI material is generally produced using high material purity and intrinsic deep-level defects to compensate residual shallow impurities.

Current commercial 4H-SiC HPSI products specifically identify the material as not vanadium-doped.

The basic objective is to keep the free-carrier concentration extremely low so that bulk substrate resistivity remains high.

Residual impurities commonly involved include:

• Nitrogen
• Boron
• Other trace impurities

Intrinsic defects can introduce deep electronic levels that compensate these residual shallow dopants.

Research on HPSI 4H-SiC has identified vacancy-related defects, divacancies and carbon vacancy–carbon antisite complexes among the defects that can contribute to the semi-insulating behavior.

This compensation mechanism allows high resistivity without relying on deliberate heavy-metal doping.

2. What Is Vanadium-Doped Semi-Insulating SiC?

The second approach is to intentionally introduce vanadium into SiC.

Vanadium produces deep energy levels inside the SiC bandgap.

These deep levels can compensate electrically active shallow donors and acceptors, reducing the number of free charge carriers.

The simplified concept is:

Residual donor/acceptor impurities → compensated by deep vanadium levels → low carrier concentration → high resistivity

Vanadium compensation has been used for many years to manufacture semi-insulating SiC.

Research has identified vanadium-related deep acceptor levels around approximately 1 eV below the conduction-band region, although the exact electrical behavior depends on vanadium concentration and other intrinsic defects.

It is important to distinguish:

Vanadium-doped

from

4H-SiC

because these describe two different characteristics.

Vanadium describes the compensation/doping method.

4H describes the SiC crystal polytype.

Vanadium-doped semi-insulating material has historically existed in several SiC polytypes. Some current commercial semi-insulating vanadium-doped substrates are, for example, offered as 6H-SiC rather than 4H-SiC.

Therefore, an RFQ should always specify both:

Polytype: 4H-SiC

and

Semi-insulating method / resistivity requirement

rather than simply requesting "semi-insulating SiC."

3. HPSI vs Vanadium-Doped SiC

The fundamental difference is how the semi-insulating electrical behavior is created.

HPSI technology became increasingly important partly because vanadium introduces additional deep-level centers.

These trap states may interact with device operation, particularly under high electric field or high-temperature conditions.

Research has reported that electrons trapped by vanadium can be thermally released and may contribute to effects such as threshold-voltage shift or back-gating in field-effect device structures. This is one reason intrinsic-defect-based HPSI material became increasingly attractive.

However, this does not mean every vanadium-doped substrate is unsuitable for RF use.

The correct comparison should consider:

• Device structure
• RF frequency
• Operating temperature
• Power density
• Epitaxial structure
• Trap distribution
• Resistivity stability
• Supplier material quality

4. Resistivity: One of the Most Important RF Parameters

For semi-insulating SiC, resistivity is a primary purchasing parameter.

Its purpose is to limit unwanted current flowing through the substrate.

Current commercial HPSI 4H-SiC specifications can reach values of at least the 10⁶–10⁸ Ω·cm range or higher depending on supplier, grade and measurement condition. One current commercial HPSI product family specifies ≥10⁸ Ω·cm for selected production material.

Vanadium-doped semi-insulating products can also exhibit extremely high resistivity.

For example, commercial vanadium-doped semi-insulating SiC materials have been reported with resistivity above 10⁹ Ω·cm, depending on polytype and product specification.

However, comparing two wafers only by the statement:

Resistivity >10⁸ Ω·cm

is not sufficient.

The buyer should also ask:

• At what temperature was resistivity measured?
• What measurement method was used?
• Is the value minimum, typical or average?
• Is resistivity uniform across the wafer?
• How stable is resistivity after high-temperature processing?
• Does the wafer contain electrically active deep traps?

For RF devices, resistivity stability can be as important as the room-temperature absolute value.

5. Why Deep-Level Defects Matter

A semiconductor wafer can have high measured resistivity and still contain electrically active defects.

Semi-insulating SiC relies on deep electronic levels to suppress free carriers.

However, deep levels can also act as traps.

Depending on device architecture, traps may capture or release carriers during:

• RF operation
• High-field operation
• Heating
• Cooling
• Bias switching
• Long-duration operation

Potential effects include:

• Current collapse
• Dynamic resistance changes
• Threshold-voltage shift
• Back-gating
• RF dispersion
• Device instability

This is why HPSI versus vanadium-doped selection cannot be reduced to a simple question of which wafer has higher resistance.

A high-quality RF substrate must combine:

high resistivity + electrical stability + controlled defects + good thermal properties

6. Crystal Defects in Semi-Insulating 4H-SiC

RF substrate quality is also influenced by crystallographic defects.

Important defect categories include:

• Micropipes
• Threading screw dislocations
• Threading edge dislocations
• Basal plane dislocations
• Stacking faults
• Polytype inclusions
• Carbon inclusions
• Surface pits
• Subsurface damage

Micropipes

Micropipes are hollow-core defects associated with large Burgers-vector screw dislocations.

Although modern SiC crystal growth has dramatically reduced micropipe density, they remain an important historical and purchasing parameter.

For RF-grade substrates, buyers should determine:

• Maximum micropipe density
• Whether the value is guaranteed or typical
• Inspection method
• Mapping area

Threading Dislocations

Threading screw dislocations and threading edge dislocations may propagate into subsequent epitaxial layers.

Their effect depends on the final device design.

Instead of specifying only:

Low dislocation density

the RFQ should identify the required defect categories and acceptance levels.

7. TTV Matters Even for RF Substrates

Electrical properties often receive the most attention for semi-insulating SiC, but wafer geometry is equally important during device fabrication.

TTV means:

Total Thickness Variation

It represents the difference between maximum and minimum wafer thickness within the specified measurement area.

High TTV can create problems during:

• Lithography
• Epitaxy
• Wafer chucking
• Backside processing
• Wafer bonding
• Thinning
• Metrology
• Automated handling

Commercial HPSI substrate specifications demonstrate that values such as TTV ≤10 µm are technically achievable for selected grades and wafer sizes, although the required value must always be agreed between buyer and supplier.

SEMI's SiC wafer standardization work also identifies thickness, tolerance, flatness, orientation and defects as fundamental physical substrate characteristics.

8. Bow and Warp

TTV describes thickness variation, but it does not describe the complete wafer shape.

Two additional parameters are important:

Bow

and

Warp

A wafer can have acceptable TTV while still exhibiting excessive global deformation.

This becomes increasingly important as wafer diameter increases.

Excessive warp may cause:

• Robot handling errors
• Chucking problems
• Focus problems
• Coating non-uniformity
• Epitaxial non-uniformity
• Inspection errors

Therefore a serious RF wafer specification should normally contain:

TTV + Bow + Warp

rather than TTV alone.

9. Surface Roughness and CMP Quality

The Si-face of a semi-insulating SiC substrate is normally prepared for subsequent epitaxial growth.

The surface must therefore have very low roughness and minimal polishing damage.

Commercial RF-oriented semi-insulating SiC substrates can achieve sub-nanometer surface roughness; current examples specify roughness below approximately 5 Å for selected products.

However, Ra alone does not completely describe an epi-ready surface.

The buyer should also inspect:

• Scratches
• Pits
• Haze
• Polishing marks
• Particles
• Contamination
• Surface damage
• Step bunching
• Edge defects

A wafer can have very low measured Ra while still containing isolated defects that affect epitaxial growth.

10. Si-Face CMP and Backside Finish

For RF epitaxy applications, the front surface is commonly the silicon face.

A typical substrate configuration may be:

Si-face: CMP, epi-ready

C-face: optical polish or specified backside finish

The exact surface preparation should always be included in the RFQ.

For example:

Front: Si-face CMP, epi-ready
Back: optical polished

Simply requesting:

Double-side polished

may not provide sufficient information about the actual CMP quality required for epitaxy.

11. Orientation: On-Axis vs Off-Axis

Another important difference between RF semi-insulating substrates and power-device substrates is orientation.

Conductive 4H-SiC power substrates are commonly supplied with an intentional off-axis orientation for homoepitaxial growth.

RF semi-insulating substrates, especially those intended as foreign substrates for nitride epitaxy, are frequently supplied on-axis.

Current commercial HPSI 4H-SiC RF products use on-axis orientations for certain substrate grades.

The RFQ should therefore clearly specify:

• Polytype
• Surface plane
• On-axis or off-axis
• Orientation tolerance
• Si-face or C-face

12. Diameter Selection

Semi-insulating SiC has historically been available in smaller diameters than mainstream conductive SiC power substrates.

Common RF substrate diameters include:

• 50.8 mm
• 76.2 mm
• 100 mm
• 150 mm

Large-diameter development continues, but availability depends strongly on:

• Polytype
• Compensation method
• Grade
• Supplier
• Defect specification

Buyers should not assume that every n-type 200 mm SiC specification is automatically available as a 200 mm HPSI wafer.

For example, current commercial product information shows 4H HPSI offerings in 100 mm and 150 mm while 200 mm availability may be associated with other SiC product families.

For this reason, diameter availability should be confirmed before finalizing a device process.

13. Typical RFQ Parameters for Semi-Insulating 4H-SiC

A professional RFQ should contain more than just diameter and resistivity.

14. Questions to Ask the Supplier

Before ordering semi-insulating SiC for RF applications, buyers should ask:

1. Is the material HPSI or intentionally vanadium-doped?
2. Is the polytype 4H or 6H?
3. What is the minimum guaranteed resistivity?
4. How is resistivity measured?
5. Is resistivity measured at room temperature only?
6. What is the wafer diameter?
7. Is the wafer on-axis?
8. What are the TTV, bow and warp limits?
9. What are the micropipe and dislocation specifications?
10. Is the Si-face CMP epi-ready?
11. What surface roughness is guaranteed?
12. Are particle and scratch maps available?
13. Is wafer-level traceability provided?
14. Can a certificate of inspection be supplied?
15. Can customized RF substrate specifications be supported?

These questions make it easier to compare quotations from different substrate suppliers.

15. HPSI or Vanadium-Doped: Which Should Be Selected?

There is no single specification suitable for every RF device.

However, for modern 4H-SiC RF substrate applications, HPSI is particularly attractive because it can provide very high resistivity without deliberate vanadium doping.

This removes one source of intentionally introduced deep-level impurity and has helped HPSI become an important material platform for advanced RF structures.

Vanadium-doped SiC can still provide excellent resistivity and remains technically relevant, especially for established device platforms and specific substrate/polytype combinations.

The correct evaluation should therefore include:

resistivity

plus

deep-level stability

plus

crystal quality

plus

geometry

plus

surface preparation

rather than selecting a wafer based on a single number.

Conclusion

Semi-insulating SiC is more than simply "high-resistivity SiC."

For RF and microwave devices, the substrate must provide a combination of:

• High bulk resistivity
• Stable electrical behavior
• High thermal conductivity
• Low crystal defect density
• Controlled TTV, bow and warp
• Low surface roughness
• Epi-ready CMP quality
• Reliable wafer-level inspection

HPSI 4H-SiC achieves semi-insulating behavior primarily through high material purity and intrinsic deep-level compensation rather than intentional vanadium doping.

Vanadium-doped SiC uses deliberately introduced deep levels to compensate residual carriers.

Both approaches can create high-resistivity substrates, but their defect physics and electrical behavior are different.

For RF substrate procurement, the most useful specification is therefore not simply:

Semi-insulating 4H-SiC, high resistivity

but a complete combination of:

Polytype + Compensation Method + Resistivity + Defect Density + TTV + Bow/Warp + Surface Roughness + CMP Quality + Inspection Criteria.

This provides both the supplier and RF device manufacturer with a clear technical basis for wafer qualification.

FAQ

What does HPSI mean in SiC wafers?

HPSI means High-Purity Semi-Insulating. In modern 4H-SiC substrates, high resistivity can be achieved through controlled material purity and intrinsic deep-level compensation without intentionally doping the crystal with vanadium.

Is HPSI the same as vanadium-doped SiC?

No. Both materials can be semi-insulating, but the compensation mechanisms are different. HPSI generally relies on intrinsic deep-level defects and high purity, while vanadium-doped material intentionally introduces vanadium-related deep levels.

What resistivity is required for semi-insulating SiC?

The required value depends on the RF device and measurement conditions. Commercial semi-insulating SiC products can specify resistivity from approximately 10⁶–10⁸ Ω·cm or substantially higher depending on material type, grade and supplier. Buyers should specify both the minimum resistivity and measurement conditions.

Is 4H-SiC always better than 6H-SiC for RF?

Not automatically. Polytype selection depends on device architecture, epitaxial process and established manufacturing platform. However, 4H-SiC has become an important commercial platform for high-purity semi-insulating RF substrates.