High-power GaN RF devices face a fundamental limitation that cannot be solved by electrical design alone: heat.

Gallium nitride high-electron-mobility transistors, or GaN HEMTs, can operate at high voltage, high frequency and high power density. These advantages make them important for applications such as radar, satellite communications, electronic warfare, 5G/6G infrastructure and high-power microwave systems.

However, as RF power density increases, heat generated near the transistor channel becomes increasingly difficult to remove.

Even when the GaN material itself maintains strong electrical performance, excessive junction temperature can reduce output power, degrade efficiency and shorten device lifetime.

This is why diamond has attracted increasing attention as a thermal platform for GaN.

With thermal conductivity exceeding 2,000 W/m·K for high-quality diamond materials, diamond offers significantly stronger heat-spreading capability than conventional semiconductor substrates. Recent research continues to identify diamond integration as one of the most promising approaches to near-junction cooling of high-power GaN HEMTs.

But simply placing GaN on diamond does not automatically create an ideal thermal solution.

The actual performance depends heavily on:

• GaN–diamond thermal boundary resistance
• Bonding-layer material
• Bonding-layer thickness
• Diamond thickness
• Diamond quality
• Interface roughness
• GaN thickness
• Device geometry
• Packaging and heat-sink design

For engineers selecting GaN-on-Diamond wafers, the interface can sometimes be just as important as the diamond itself.

Why High-Power GaN RF Devices Need Better Thermal Management

A GaN HEMT generates much of its heat in a very small region close to the gate on the drain side.

This creates an extremely high local heat flux.

The problem is therefore different from simply cooling an entire circuit board.

Heat must first move away from the active GaN channel, pass through the semiconductor structure and interface layers, and then spread into the substrate and package.

A simplified thermal path can be represented as:

GaN HEMT channel
↓
GaN epitaxial layer
↓
GaN/Diamond interface
↓
Diamond heat-spreading layer
↓
Package
↓
Heat sink

Every layer adds thermal resistance.

If any one layer has poor thermal conductivity or a high interface resistance, the benefit of the diamond substrate can be reduced substantially.

This is why researchers increasingly focus on near-junction thermal management rather than only improving the external heat sink.

DARPA's earlier Near Junction Thermal Transport program specifically targeted the region within approximately 100 µm of high-power electronic junctions, including the use of diamond substrates and removal of low-thermal-conductivity transition layers. Its current THREADS program continues to address thermal barriers that restrict RF power output.

What Is a GaN-on-Diamond Wafer?

A GaN-on-Diamond wafer is a heterogeneous semiconductor structure in which a GaN device or epitaxial layer is thermally integrated with diamond.

The structure may take several forms.

A simplified bonded structure could be:

GaN device layer
↓
Ultrathin bonding/intermediate layer
↓
Diamond substrate

Another approach may use:

AlGaN/GaN HEMT structure
↓
GaN buffer
↓
Nucleation/interfacial layer
↓
CVD diamond

Diamond can also be added from the top side of a finished or partially fabricated GaN device to create a near-channel heat spreader.

These different structures should not be treated as identical.

Their thermal performance depends strongly on where the diamond is located relative to the heat-generating channel.

Why Diamond Is Attractive for RF GaN

High-quality diamond offers several attractive characteristics for thermal management:

• Extremely high thermal conductivity
• High electrical resistivity
• High breakdown strength
• Excellent mechanical hardness
• Good chemical stability
• Strong heat-spreading capability

For high-power RF devices, thermal conductivity is the main attraction.

By comparison, commonly used substrates provide significantly lower bulk thermal conductivity.

However, bulk material thermal conductivity is only part of the story.

A GaN HEMT does not experience the theoretical bulk thermal conductivity of diamond directly.

Heat must first cross the GaN–diamond interface.

That makes thermal boundary resistance one of the key specifications of GaN-on-Diamond technology.

What Is Thermal Boundary Resistance?

Thermal Boundary Resistance, commonly abbreviated as TBR, describes resistance to heat flow across the interface between two different materials.

For GaN-on-Diamond, it can be written conceptually as:

GaN
→ interface
→ Diamond

Even if both materials conduct heat effectively, phonons carrying thermal energy may not transfer efficiently across the interface.

This happens because GaN and diamond have very different:

• Crystal structures
• Phonon spectra
• Bonding characteristics
• Acoustic properties

Additional bonding layers can increase the complexity further.

TBR is commonly expressed in:

m²·K/GW

A lower value generally indicates more efficient heat transfer through the interface.

Why TBR Matters So Much

Consider two GaN-on-Diamond structures using exactly the same diamond substrate.

Structure A has a thin, well-controlled interface.

Structure B contains a thicker amorphous bonding layer with poor thermal conductivity.

Even though the diamond is identical, Structure A may remove heat from the GaN channel considerably more efficiently.

This is why it can be misleading to specify only:

“Diamond thermal conductivity >2000 W/m·K.”

For device-level thermal design, buyers should also consider:

• Interface material
• Interface thickness
• TBR or TBC
• Bonding voids
• Surface roughness

How Low Can GaN-on-Diamond TBR Become?

This remains an active research area.

A 2026 study reported GaN-on-Diamond structures produced using an in-situ surface-reconstruction process combined with surface-activated bonding.

The researchers reduced diamond RMS surface roughness to below 0.5 nm and used an approximately 8 nm silicon interlayer.

The resulting structure achieved a reported TBR of approximately 7.7 ± 0.6 m²·K/GW.

This is significant because it demonstrates that interface engineering can dramatically influence the thermal performance of GaN-on-Diamond structures.

It also illustrates an important principle:

A high-performance GaN-on-Diamond wafer requires both a good diamond substrate and a good GaN–diamond interface.

Bonding Methods for GaN-on-Diamond

There are several approaches to integrating GaN with diamond.

Each has advantages and technical challenges.

1. Surface-Activated Bonding

Surface-Activated Bonding, or SAB, can join two materials at or near room temperature after surface activation.

A thin intermediate layer may be used.

Advantages can include:

• Low bonding temperature
• Reduced thermal stress
• Compatibility with heterogeneous materials
• Thin bonding interfaces
• Potentially low TBR

Room-temperature bonding is particularly attractive because GaN and diamond have different thermal expansion behavior.

High-temperature bonding may introduce additional stress during cooling.

Recent work using modified SAB has also investigated AlN and silicon interlayers for N-polar GaN/Diamond structures.

A 2026 study reported approximately 31 m²·K/GW TBR using an AlN-containing interface and measured RF loss of about 0.32 dB/mm at 40 GHz, demonstrating that the bonding interface affects both thermal and RF properties.

2. Direct Bonding

Direct bonding attempts to minimize or eliminate conventional adhesive layers.

The major goal is to shorten the thermal path between GaN and diamond.

Potential advantages include:

• Reduced interface thickness
• Lower thermal resistance
• No thick organic adhesive
• Better high-temperature stability

However, direct bonding normally requires extremely good surface preparation.

Important parameters include:

• Surface roughness
• Flatness
• Cleanliness
• Particle control
• Surface chemistry

Even small voids can reduce both bonding strength and thermal performance.

3. Van der Waals Bonding

Direct van der Waals bonding has also been investigated for GaN/Diamond integration.

One advantage is avoiding a thick low-thermal-conductivity interlayer.

However, achieving reliable contact over a large area remains difficult because wafer surfaces must be exceptionally smooth and clean.

Research comparing direct and interlayer-assisted GaN/Diamond integration shows why total interface resistance—not simply the existence of a bonding layer—must be evaluated.

4. Diamond Growth on GaN

Another strategy is to remove the original substrate and grow diamond relatively close to the GaN device layer.

A simplified flow can include:

GaN epitaxy growth
↓
Temporary carrier bonding
↓
Original substrate removal
↓
GaN backside preparation
↓
Nucleation/interlayer formation
↓
CVD diamond growth

This can place diamond very close to the active device.

However, diamond CVD conditions can be aggressive.

The GaN surface must be protected from:

• Plasma damage
• Hydrogen exposure
• High temperature
• Mechanical stress

An appropriate nucleation or dielectric layer may therefore be required.

The challenge is that this protective layer can itself increase thermal boundary resistance.

The Bonding-Layer Trade-Off

An ideal bonding layer should provide:

• Strong adhesion
• Low thermal resistance
• Low RF loss
• Minimal thickness
• High thermal stability
• Low mechanical stress

Unfortunately, these requirements can conflict.

A thicker bonding layer may improve mechanical reliability but increase thermal resistance.

A thinner layer may improve heat flow but demand much better surface preparation.

Potential interlayer materials include:

• Silicon
• AlN
• SiN
• SiO₂
• Other dielectric or amorphous layers

The interlayer therefore needs to be optimized as part of the full device architecture.

Diamond Thickness: Is Thicker Always Better?

Not necessarily.

Increasing diamond thickness can improve lateral heat spreading, especially when the heat source is highly localized.

However, thermal improvement does not increase indefinitely with thickness.

Once the diamond layer is sufficiently thick to spread heat effectively, other thermal resistances may dominate.

These can include:

• GaN layer resistance
• GaN/Diamond TBR
• Die attach
• Package
• Thermal interface material
• Heat sink

This means that doubling diamond thickness does not necessarily reduce device junction temperature by half.

Thin Diamond Layers

Thin diamond may offer:

• Shorter fabrication time
• Lower material cost
• Easier integration
• Lower mechanical stress

But if the film is too thin, lateral heat spreading may be limited.

Thick Diamond Layers

Thicker diamond may provide:

• Improved heat spreading
• Greater mechanical support
• Better handling after substrate removal

But disadvantages can include:

• Higher CVD growth cost
• Longer processing time
• Increased stress
• More difficult wafer processing

The optimum thickness must therefore be calculated based on the device heat-flux density and total package thermal architecture.

There is no universal “best diamond thickness” for every GaN RF device.

Diamond Quality Also Matters

Not all diamond films provide the same thermal performance.

Thermal conductivity depends on factors such as:

• Grain size
• Grain boundaries
• Defects
• Impurities
• Film thickness
• Growth method
• Crystal quality

Single-crystal diamond generally offers extremely high thermal conductivity.

Polycrystalline CVD diamond can also provide excellent heat spreading, but its effective thermal conductivity depends strongly on microstructure.

Nanocrystalline regions near the nucleation interface can exhibit lower thermal conductivity than larger-grain material farther from the interface.

For this reason, simply specifying “CVD diamond” is not enough for high-performance RF design.

GaN-on-Diamond vs GaN-on-SiC

GaN-on-SiC remains one of the most important platforms for high-power RF devices.

SiC provides:

• Good thermal conductivity
• Established GaN epitaxy
• High mechanical stability
• Mature RF manufacturing experience

GaN-on-Diamond aims to push thermal performance further.

The primary potential advantage is stronger heat spreading near the GaN channel.

However, GaN-on-Diamond introduces additional integration complexity.

GaN-on-SiC

Advantages:

• Mature technology
• Established wafer supply chain
• Proven RF device processing
• Good thermal performance

Challenges:

• Thermal conductivity lower than high-quality diamond
• Junction temperature can still limit extreme power density

GaN-on-Diamond

Advantages:

• Exceptional heat-spreading potential
• Lower device temperature at high heat flux
• Potential for increased RF power density
• Particularly attractive for near-junction cooling

Challenges:

• Bonding complexity
• TBR control
• Diamond processing cost
• Wafer-scale integration
• Thermal stress
• Yield

GaN-on-Diamond should therefore not simply be viewed as a direct replacement for every GaN-on-SiC device.

Its strongest value is likely to appear where thermal limitations justify the additional integration complexity.

Why Junction Temperature Matters

Reducing junction temperature can influence several device characteristics.

Potential benefits include:

• Higher RF output power
• Improved power-added efficiency
• Reduced performance degradation
• Improved reliability
• Higher allowable power density

Self-heating can change electron transport and electrical behavior inside the GaN HEMT.

As channel temperature rises, device performance may deteriorate even before catastrophic failure occurs.

A 2026 review of diamond-capped GaN HEMTs highlights self-heating as a major limitation in high-power RF operation and identifies diamond integration as a promising near-junction thermal solution.

Top-Side Diamond Cooling

GaN-on-Diamond does not always require replacing the original substrate.

Another emerging strategy is depositing diamond above the GaN HEMT.

The structure may resemble:

Diamond heat spreader
↓
Dielectric/interfacial layer
↓
AlGaN barrier
↓
GaN channel
↓
Original substrate

This allows heat to escape from both sides of the device.

Research on top-side diamond integration has shown that the dielectric interlayer itself contributes significantly to total thermal boundary resistance.

A 2025 study using a polycrystalline diamond heat spreader on AlGaN/GaN-on-SiC therefore evaluated the combined influence of the interface and AlGaN barrier rather than considering diamond conductivity alone.

Future high-power GaN RF devices could potentially use both:

• Diamond substrate cooling
• Top-side diamond heat spreading

This creates a more three-dimensional thermal management architecture.

Wafer-Scale Manufacturing Challenges

Moving GaN-on-Diamond from small research samples to commercial wafers introduces additional difficulties.

Surface Flatness

Bonding requires very good wafer flatness.

Local surface height differences can produce:

• Voids
• Weak bonding
• Non-uniform interface thickness

Surface Roughness

Sub-nanometer roughness may be required for some advanced bonding approaches.

The 2026 surface-reconstruction study reduced diamond RMS roughness below 0.5 nm before room-temperature bonding.

Particles

A single particle can prevent local contact and create a bonding void.

Particle control therefore becomes increasingly difficult as wafer diameter increases.

Wafer Bow

GaN, bonding materials and diamond have different mechanical and thermal properties.

Residual stress can lead to:

• Bow
• Warp
• Cracking
• Delamination

Bonding Yield

A good laboratory sample does not automatically translate into wafer-scale production.

Commercial manufacturing requires consistent bonding across nearly the entire usable wafer area.

Typical GaN-on-Diamond RF Applications

GaN-on-Diamond is particularly attractive for applications with extremely high RF power density.

Examples include:

Radar

High-power radar transmitters require RF amplifiers capable of maintaining performance under severe thermal loads.

Electronic Warfare

Electronic warfare systems often require high-output RF power combined with compact system size.

Satellite Communications

Thermal management is especially important in space systems where conventional cooling approaches are limited.

High-Power Microwave Systems

Diamond heat spreading may allow GaN devices to operate at higher power density while controlling junction temperature.

Future 5G and 6G Infrastructure

Higher-frequency RF systems may benefit from advanced thermal management as device dimensions decrease and local heat flux increases.

What Should Buyers Specify for a GaN-on-Diamond Wafer?

A useful RFQ should include much more than wafer diameter.

Wafer Diameter

Specify:

• 2 inch
• 3 inch
• 4 inch
• 6 inch
• Custom size

Availability may depend strongly on the integration method.

GaN Structure

Specify whether the required material is:

• GaN layer
• AlGaN/GaN HEMT structure
• N-polar GaN
• Ga-polar GaN
• Custom epi structure

GaN Thickness

Provide:

• Device-layer thickness
• Buffer thickness
• Total transferred GaN thickness

Diamond Type

Specify:

• Single-crystal diamond
• Polycrystalline CVD diamond
• Nanocrystalline diamond
• Diamond film on carrier

Diamond Thickness

Specify the required thickness or target thermal performance.

Do not select thickness based only on the assumption that thicker diamond is always better.

Bonding Method

Discuss whether the structure uses:

• Direct bonding
• Surface-activated bonding
• Modified SAB
• CVD diamond growth
• Other heterogeneous integration methods

Intermediate Layer

Specify:

• Material
• Thickness
• Maximum allowable thermal resistance

For high-power RF applications, even nanometer-scale interface layers may influence the total thermal path.

TBR

Where thermal performance is critical, buyers should consider defining a target TBR.

The test method should also be specified.

Common thermal characterization methods include:

• TDTR
• TTR
• Raman thermometry
• Thermoreflectance techniques

Surface Quality

Specify:

• RMS roughness
• Scratch requirements
• Particle requirements
• Bonding void limits

Geometry

Important parameters include:

• TTV
• Bow
• Warp
• Thickness tolerance

RF Requirements

Depending on the project, requirements may also include:

• RF insertion loss
• Dielectric properties
• Frequency range
• Carrier mobility
• Sheet resistance

Example RFQ Structure

A GaN-on-Diamond development request could include information such as:

Wafer diameter: 100 mm

GaN structure: AlGaN/GaN HEMT

GaN thickness: custom

Diamond: CVD diamond

Diamond thickness: according to thermal design

Bonding method: room-temperature direct or surface-activated bonding

Bonding interlayer: ultrathin Si or AlN-based structure

Interface: low-TBR requirement

Surface: device-grade

Geometry: controlled TTV, bow and warp

Application: high-power RF amplifier

Frequency range: customer specified

Providing the target device application is particularly useful.

A GaN-on-Diamond structure for a high-power X-band amplifier may not use the same optimum structure as one designed for millimeter-wave devices.

Thermal Simulation Should Come Before Final Wafer Design

Because GaN-on-Diamond performance depends on many interconnected parameters, thermal simulation is highly valuable before freezing the material structure.

A realistic simulation should consider:

• Gate geometry
• Heat-source dimensions
• GaN thickness
• GaN thermal conductivity
• Interface TBR
• Diamond thickness
• Diamond thermal conductivity
• Device pitch
• Package thermal resistance
• Heat-sink temperature

A common mistake is to simulate diamond using only its bulk thermal conductivity while assuming a perfect interface.

This can significantly overestimate cooling performance.

The GaN/Diamond interface should be included explicitly.

Current Research Direction in 2026

The industry is increasingly moving away from viewing thermal management as a package-level problem only.

In June 2026, DARPA described its THREADS program as targeting materials, device architectures and thermal-management approaches that can remove heat more efficiently from RF electronics and enable higher power output without proportional increases in system size and complexity.

At the materials level, recent GaN/Diamond research is focusing heavily on:

• Lower TBR
• Room-temperature heterogeneous bonding
• Ultrathin interlayers
• Improved diamond surface preparation
• N-polar GaN integration
• Lower RF loss
• Top-side diamond cooling

The 2026 report of approximately 7.7 m²·K/GW TBR using an ultrathin Si bonding layer is particularly relevant because it demonstrates how surface engineering and bonding technology can directly improve thermal transport.

These developments suggest that the next major improvement in GaN RF performance may come not only from transistor scaling or higher-quality GaN epitaxy, but also from engineering the thermal path immediately beneath and above the active channel.

Conclusion

GaN-on-Diamond wafers offer one of the most promising material platforms for overcoming the thermal limitations of high-power RF GaN devices.

Diamond provides exceptional thermal conductivity, but diamond thermal conductivity alone does not determine device cooling performance.

For a practical GaN-on-Diamond structure, engineers must consider the complete thermal path:

GaN channel
→ GaN layer
→ bonding interface
→ diamond
→ package
→ heat sink

Among these elements, GaN–diamond thermal boundary resistance is especially important.

A poorly designed interface can prevent the device from taking full advantage of diamond's thermal conductivity.

Diamond thickness must also be optimized rather than simply maximized. Once the diamond provides sufficient heat spreading, interface resistance and packaging may become the dominant thermal limitations.

For high-power RF projects, the most important GaN-on-Diamond specifications therefore include:

• GaN epi structure
• Diamond type
• Diamond thickness
• Bonding method
• Intermediate-layer material and thickness
• Thermal boundary resistance
• Surface roughness
• Bonding voids
• TTV, bow and warp
• RF loss
• Target device power density

As RF GaN devices continue toward higher frequencies and power densities, heterogeneous GaN–diamond integration is likely to become increasingly important wherever conventional substrate and package cooling approaches can no longer maintain an acceptable junction temperature.