Diamond Thermal Boundary Resistance: Why Conductivity Is Not Enough

A high-conductivity diamond plate does not guarantee a cooler device. Heat has to enter the plate and leave it again. Every bond or contact on that route can consume part of the temperature budget.

Trace the complete heat path

1. Heat originates in a device hotspot and travels through the device material.
2. It crosses a bond or thermal-interface material (TIM) into the diamond spreader.
3. After spreading through diamond, it crosses another connection into a heat sink or cold plate and eventually the coolant or air.

If a bond is voided or highly resistive, improving the diamond alone may have little effect at the hotspot.

Four quantities that should not be conflated

Quantity	Meaning	Unit
Thermal conductivity, k	Ability of a bulk material to conduct heat	W/(m K)
Material-layer thermal resistance	Resistance of a stated layer thickness and contact area	K/W
Area-normalized thermal boundary resistance, TBR	Resistance attributed to a contact or interface per unit area	m2 K/W
Thermal boundary conductance, TBC	Conductance of that interface per unit area	W/(m2 K)

For ideal one-dimensional steady conduction, a material layer has R = t/(kA), where t is thickness and A is area. Its area-normalized resistance is t/k. For the same interface definition, TBC = 1/TBR. An actual TIM or bond has a finite-thickness material layer and may also have resistance at each of its two contact boundaries. Real hotspots introduce three-dimensional spreading resistance, so a sum of one-dimensional terms is not a complete device model.

Put layer and interface values on the same scale

For the calculated examples below, assume diamond k = 1500 W/(m K) and TIM k = 3 W/(m K). These are chosen inputs, not measurements of a particular product.

Item	Area-normalized thermal resistance
Calculated 1 um diamond layer	Approximately 6.7 x 10^-10 m2 K/W
Calculated 100 um diamond layer	Approximately 6.7 x 10^-8 m2 K/W
Bonded GaN/diamond interface with approximately 2.5 nm SiOx interlayer	Reported 8.3 x 10^-9 m2 K/W
Calculated 25 um TIM layer at 3 W/(m K)	Approximately 8.3 x 10^-6 m2 K/W

On this equal-area basis, the reported bonded interface is equivalent to roughly 12 um of the assumed diamond material. The hypothetical TIM is roughly 120 times the resistance of the 100 um diamond layer. These ratios explain why the connection can dominate a comparison, but they do not predict an actual junction-temperature reduction.

Bonding and growth place the bottleneck differently

GaN-on-diamond route	How diamond is integrated	Thermal concern
Bonding	A GaN layer is joined to diamond	Bond quality, local voids, roughness, and thickness of the interfacial layer
Growth	Diamond is deposited on the back of a GaN structure	Dielectric transition layer and the initially grown, small-grain diamond near the interface

In a surface-activated GaN/diamond bonding study, an approximately 2.5 nm amorphous SiOx interlayer corresponded to 8.3 m2 K/GW, or 8.3 x 10^-9 m2 K/W. Increasing the layer to approximately 5.3 nm corresponded to 34 m2 K/GW. These are measured results for particular samples and test conditions, not generic product specifications. In the growth route, the near-interface material deserves separate attention; polycrystalline diamond thermal modeling explains why the nucleation region can matter. See also single-crystal versus polycrystalline CVD diamond.

How to judge a claimed improvement

1. Compare devices under matched power, hotspot location, package architecture, and cooling conditions.
2. Report diamond thickness, contact area, joining method, and interface test results separately from bulk conductivity.
3. Check spatial uniformity. A single measurement may miss local voids, and interpretation of thermal-reflectance methods depends on the model. A study of silicon-based fusion bonds is a measurement-method example, not a source of GaN/diamond values.
4. Return to device results: junction or hotspot temperature, reliability, and lot-to-lot consistency.

For material choices, see diamond material grades and wafer evaluation.

Is lower TBR always preferable? All else equal, generally yes. Bond strength, electrical isolation, thermal-expansion mismatch, aging, and cost still matter.

Why measure the TIM if diamond conducts heat so well? Heat must cross the TIM. Its thickness, actual contact area, voids, and aging can dominate the assembled thermal resistance.

This article explains the thermal-resistance terms; it does not specify a package design or material purchase.