Thermal Modeling of GaN HEMT Devices With Diamond Heat-Spreader

At a Glance

Metadata	Details
Publication Date	2020-01-01
Journal	IEEE Journal of the Electron Devices Society
Authors	Marzieh Mahrokh, Hongyu Yu, Yuejin Guo
Institutions	Shenzhen Third People’s Hospital, Southern University of Science and Technology
Citations	15
Analysis	Full AI Review Included

Technical Documentation & Analysis: Diamond Heat Spreaders for GaN HEMT Devices

Research Paper Analyzed: Thermal Modeling of GaN HEMT Devices With Diamond Heat-Spreader (Mahrokh et al., JEDS 2020)

Executive Summary

This analysis confirms the critical role of high-thermal-conductivity Single-Crystal Diamond (SCD) in managing heat dissipation for next-generation, high-power GaN HEMT devices, directly aligning with 6CCVD’s core material offerings.

Superior Thermal Performance: Single-Crystal CVD Diamond (K ≈ 2000 W/mK) is validated as the optimal heat spreader, significantly outperforming SiC (K ≈ 420 W/mK) for high areal power densities (10 W/mm).
TBR Sensitivity is Key: The study highlights that Thermal Boundary Resistance (TBR) at the GaN/Diamond interface is the primary limiting factor. GaN-on-Diamond structures exhibit high junction temperature sensitivity (1.28°C per unit of TBR).
Material Quality Mandate: Achieving the full thermal benefit of GaN-on-Diamond requires extremely low TBR, necessitating high-quality bonding or direct growth, which relies on ultra-smooth, high-purity SCD substrates.
Critical TBR Threshold: The merit of direct GaN-on-Diamond integration over GaN/SiC-on-Diamond is lost if the TBR exceeds a critical threshold (22 m²K/GW to 35 m²K/GW).
6CCVD Value Proposition: 6CCVD specializes in providing the necessary high-pgrade SCD wafers (up to 500 µm thick, Ra < 1 nm polish) and custom metalization services required to minimize TBR and maximize device reliability.

Technical Specifications

The following hard data points were extracted from the thermal modeling analysis, defining the requirements for high-performance GaN/Diamond integration.

Parameter	Value	Unit	Context
CVD Diamond Thermal Conductivity (K)	2000	W/mK	@ 300°K (SCD)
SiC Thermal Conductivity (K)	420	W/mK	@ 300°K
Areal Power Density (P_A)	10	W/mm	Simulation input for HEMT
GaN Layer Thickness	1	µm	Active device layer
CVD-Diamond Heat Spreader Thickness	100	µm	Modeled thickness
GaN/SiC Interface TBR (Fixed)	4.4	m²K/GW	Baseline value
GaN/Diamond Interface TBR Range	0 to 75	m²K/GW	Simulation range
Junction Temp Sensitivity (GaN-on-D)	1.28	°C per unit TBR	High sensitivity case (critical interface)
Junction Temp Sensitivity (GaN/SiC-on-D)	0.43	°C per 10 units TBR	Low sensitivity case
Critical TBR Crossover Point	22 to 35	m²K/GW	Limit where GaN-on-Diamond loses advantage over 50 µm SiC structure
Base Plate/Carrier Material	CuW	1.4 mm	Attached via 25 µm AuSn solder

Key Methodologies

The thermal performance comparison was conducted using Finite Volume Method (FVM) simulations in ANSYS Icepak, focusing on two primary integration methods and two thermal models.

Simulation Platform: Finite Volume Method (FVM) performed using ANSYS Icepak.
Device Structure: 10-finger GaN HEMT device (10 x 125 µm hotspot geometry) with a 20 µm gate pitch.
Integration Methods Modeled:
- GaN/SiC-on-Diamond: GaN-on-SiC substrate bonded to SCD heat spreader.
- GaN-on-Diamond: Host substrate and nucleation layers etched away, GaN bonded directly to SCD.
Thermal Models Compared:
- Constant-K Model: Uses room-temperature thermal conductivity values (overestimates performance).
- Temperature-Dependent-K Model: Accounts for the reduction in thermal conductivity (K) as junction temperature rises (more accurate for high-power operation).
Material Stacks:
- GaN (1 µm) / SiC (100 µm or 50 µm) / SCD (100 µm) / AuSn (25 µm) / CuW (1.4 mm).
- GaN (1 µm) / SCD (100 µm) / AuSn (25 µm) / CuW (1.4 mm).
Thermal Boundary Conditions: A fixed temperature of 25°C (or 35°C for comparative analysis) was set at the bottom side of the CuW carrier.

6CCVD Solutions & Capabilities

The research confirms that the success of GaN-on-Diamond technology hinges entirely on the quality and processing of the CVD diamond heat spreader, specifically its thermal conductivity and the resulting interface TBR. 6CCVD is uniquely positioned to supply the materials and processing required to meet these stringent demands.

Applicable Materials

To replicate or extend this research, engineers require diamond with the highest possible thermal conductivity and surface quality to ensure minimal TBR.

6CCVD Material	Specification	Application Justification
Optical Grade Single Crystal Diamond (SCD)	K > 2000 W/mK, Low Defect Density	Required to match the modeled 2000 W/mK performance and minimize phonon scattering near the interface.
High-Purity Polycrystalline Diamond (PCD)	K up to 1800 W/mK (Inch-size wafers)	Suitable for large-area heat spreading applications (up to 125mm) where the highest SCD purity is not cost-effective, but high K is still essential.
Custom Thickness Plates	SCD (0.1 µm - 500 µm)	The paper modeled 100 µm thick heat spreaders; 6CCVD can supply custom thicknesses to optimize thermal resistance and cost.

Customization Potential for Low-TBR Integration

The paper emphasizes that low TBR is achieved through high-quality bonding (e.g., Thermo-Compression Bonding, Surface Activated Bonding). This requires exceptional surface preparation and precise metalization stacks, both of which are 6CCVD core capabilities.

Precision Polishing: To facilitate low-TBR bonding, the diamond surface must be ultra-smooth. 6CCVD guarantees Ra < 1 nm polishing for SCD and Ra < 5 nm for inch-size PCD, ensuring optimal surface contact for bonding.
Custom Dimensions: The modeled heat spreader was 7x7 mm². 6CCVD offers custom laser cutting and shaping services for both SCD and PCD plates/wafers up to 125mm in diameter.
Interface Metalization: The model utilized AuSn solder bonding. 6CCVD offers comprehensive in-house metalization services, including the deposition of critical adhesion and barrier layers (e.g., Ti/Pt/Au, W, Cu) necessary for robust, low-resistance bonding interfaces prior to AuSn application.

Engineering Support

The high sensitivity of junction temperature to TBR (1.28°C per unit TBR for GaN-on-Diamond) makes material selection and interface engineering paramount.

6CCVD’s in-house PhD team provides expert consultation on:

Thermal Stack Optimization: Assisting researchers in selecting the optimal diamond thickness and grade to minimize overall thermal resistance for similar GaN HEMT Thermal Management projects.
Interface Engineering: Advising on appropriate metalization schemes (e.g., Ti/Pt/Au vs. Ti/W/Au) to achieve the lowest possible TBR for specific bonding techniques (TCB, SAB).
Material Sourcing: Ensuring global supply chain reliability with DDU default shipping and DDP options available worldwide.

Call to Action: For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

Harvesting the potential performance of GaN-based devices in terms of the areal power density and reliability, relies on the efficiency of their thermal management. Integration of extremely high thermal conductivity Single-crystalline CVD-diamond serves as an efficient solution to their strict thermal requirements. However, the major challenge lies in the Thermal Boundary Resistance (TBR) at the interface of GaN/Diamond or SiC/Diamond. Junction temperature of the device shows a sensitivity of 1.28°C for every unit of TBR for GaN-on-Diamond compared to 0.43°C for every 10 units of TBR for GaN/SiC-on-Diamond. Finite Volume Thermal Analysis has shown a limit of around 22 m<sup>2</sup>K/GW beyond which the merit of proximity to the heat-source for GaN-on-Diamond can no more outperform GaN/SiC-on-Diamond. Besides, due to the temperature dependency of the thermal conductivity K, an increase in the temperature causes an increase in the thermal resistivity of the device which is more significant in high power operations. Simplified assumption of constant K overestimates the device performance by resulting in 17.4°C lower junction temperature for the areal power density of 10W/mm. Other part of the project regarding the in-house growth of CVD-diamond to be bonded to the GaN device has been simultaneously in progress.

Tech Support

Original Source

DOI: https://doi.org/10.1109/jeds.2020.3023081

References

2008 - 55% PAE and high power Ka-band GaN HEMTs with linearized transconductance via n+ GaN source contact ledge [Crossref]