Reduction of self-heating effects in GaN HEMT via h-BN passivation and lift-off transfer to diamond substrate - A simulation study

At a Glance

Metadata	Details
Publication Date	2024-01-14
Journal	Materials Science and Engineering B
Authors	Fatima Zahrae Tijent, Mustapha Faqir, Paul L. Voss, Jean‐Paul Salvestrini, A. Ougazzaden
Institutions	International University of Rabat, Georgia Tech Lorraine
Citations	7
Analysis	Full AI Review Included

Technical Documentation & Analysis: GaN HEMT Thermal Management via Diamond Transfer

This document analyzes the research paper “Reduction of self-heating effects in GaN HEMT via h-BN passivation and lift-off transfer to diamond substrate: a simulation study” to highlight the critical role of high-quality diamond substrates and demonstrate 6CCVD’s capability to supply materials necessary for realizing these advanced thermal management solutions.

Executive Summary

The numerical simulation confirms that transferring GaN High Electron Mobility Transistors (HEMTs) onto a diamond substrate is the most effective strategy for mitigating Self-Heating Effects (SHEs) in high-power applications.

Thermal Performance: The integration of GaN HEMT onto a diamond substrate (via h-BN lift-off) reduced the maximum lattice temperature ($T_{L,max}$) from $507 \text{ K}$ (on sapphire) to $372 \text{ K}$.
Thermal Resistance Reduction: The total thermal resistance ($R_{th}$) was reduced by a factor of 5, dropping from $27 \text{ K.mm/W}$ (sapphire) to $5.5 \text{ K.mm/W}$ (diamond).
Electrical Performance Gain: This thermal improvement resulted in a $47 \text{ %}$ enhancement in both drain current and transconductance.
Reliability Improvement: The drain current reduction at high bias ($V_{DS}=40 \text{ V}$) was significantly lowered from $46 \text{ %}$ (sapphire) to $18 \text{ %}$ (diamond), extending device lifetime and reliability.
Process Advantage: The h-BN lift-off technique enables room-temperature bonding of GaN to diamond, successfully bypassing the thermal coefficient mismatch issues (wafer cracking, stress accumulation) associated with high-temperature GaN growth on diamond.
Material Requirement: Achieving the simulated performance requires ultra-high thermal conductivity diamond substrates with extremely low Thermal Boundary Resistance (TBR) interfaces ($10^{-8} \text{ m}^2\text{K W}^{-1}$).

Technical Specifications

The following table summarizes the critical performance metrics and material parameters extracted from the simulation study, demonstrating the performance gap between conventional and diamond-based GaN HEMTs.

Parameter	Value	Unit	Context
Diamond Thermal Conductivity ($k_{300}$)	$11.48$	W/cm.K	In-plane value used in simulation (Literature value $\approx 2000 \text{ W/mK}$)
Max Lattice Temperature ($T_{L,max}$)	$507$	K	Baseline: SiO₂/GaN/Sapphire HEMT
Max Lattice Temperature ($T_{L,max}$)	$372$	K	Optimized: h-BN/GaN/Diamond HEMT
Total Thermal Resistance ($R_{th}$)	$27$	K.mm/W	Baseline: GaN/Sapphire HEMT
Total Thermal Resistance ($R_{th}$)	$5.5$	K.mm/W	Optimized: h-BN/GaN/Diamond HEMT
Drain Current/Transconductance Improvement	$47$	%	Result of transfer to diamond
Drain Current Drop at $V_{DS}=40 \text{ V}$	$18$	%	h-BN/GaN/Diamond HEMT
Thermal Boundary Resistance (TBR)	$10^{-8}$	m²K W^-1	Required for GaN/Diamond interface simulation
Transient Falling Time Reduction	$750 \text{ ns}$ vs $3.3 \text{ µs}$	Time	Diamond HEMT vs. Sapphire HEMT
GaN Buffer Layer Thickness	$2$	µm	Undoped GaN
AlGaN Barrier Layer Thickness	$20$	nm	Al_0.3Ga_0.7N

Key Methodologies

The study utilized numerical simulations to model the electrothermal behavior of the GaN HEMT structures. The key steps and parameters involved in the device design and transfer process are outlined below.

Simulation and Device Structure

Software: Atlas Silvaco TCAD software used for numerical simulation.
Device Type: GaN HEMT (GaN/AlN/Al_0.3Ga_0.7N/GaN structure).
Key Dimensions:
- Gate Length: $1.5 \text{ µm}$.
- Source-Drain Spacing: $6 \text{ µm}$.
- Gate Width: $100 \text{ µm}$.
Thermal Modeling: Temperature-dependent models were used for mobility, thermal conductivity, and heat capacity.
Interface Requirement: The simulation relied on achieving an ultra-low Thermal Boundary Resistance (TBR) of $10^{-8} \text{ m}^2\text{K W}^{-1}$ at the GaN/Diamond interface to accurately model high heat dissipation.

h-BN Lift-Off Transfer Process

The h-BN lift-off technique was simulated as the mechanism to transfer the GaN HEMT structure from the growth substrate (sapphire) to the high thermal conductivity substrate (diamond) at room temperature.

Epitaxy: GaN epilayers are grown on an h-BN/sapphire template (MOCVD).
Device Fabrication: Standard processing steps (photolithography, ICP dry etching, chemical etching, e-beam evaporation) are used to define the Source, Gate, and Drain electrodes.
Transfer Preparation: A commercial water-dissolvable tape is applied to the top surface of the fully processed device.
Lift-Off: A mechanical force is applied to the tape, causing the device layer to release from the sapphire substrate at the h-BN interface.
Bonding: The released GaN HEMT is bonded to the diamond substrate at room temperature.
Interface Layer: Benzo-cyclobutene (BCB) polymer is proposed as an interfacial layer to enhance adhesion and compensate for the inherent roughness of the diamond substrate.
Final Step: The water-dissolvable tape is removed via dissolution in warm water.

6CCVD Solutions & Capabilities

The successful realization of GaN-on-Diamond HEMTs requires diamond substrates that meet stringent thermal, dimensional, and surface quality specifications. 6CCVD, an expert in MPCVD diamond, provides the necessary materials and customization services to move this simulated breakthrough into practical engineering reality.

Applicable Materials for Replication and Extension

To achieve the simulated $5.5 \text{ K.mm/W}$ thermal resistance and $372 \text{ K}$ operating temperature, the highest quality diamond is essential.

Research Requirement	6CCVD Solution	Material Specification
Ultra-High Thermal Conductivity	Optical Grade SCD	Single Crystal Diamond (SCD) offers the highest purity and thermal conductivity ($\approx 2000 \text{ W/mK}$), ideal for maximizing heat spreading efficiency.
Scalability & Cost Efficiency	High Thermal Grade PCD	Polycrystalline Diamond (PCD) plates, available up to $125 \text{ mm}$ diameter, provide excellent thermal properties for large-scale production and wafer-level integration.
Interface Adhesion/Roughness	Polished SCD/PCD	SCD substrates polished to $R_a < 1 \text{ nm}$ and inch-size PCD polished to $R_a < 5 \text{ nm}$. This ultra-smooth surface is critical for achieving the required low TBR ($10^{-8} \text{ m}^2\text{K W}^{-1}$) necessary for effective room-temperature bonding (e.g., via BCB).
Interfacial Layer Integration	Boron-Doped Diamond (BDD)	For applications requiring active thermal or electrical interfaces, 6CCVD offers BDD layers, which can be integrated as thin films ($0.1 \text{ µm}$) or thick substrates ($500 \text{ µm}$).

Customization Potential for GaN-on-Diamond Integration

The h-BN lift-off process requires precise handling and integration of the diamond substrate with the transferred GaN layer. 6CCVD offers comprehensive customization capabilities tailored for advanced HEMT fabrication:

Custom Dimensions and Thickness: We supply diamond plates and wafers in custom dimensions up to $125 \text{ mm}$ (PCD) and thicknesses ranging from $0.1 \text{ µm}$ to $10 \text{ mm}$ (Substrates). This flexibility supports both R&D prototyping and high-volume manufacturing scale-up.
Precision Polishing: The paper explicitly notes that diamond roughness is a challenge for room-temperature bonding. 6CCVD guarantees industry-leading surface roughness specifications ($R_a < 1 \text{ nm}$ for SCD), ensuring optimal adhesion and minimal thermal boundary resistance.
Integrated Metalization: While the paper used e-beam evaporation for S/G/D electrodes, 6CCVD offers in-house metalization services (Au, Pt, Pd, Ti, W, Cu) on the diamond substrate prior to bonding, enabling complex back-side contacts or heat spreader integration.

Engineering Support

6CCVD’s in-house PhD team specializes in the electrothermal properties of MPCVD diamond. We offer expert consultation to assist researchers and engineers in selecting the optimal diamond material (SCD vs. PCD), surface finish, and thickness required to replicate or extend this GaN HEMT thermal management research. We ensure the diamond substrate meets the stringent specifications needed to realize the simulated $47 \text{ %}$ performance improvement.

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

Tech Support

Original Source

DOI: https://doi.org/10.1016/j.mseb.2024.117185

References

2005 - High-power AlGaN/GaN HEMTs for Ka-band applications [Crossref]
2015 - Implementation of high-power-density X-band AlGaN/GaN high electron mobility transistors in a millimeter-wave monolithic microwave integrated circuit process [Crossref]
2002 - AlGaN/GaN HEMTs - an overview of device operation and applications [Crossref]
2008 - GaN-based RF power devices and amplifiers [Crossref]
2022 - Active thermal management of GaN-on-SiC HEMT with embedded microfluidic cooling [Crossref]
2006 - Improved thermal performance of AlGaN/GaN HEMTs by an optimized flip-chip design [Crossref]
2003 - Thermal management of AlGaN-GaN HFETs on sapphire using flip-chip bonding with epoxy underfill [Crossref]
1987 - The intrinsic thermal conductivity of AIN [Crossref]
2010 - AlN passivation over AlGaN/GaN HFETs for surface heat spreading [Crossref]