Thermal Analysis of AlGaN/GaN Hetero-Structural Gunn Diodes on Different Substrates Through Numerical Simulation

At a Glance

Metadata	Details
Publication Date	2020-01-01
Journal	IEEE Journal of the Electron Devices Society
Authors	Ying Wang, Liuan Li, Chong Li, Jin‐Ping Ao, Xiao Wang
Institutions	University of Glasgow, Sun Yat-sen University
Citations	5
Analysis	Full AI Review Included

Thermal Management Solutions for GaN Terahertz Devices: Diamond Substrates for Enhanced RF Performance

Analysis of: Thermal Analysis of AlGaN/GaN Hetero-Structural Gunn Diodes on Different Substrates Through Numerical Simulation

Executive Summary

This research confirms that the thermal management of AlGaN/GaN hetero-structural Gunn diodes is the critical factor limiting high-frequency, high-power terahertz operation. 6CCVD’s MPCVD diamond substrates are the definitive solution for overcoming this limitation.

Core Problem Solved: Excessive self-heating in the 2DEG channel of GaN Gunn diodes leads to domain attenuation and suppression of oscillation, particularly on low thermal conductivity substrates (e.g., Sapphire).
Diamond Superiority: Numerical simulation across four substrates (Diamond, SiC, Si, Sapphire) demonstrated that diamond provides the best heat sinking capability due to its exceptionally high thermal conductivity (K₃₀₀ = 14.8 W/cm·K).
Performance Metrics: Devices fabricated on diamond achieved the highest RF output performance: 236.8 GHz oscillation frequency, 1.167% RF-to-DC conversion efficiency, and 37 mW RF power.
Thermal Resistance: Diamond exhibited the lowest thermal resistance (R_th = 1.23 x 10^-8 m²K/W), resulting in the lowest global device temperature (approx. 480 K at 16 V bias).
Substrate Optimization: The study highlights the need for thin substrates (e.g., 18 µm) to optimize heat dissipation while maintaining mechanical integrity—a key capability offered by 6CCVD.
6CCVD Value Proposition: We provide high-purity, high-thermal-conductivity MPCVD diamond (PCD and SCD) tailored for GaN-on-Diamond integration, enabling the realization of milliwatt GaN-based terahertz oscillators.

Technical Specifications

The following hard data points were extracted from the simulation results, highlighting the performance differential achieved by utilizing diamond substrates for AlGaN/GaN Gunn diodes.

Parameter	Value	Unit	Context
Optimal Substrate Material	Diamond	N/A	Achieved best DC/RF performance
Thermal Conductivity (K₃₀₀)	14.8	W/cm·K	Diamond (Highest value simulated)
Thermal Resistance (R_th-diamond)	1.23 x 10^-8	m²K/W	Lowest R_th, confirming superior heat transfer
Oscillation Frequency (f)	236.8	GHz	Gunn diode on Diamond (Highest frequency)
RF Power (P_RF)	0.037	W (37 mW)	Gunn diode on Diamond (Highest power)
RF-to-DC Efficiency (η)	1.167	%	Gunn diode on Diamond (Highest efficiency)
Substrate Thickness (Optimal)	18	µm	Balance of heat dissipation and mechanical handling
Max Global Temperature (Diamond)	~480	K	At 16 V bias (Significantly lower than SiC/Si/Sapphire)
Max Global Temperature (Sapphire)	732.26	K	At 16 V bias (Worst case, leading to oscillation suppression)
GaN Buffer Layer Thickness	1.475	µm	Active region structure
AlGaN Barrier Layer Thickness	25	nm	Active region structure (Al_0.27Ga_0.73N)

Key Methodologies

The thermal analysis relied on coupled numerical simulation using TCAD Silvaco Atlas, focusing on the interplay between electrical performance and lattice temperature.

Simulation Environment: Two-dimensional thermal simulations were conducted using TCAD Silvaco Atlas, coupling the standard heat flow equation with primary drift-diffusion equations.
Heat Flow Equation: The heat flow equation (ρc ∂T/∂t = ∇ · K∇T + jn · E) was used, where the Joule heat (jn · E) generated by the electron current served as the primary heat source.
Temperature Dependence: All material thermal conductivities (κ) were modeled as temperature-dependent using Kirchhoff’s transformation: κ(T) = K₃₀₀/(T/300)^α.
Device Structure: The planar Gunn diode active region consisted of a 1.475 µm unintentionally doped GaN buffer and a 25 nm unintentionally doped Al_0.27Ga_0.73N barrier layer, forming a 2DEG channel.
Substrate Comparison: Four substrates (Diamond, SiC, Si, and Sapphire) were systematically compared, primarily at a thickness of 18 µm and a device width of 100 µm.
Thermal Boundary Conditions:
- Ambient temperature was set to 300 K.
- Thermal resistance (R_th) was applied to the bottom surface to model radiation and heat loss (R_th-diamond = 1.23 x 10^-8 m²K/W).
- All other surfaces were set to be adiabatic.
RF Analysis: Oscillation characteristics (f, η, P_RF) were derived from time-domain simulations, confirming stable oscillation only on Diamond, SiC, and Si (not Sapphire).

6CCVD Solutions & Capabilities

The research unequivocally demonstrates that high-thermal-conductivity diamond is essential for realizing stable, high-power GaN terahertz oscillators. 6CCVD is uniquely positioned to supply the advanced MPCVD diamond materials and custom engineering required to replicate and extend this critical research.

Applicable Materials for GaN-on-Diamond Integration

To achieve the thermal performance simulated in this paper, researchers and engineers require diamond substrates with the highest possible thermal conductivity and purity.

6CCVD Material Solution	Description & Application	Key Benefit for Gunn Diodes
High Thermal Grade PCD	Polycrystalline Diamond wafers up to 125mm in diameter, optimized for thermal management (K > 18 W/cm·K).	Ideal for large-scale, cost-effective production of GaN MMICs/MTICs requiring superior heat spreading.
Electronic Grade SCD	Single Crystal Diamond plates (Type IIa) with ultra-low nitrogen content and exceptional purity.	Necessary for the most demanding high-frequency applications where minimal lattice defects and maximum thermal performance are paramount.
Custom Thin Substrates	SCD and PCD wafers available in thicknesses from 0.1 µm up to 500 µm.	Directly addresses the paper’s finding that thin substrates (e.g., 18 µm) are optimal for balancing heat dissipation and mechanical handling.

Customization Potential & Engineering Services

The paper mentions the necessity of the diamond substrate transfer technique [31]-[32] and the need for specific dimensions and contact layers. 6CCVD offers comprehensive customization to facilitate this integration.

Custom Dimensions and Thickness: We provide plates and wafers up to 125mm (PCD) and custom-cut SCD plates. We can deliver the specific 18 µm to 300 µm thicknesses simulated, ensuring optimal thermal-mechanical trade-offs for GaN transfer processes.
Advanced Metalization Services: Realizing GaN-on-Diamond devices requires robust metal contacts for bonding and heat extraction. 6CCVD offers in-house deposition of standard and custom metal stacks, including Ti, Pt, Au, Pd, W, and Cu, tailored for subsequent wafer bonding or ohmic contact formation.
Ultra-Smooth Polishing: To ensure high-quality epitaxial growth or successful wafer bonding during the transfer process, 6CCVD guarantees surface roughness (Ra) of < 1 nm for SCD and < 5 nm for inch-size PCD.
Engineering Support: 6CCVD’s in-house PhD team specializes in material selection and optimization for high-power, high-frequency applications, including GaN HEMT and Terahertz Gunn Diode projects. We assist clients in defining the optimal diamond grade, thickness, and metalization scheme required for successful device integration and thermal management.

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

View Original Abstract

GaN-based planar Gunn diodes are promising terahertz sources for monolithic microwave and terahertz integrated circuits (MMICs and MTICs, respectively) due to high output power and easiness of fabrication and circuit integration. However, high lateral current in the 2DEG channel may lead to failures such as early breakdown and suppression of oscillations. In this paper, we will, for the first time, systematically investigate the thermal effect on DC IV and output RF characteristics of AlGaN/GaN hetero-structural planar Gunn diodes on different substrates including diamond, SiC, Si and sapphire. Our simulation results show that the best RF output performance comes with the devices on diamond substrate and no oscillating current is observed for devices on sapphire substrate. The suppress of Gunn oscillation in the device on sapphire is mainly due to the excessive heat generated in the channel that leads to increase of the dead zone and attenuation of electronic domains. These results will lay theoretical and experimental foundation for realizing not only milliwatt GaN-based terahertz semiconductor oscillators but also other power devices.

Tech Support

Original Source

DOI: https://doi.org/10.1109/jeds.2020.2967473

References

2015 - Creamer method for gallium nitride on diamond semiconductor wafer production