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, Northwestern Polytechnical University
Citations	5
Analysis	Full AI Review Included

Executive Summary

Core Challenge: Self-heating in AlGaN/GaN planar Gunn diodes severely limits performance, causing attenuation of electronic domains and suppression of high-frequency oscillations, restricting their use as Terahertz (THz) sources.
Methodology: Two-dimensional electro-thermal numerical simulations (TCAD Silvaco Atlas) were performed to systematically analyze the DC I-V and RF output characteristics of 1 µm channel length Gunn diodes on four different substrates: Diamond, SiC, Si, and Sapphire.
Optimal Performance: The Diamond substrate demonstrated the best heat dissipation capability, resulting in the highest current, lowest operating temperature, and superior RF output.
Key RF Metrics (Diamond): The device achieved an oscillation frequency of 236.8 GHz, an RF power (P_RF) of 37 mW, and an RF-to-DC conversion efficiency (η) of 1.167%.
Thermal Failure Mode: The Sapphire substrate exhibited the worst heat sinking, leading to global device temperatures exceeding 700 K and complete suppression of Gunn oscillation due to excessive dead zone elongation.
Mitigation Strategy: Effective suppression of heat generation, either through the use of high thermal conductivity substrates (like diamond) or narrow pulse width operation (nanoseconds), is essential for realizing stable, milliwatt-level GaN THz oscillators.

Technical Specifications

Parameter	Value	Unit	Context
Best Oscillation Frequency (f)	236.8	GHz	Diamond substrate (1 µm channel).
Peak RF Power (P_RF)	37	mW	Diamond substrate.
Peak RF-to-DC Efficiency (η)	1.167	%	Diamond substrate.
Worst Oscillation Frequency (f)	895.1	GHz	Si substrate (Note: High frequency due to shortened effective travel distance/elongated dead zone).
Diamond Thermal Conductivity (κ₃₀₀)	14.8	W/cm·K	Highest conductivity analyzed (at 300 K).
Sapphire Thermal Conductivity (κ₃₀₀)	0.35	W/cm·K	Lowest conductivity analyzed (at 300 K).
Sapphire Device Temperature	732.26	K	Global temperature at 16 V bias (Oscillation suppressed).
Diamond Thermal Resistance (R_th)	1.23 x 10^-8	m²K/W	Lowest thermal resistance analyzed.
Sapphire Thermal Resistance (R_th)	18.78 x 10^-8	m²K/W	Highest thermal resistance analyzed.
Active Channel Length (L_DC)	1	µm	Standard device geometry.
AlGaN Barrier Composition	Al_0.27Ga_0.73N	N/A	25 nm thickness.
GaN Buffer Thickness	1.475	µm	Undoped layer thickness.
Substrate Thickness (Standard)	18	µm	Used for primary comparison.

Key Methodologies

Simulation Platform: Two-dimensional (2D) electro-thermal simulations were conducted using the commercially available TCAD Silvaco Atlas software.
Model Coupling: The standard heat flow equation was coupled directly to the primary drift-diffusion equations to accurately model the interaction between electron transport (Gunn effect) and lattice heating (Joule heating).
Thermal Conductivity Modeling: Temperature-dependent thermal conductivities, κ(T), were implemented using Kirchhoff’s transformation: κ(T) = κ₃₀₀ / (T/300)^α, where κ₃₀₀ and the temperature dependence coefficient (α) were specific to each material (AlGaN, GaN, Si₃N₄, and substrates).
Substrate Comparison: Four substrate materials (Diamond, SiC, Si, Sapphire) were systematically compared, typically using a fixed substrate thickness of 18 µm to isolate the effect of thermal conductivity.
Boundary Conditions: The ambient temperature was fixed at 300 K. All surfaces, except the bottom of the substrate, were set as adiabatic. Heat loss at the bottom was modeled using substrate-specific thermal resistance (R_th) values.
Transient Analysis: Pulsed I-V characteristics were simulated using drain voltage pulse widths ranging from 1 x 10^-10 s (nanoseconds) up to 1 x 10^-4 s to determine the time scale required for thermal accumulation to suppress the Negative Differential Resistance (NDR) effect.

Commercial Applications

Terahertz (THz) Signal Generation: Realization of compact, high-power, low phase noise signal sources for the 0.1 THz to 2 THz frequency band, addressing the long-term deficiency of powerful THz radiation.
Monolithic Integrated Circuits (MMICs/MTICs): The planar geometry of the Gunn diode is compatible with standard fabrication processes, enabling easy integration into complex high-frequency monolithic circuits.
High-Speed Wireless Communication: Development of high-frequency transmitters and local oscillators for next-generation 6G and beyond communication systems requiring THz bandwidth.
High-Power GaN-on-Diamond Technology: Provides theoretical validation for the use of diamond substrate transfer techniques (e.g., wafer bonding) to manage extreme heat flux in high-power GaN HEMT and diode devices.
Advanced Sensing and Spectroscopy: Enabling compact, high-efficiency THz sources for applications in non-destructive testing, security screening, and chemical/biological spectroscopy.

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