Thermal Performance Improvement of AlGaN/GaN HEMTs Using Nanocrystalline Diamond Capping Layers

At a Glance

Metadata	Details
Publication Date	2022-09-07
Journal	Micromachines
Authors	Huaixin Guo, Yizhuang Li, Xinxin Yu, Jianjun Zhou, Yuechan Kong
Citations	12
Analysis	Full AI Review Included

Executive Summary

This study successfully demonstrates the integration of Nanocrystalline Diamond Capping (NDC) layers onto AlGaN/GaN High-Electron Mobility Transistors (HEMTs) to significantly enhance thermal management and electrical performance for RF applications.

Thermal Resistance Reduction: The NDC-GaN HEMTs achieved a 21.4% reduction in thermal resistance (R) compared to conventional SiN-passivated devices (20.54 K/W vs. 26.12 K/W).
Output Current Improvement: The maximum drain current (I_DS) at V_GS = 1 V showed a 27.9% improvement, reaching 950.45 mA/mm, directly attributed to reduced self-heating and less temperature-dependent mobility degradation.
RF Performance Gains: Small signal gain at 10 GHz was improved by an average of 36.7% (NDC: 10.83-11.80 dB) over SiN-GaN HEMTs.
High-Frequency Operation: The cut-off frequency (f_T) was slightly improved by 1.8%, reaching 34.6 GHz, demonstrating compatibility with high-speed RF requirements.
Novel Integration Approach: The “diamond-before-gate” approach was adopted, utilizing a multi-step etching technique and a thin (20 nm) SiN isolation layer to ensure compatibility between the high-temperature NDC growth (710 °C) and the subsequent Schottky gate fabrication on 0.3 µm gate length devices.
Mechanism Validation: Finite element method simulations confirmed that the NDC layer acts as a key thermal spreader, effectively solving heat accumulation near the hot spot.

Technical Specifications

Parameter	Value	Unit	Context
Thermal Resistance (R)	20.54	K/W	NDC-GaN HEMTs
Thermal Resistance (R)	26.12	K/W	SiN-GaN HEMTs (Conventional)
R Reduction	21.4	%	NDC vs. SiN
Max Drain Current (I_DS)	950.45	mA/mm	NDC-GaN, V_GS = 1 V
Max Drain Current (I_DS)	743.28	mA/mm	SiN-GaN, V_GS = 1 V
I_DS Improvement	27.9	%	NDC vs. SiN
Cut-Off Frequency (f_T)	34.6	GHz	NDC-GaN
Cut-Off Frequency (f_T)	34.0	GHz	SiN-GaN
f_T Improvement	1.8	%	NDC vs. SiN
Small Signal Gain (10 GHz)	10.83-11.80	dB	NDC-GaN
Small Signal Gain (10 GHz)	7.91-8.55	dB	SiN-GaN
Gain Improvement (Average)	36.7	%	NDC vs. SiN
NDC Layer Thickness	500	nm	Grown by MWCVD
SiN Isolation Layer Thickness	20	nm	Deposited by PECVD (between AlGaN and NDC)
Gate Length (L_G)	0.3	µm	Actual fabricated length: 0.3247 µm
Total Gate Width (W_G)	250	µm	Double gate fingers (2 x 125 µm)
NDC Growth Temperature	710	°C	Microwave Chemical Vapor Deposition (MWCVD)
NDC Growth Rate	80	nm/h	Controlled for high quality/thermal conductivity
Junction Temperature (T_J)	181.9	°C	NDC-GaN (at P_diss = 5.69 W)
Junction Temperature (T_J)	182.0	°C	SiN-GaN (at P_diss = 4.48 W)

Key Methodologies

The fabrication utilized a “diamond-before-gate” approach on traditional SiC substrate GaN HEMTs, focusing on resolving compatibility issues between high-temperature diamond growth and the Schottky gate process.

Mesa Isolation and Ohmic Contact: Conventional processing was used to define the active area and deposit the 200 nm thick source and drain ohmic metal.
SiN Isolation Layer Deposition: A thin 20 nm SiN film was deposited via Plasma Enhanced Chemical Vapor Deposition (PECVD). This layer served two critical functions: protecting the AlGaN barrier during subsequent diamond growth and minimizing interfacial thermal resistance.
Nanocrystalline Diamond (NDC) Growth: The 500 nm NDC capping layer was grown using Microwave Chemical Vapor Deposition (MWCVD) at a temperature of 710 °C. The low growth rate (80 nm/h) was maintained to ensure high thermal conductivity of the NDC film.
Multi-Step Diamond Etching (Gate Region): A complex, three-step Inductively Coupled Plasma (ICP) etching process was developed to define the 0.3 µm gate region:
- Step 1 (Bulk Removal): Rapid etching using O₂/Ar atmospheres to ensure verticality and remove approximately 80% of the NDC thickness.
- Step 2 (Surface Quality): High-quality surface etching using pure O₂ atmosphere to smooth the surface and ease burrs.
- Step 3 (Isolation Layer Etch): Low-power ICP etching using O₂ atmosphere and over-etching to completely remove the 20 nm SiN isolation layer without damaging the underlying AlGaN barrier.
Schottky Gate Preparation: The gate metal (actual length 0.3247 µm) was prepared using an e-beam evaporation method.
Metal Interconnection: Final source, gate, and drain metal interconnection was implemented using Au deposition.

Commercial Applications

The demonstrated thermal management and performance improvements make this technology highly relevant for applications requiring high power density, high efficiency, and enhanced reliability in the RF and microwave spectrum.

High Power RF Electronics: Essential for minimizing channel temperature rise in high-power amplifiers (HPAs) and monolithic microwave integrated circuits (MMICs).
5G and 6G Wireless Infrastructure: Enabling higher output power and greater reliability in base station transmitters and active antenna systems.
Radar Systems: Applicable in military, aerospace, and automotive radar modules where thermal stability under high-duty cycles is critical.
Satellite Communications: Improving the efficiency and lifespan of solid-state power amplifiers (SSPAs) used in satellite transponders.
Thermal Management Solutions: The NDC layer serves as a high thermal conductivity heat spreader, a core product category for advanced semiconductor packaging.

View Original Abstract

Nanocrystalline diamond capping layers have been demonstrated to improve thermal management for AlGaN/GaN HEMTs. To improve the RF devices, the application of the technology, the technological approaches and device characteristics of AlGaN/GaN HEMTs with gate length less than 0.5 μm using nanocrystalline diamond capping layers have been studied systematically. The approach of diamond-before-gate has been adopted to resolve the growth of nanocrystalline diamond capping layers and compatibility with the Schottky gate of GaN HEMTs, and the processes of diamond multi-step etching technique and AlGaN barrier protection are presented to improve the technological challenge of gate metal. The GaN HEMTs with nanocrystalline diamond passivated structure have been successfully prepared; the heat dissipation capability and electrical characteristics have been evaluated. The results show the that thermal resistance of GaN HEMTs with nanocrystalline diamond passivated structure is lower than conventional SiN-GaN HEMTs by 21.4%, and the mechanism of heat transfer for NDC-GaN HEMTs is revealed by simulation method in theory. Meanwhile, the GaN HEMTs with nanocrystalline diamond passivated structure has excellent output, small signal gain and cut-off frequency characteristics, especially the current-voltage, which has a 27.9% improvement than conventional SiN-GaN HEMTs. The nanocrystalline diamond capping layers for GaN HEMTs has significant performance advantages over the conventional SiN passivated structure.

Tech Support

Original Source

DOI: https://doi.org/10.3390/mi13091486

References

2017 - Nanocrystalline diamond integration with III-Nitride HEMTs [Crossref]
2021 - A numerical investigation of heat suppression in HEMT for power electronics application [Crossref]
2021 - Thermal stress modelling of diamond on GaN/III-Nitride membranes [Crossref]
2020 - Integration of GaN and diamond using epitaxial lateral overgrowth [Crossref]
2017 - Effect of self-heating on electrical characteristics of AlGaN/ GaN HEMT on Si (111) substrate [Crossref]
2019 - Selective area deposition of hot filament CVD diamond on 100 mm MOCVD grown AlGaN/GaN wafers [Crossref]
2013 - Impact of intrinsic stress in diamond capping layers on the electrical behavior of AlGaN/GaN HEMTs [Crossref]