Skip to content
6CCVD Research

Preparation and Characterization of GaN-on-Si HEMTs with Nanocrystalline Diamond Passivation

At a Glance

Section titled “At a Glance”
MetadataDetails
Publication Date2025-02-28
JournalCrystals
AuthorsYu Fu, Songyuan Song, Zeyang Ren, Liaoliang Zhu, Jinfeng Zhang
InstitutionsWuhu Institute of Technology, Xidian University
Citations1
AnalysisFull AI Review Included

Preparation and Characterization of GaN-on-Si HEMTs with Nanocrystalline Diamond Passivation

Section titled “Preparation and Characterization of GaN-on-Si HEMTs with Nanocrystalline Diamond Passivation”

Executive Summary

Section titled “Executive Summary”

This study successfully implemented a low-temperature nanocrystalline diamond (NCD) passivation layer on AlGaN/GaN-on-Si High-Electron Mobility Transistors (HEMTs) to significantly enhance thermal management and electrical performance.

  • Thermal Management Improvement: The NCD passivation layer resulted in a 36% improvement in heat dissipation efficiency, reducing the thermal resistance slope from 25.88 °C/(W·mm) to 16.38 °C/(W·mm).
  • Low-Temperature Process: NCD growth was achieved at a relatively low temperature of 650 °C using Microwave Plasma Chemical Vapor Deposition (MPCVD), minimizing potential degradation of the AlGaN/GaN heterojunction.
  • Current Density Boost: Maximum saturation current density (IDmax) increased by 24%, rising from 447 mA/mm to 555 mA/mm.
  • On-Resistance Reduction: On-resistance (Ron) was reduced by approximately 34%, dropping from 19.9 Ω·mm to 13.2 Ω·mm.
  • Breakdown Voltage Enhancement: The off-state breakdown voltage (Vbr) increased by 100 V (from 400 V to 500 V), attributed to the NCD layer forming a beneficial T-shaped gate structure that optimizes the electric field.
  • Material Quality: The NCD film thickness ranged from 250-383 nm, exhibiting good uniformity with grain sizes around 240 nm.

Technical Specifications

Section titled “Technical Specifications”
ParameterValueUnitContext
SubstrateSi (4-inch)N/AWafer material
Barrier Layer20 nm Al0.21Ga0.79NnmEpi-structure
Channel Layer190 nm GaNnmEpi-structure
Buffer Layer4.49 ”m GaN”mEpi-structure
Gate Length (LG)2”mDevice geometry
Gate Width (WG)100”mDevice geometry
NCD Passivation Thickness250-383nmGrown film range
NCD Grain Size~240nmMeasured morphology
NCD Growth Temperature650°CMPCVD process
Thermal Conductivity (Single-Crystal Diamond)>2000W/(m·K)Reference value
Thermal Conductivity (NCD)20 to 200W/(m·K)Reference value
Diamond Raman Peak1334.7cm-1Confirms diamond growth
IDmax (w/ NCD)555mA/mmMaximum saturation current
IDmax (w/o NCD)447mA/mmBaseline current
Ron (w/ NCD)13.2Ω·mmOn-resistance
Ron (w/o NCD)19.9Ω·mmBaseline resistance
Gm,max (w/ NCD)97.0mS/mmPeak transconductance
Vth (Both Devices)-2.8VThreshold voltage
Vbr (w/ NCD)500VOff-state breakdown voltage
Vbr (w/o NCD)400VBaseline breakdown voltage
Thermal Resistance (w/ NCD)16.38°C/(W·mm)Heat dissipation efficiency slope
Thermal Resistance (w/o NCD)25.88°C/(W·mm)Baseline thermal slope
Ohmic Contact Resistance (Rc)2.44Ω·mmAfter NCD growth

Key Methodologies

Section titled “Key Methodologies”

The device fabrication utilized a strategy where the NCD passivation layer was grown before the gate metal deposition to ensure compatibility with the high-temperature MPCVD process.

  1. Mesa Isolation and Ohmic Contact Formation:

    • Mesa isolation was performed using chlorine-based dry etching.
    • Source/Drain Ohmic contacts were formed by depositing a Ti/Al/Ni/Au multilayer stack (20/150/50/100 nm) via electron beam evaporation.
    • Rapid Thermal Annealing (RTA) was performed at 835 °C for 30 s in a pure N2 atmosphere.

  2. SiNx Protection Layer Deposition:

    • A 50 nm-thick SiNx layer was deposited via ICP-CVD at 130 °C to protect the AlGaN/GaN surface during the subsequent NCD growth.

  3. NCD Seeding and Growth (MPCVD):

    • Nanocrystalline diamond seed suspension was spin-coated (2000 rpm for 30 s) and dried at 80 °C.
    • NCD growth was performed in a two-step MPCVD process at 650 °C, maintaining gas flows of H2 (300 sccm), CH4 (12 sccm), and N2 (0.05 sccm).
    • Step 1 (Nucleation): 120 s duration, 2.0 kW microwave power, 90 mbar pressure.
    • Step 2 (Main Growth): 15 min duration, 3.2 kW microwave power, 135 mbar pressure.

  4. Gate Definition and Metal Deposition:

    • A 200 nm-thick SiNx hard mask was deposited via ICP-CVD.
    • The SiNx hard mask was selectively etched (CF4 plasma) to expose the gate area covered by NCD.
    • The exposed NCD was removed using oxygen plasma etching.
    • Gate metal (Ni/Au, 20/200 nm) was deposited via lift-off to complete the device structure.

  5. Characterization:

    • Material quality was verified using SEM (morphology, thickness) and Raman spectroscopy (diamond peak at 1334.7 cm-1).
    • DC characteristics (IDmax, Ron, Gm) were measured using a KEYSIGHT B1500A.
    • Junction temperature and heat dissipation efficiency were measured using a FOTRIC infrared thermal imaging system under varying DC power levels.

Commercial Applications

Section titled “Commercial Applications”

The integration of nanocrystalline diamond passivation directly above the active region of GaN HEMTs provides a critical solution for thermal bottlenecks, enabling higher power density and reliability in demanding electronic systems.

  • High-Frequency/RF Systems:
    • 5G/6G Base Stations and Transmitters: GaN HEMTs are essential for high-power amplification; improved thermal management allows for higher output power and reduced cooling requirements in compact modules.
    • Radar Systems: Enhanced heat dissipation is crucial for high-duty-cycle, high-power pulsed radar applications (e.g., defense and aerospace).
  • Power Electronics and Conversion:
    • High-Efficiency Power Converters: Used in data centers, electric vehicles (EVs), and industrial motor drives, where minimizing junction temperature is key to maximizing efficiency and lifespan.
  • Wireless Energy Transmission:
    • High-power GaN devices used in wireless charging systems benefit from the ability to operate at higher power densities without thermal runaway.
  • Integrated Technology:
    • The compatibility of the NCD passivation process with existing GaN fabrication lines makes this a viable strategy for next-generation wide-bandgap semiconductor devices and integrated circuits.
View Original Abstract

Thermal accumulation under high output power densities is one of the most significant challenges for GaN power devices. Diamond, with its ultra-high thermal conductivity, offers great potential for improving heat dissipation in high-power GaN devices. In this study, nanocrystalline diamond (NCD) passivated high-electron mobility transistors (HEMTs) based on AlGaN/GaN-on-Si heterostructures were fabricated with a gate length of 2 Όm. The NCD film has a thickness of 250-383 nm and a uniform morphology with a grain size of mostly ~240 nm. Compared to the devices without NCD passivation, those devices with the NCD passivation layer show an increase in current density from 447 mA/mm to 555 mA/mm, a reduction in on-resistance from 20 Ω·mm to 13 Ω·mm, and a noticeable suppression of current degradation at high-drain voltages. Junction temperature measurements under varied output power densities reveal a 36% improvement in heat dissipation efficiency with the NCD passivation. These results fully demonstrate the promising potential of NCD for enhancing heat dissipation in high-power GaN devices.

Tech Support

Section titled “Tech Support”

Original Source

Section titled “Original Source”

References

Section titled “References”
  1. 2020 - Prospects for Wide Bandgap and Ultrawide Bandgap CMOS Devices [Crossref]
  2. 2022 - Improved Stability of GaN MIS-HEMT With 5-nm Plasma-Enhanced Atomic Layer Deposition SiN Gate Dielectric [Crossref]
  3. 2023 - Overview of Wide/Ultrawide Bandgap Power Semiconductor Devices for Distributed Energy Resources [Crossref]
  4. 2016 - Analysis of the modulation mechanisms of the electric field and breakdown performance in AlGaN/GaN HEMT with a T-shaped field-plate [Crossref]
  5. 2021 - Fabrication and characterization of GaN HEMTs grown on SiC substrates with different orientations [Crossref]
  6. 2022 - A 32-A, 5-V-Input, 94.2% Peak Efficiency High-Frequency Power Converter Module Featuring Package-Integrated Low-Voltage GaN nMOS Power Transistors [Crossref]
  7. 2021 - A GaN BCM AC-DC Converter for Sub-1 V Electromagnetic Energy Harvesting With Enhanced Output Power [Crossref]
  8. 2023 - Cooling future system-on-chips with diamond inter-tiers [Crossref]
  9. 2022 - GaN MMICs on a diamond heat spreader with through-substrate vias fabricated by deep dry etching process [Crossref]

Reads Similar to This

Crystalline Interlayers for Reducing the Effective Thermal Boundary Resistance in GaN-on-Diamond Pulsed laser milling process of CVD single crystal diamond Hybrid sp2+sp3carbon phases created from carbon nanotubes