Unique opportunity to harness polarization in GaN to override the conventional power electronics figure-of-merits

At a Glance

Metadata	Details
Publication Date	2015-06-01
Authors	Huili Grace Xing, Bo Song, Mingda Zhu, Zongyang Hu, Meng Qi
Institutions	Cornell University, University of Notre Dame
Citations	6
Analysis	Full AI Review Included

6CCVD Technical Analysis: Harnessing Polarization in WBG Semiconductors for Power Electronics

Executive Summary

This paper investigates advanced device architectures in Gallium Nitride (GaN) using polarization engineering to significantly surpass the conventional unipolar trade-off between specific on-resistance (R_on,sp) and breakdown voltage (BV). Diamond is identified as the ultimate WBG candidate for this performance regime, making this research highly relevant to 6CCVD’s core materials.

Objective Achievement: GaN polarization doping (Pi-doping) enables ideal dopant behavior (no temperature or frequency dependence), resolving a critical challenge faced by other Wide Bandgap Semiconductors (WBGs) like deep dopants in SiC and diamond.
Architectural Solutions: Two primary device concepts are introduced: the vertical PolarMOS and the lateral Polarization-Doped Super Junction (LPSJ).
Performance Breakthrough: Modeling confirms the LPSJ structure, utilizing compositionally graded AlGaN, achieves a greater than 10X reduction in R_on,sp compared to conventional GaN junctions for breakdown voltages exceeding 2 kV.
Material Precision: The success of both architectures relies on ultra-precise compositional grading and layer thickness control (e.g., 0.125 µm layers) achieved via advanced epitaxy (MBE/MOCVD).
Diamond Opportunity: The paper explicitly names diamond alongside GaN and SiC as a leading material focus for next-generation power switching. Diamond’s intrinsic properties (highest E_b) position it as the theoretical successor to GaN/SiC in achieving the ultimate R_on,sp-BV limits.
6CCVD Relevance: Replication and extension of this high-voltage research require materials offering maximum breakdown fields and superior thermal management capabilities—the definition of 6CCVD’s Single Crystal Diamond (SCD) and Polycrystalline Diamond (PCD) platforms.

Technical Specifications

The following hard specifications and performance projections are extracted from the analysis of polarization-doped GaN architectures:

Parameter	Value	Unit	Context
Target Breakdown Voltage (BV)	> 2	kV	LPSJ modeling for high-power applications
R_on,sp Reduction (LPSJ)	> 10	X	Compared to conventional GaN junctions at BV > 2 kV
Optimal Al Composition (LPSJ)	0.3	-	X_Al leading to 10X R_on,sp reduction
LPSJ Pillar/Grading Thickness (d)	0.125	µm	Optimal layer thickness for 2D modeling
Desired Doping Concentration (Power Switching)	< 10¹⁷	cm^-3	Required for high breakdown voltage performance
Prior Research Concentration Focus	> 10¹⁸	cm^-3	Focus of earlier high-current research
Polarization-Doped Mobility (µ)	586	cm²/(V.s)	Simulated mobility for 2D LPSJ structure
PolarMOS Threshold Voltage (V_th)	> 2	V	Achieved via gate dielectric layer
Diamond Breakdown Field (Theoretical E_b)	5 - 10	MV/cm	Highest WBG material (GaN is ~3.3 MV/cm)

Key Methodologies

The core innovations enabling the extreme performance characteristics detailed in the paper rely on precise material engineering, primarily utilizing advanced epitaxial deposition techniques:

Compositional Grading: Linear grading of Al composition in Al_xInGaN layers to intentionally control the spontaneous and piezoelectric polarization charges, generating a 3-dimensionally distributed charge.
Polarization Doping (Pi-Doping): Utilizing the induced positive or negative charges associated with the crystal lattice (due to grading) to attract mobile negative (electrons) or positive (holes) charge, resulting in full carrier ionization without thermal excitation limitations (ideal dopant behavior).
Super Junction (SJ) Formation: Epitaxially growing balanced n/p pillar regions using sequential compositional grading (GaN to AlGaN, then back to GaN) to suppress charge imbalance without needing high-energy ion implantation (a method typically used in Si SJ fabrication).
Advanced Epitaxy (MBE/MOCVD): Employing techniques like MBE or MOCVD to ensure ultra-precise control over Al composition and layer thicknesses, critical for balancing the n/p pillar charges required for the flat electrical field profile in the LPSJ drift region.
Gate Dielectric Integration: Utilizing a gate dielectric layer to achieve high threshold voltages (V_th > 2 V) and minimize off-state leakage in the PolarMOS structure.

6CCVD Solutions & Capabilities

As specialists in CVD diamond materials, 6CCVD offers the necessary foundation and advanced fabrication services to either accelerate research into diamond-based WBGs or to provide critical component integration (thermal management, BDD electrodes) for high-power GaN systems like the LPSJ and PolarMOS.

Applicable Materials

To replicate the high-breakdown performance regime explored in this GaN paper, and specifically to pursue the ultimate theoretical limits identified for WBGs, 6CCVD Single Crystal Diamond (SCD) is the definitive material solution.

6CCVD Material	Recommended Grade	Application Relevance
Optical Grade SCD	High Purity, Low Nitrogen (Electronic Grade)	Required for fundamental research into diamond power devices, capitalizing on diamond’s E_b (5-10 MV/cm) to surpass the theoretical limits of GaN and SiC.
Boron-Doped Diamond (BDD)	Heavy Doping (Metallic)	Used for ohmic contacts, high-quality electrodes, and specialized sensors required in high-current/high-field environments where the contacts described (n⁺/p⁺ cathode/anode) are integrated.
Polycrystalline Diamond (PCD)	Thermal Management Grade	Ideal substrate for integrating high-power GaN LPSJ or PolarMOS modules, providing superior heat spreading for devices operating near the 2 kV threshold.

Customization Potential for WBG Development

The GaN paper highlights the necessity of precise layer control (e.g., 0.125 µm layers) and complex electrode structures. 6CCVD’s in-house capabilities directly address these precision requirements for developing next-generation diamond or integrated GaN/Diamond devices:

Requirement from Research	6CCVD Custom Capability	Benefit to Project
Precision Thickness Control	SCD and PCD thicknesses from 0.1 µm up to 500 µm (wafers) or 10 mm (substrates).	Ensures optimal layer thickness for achieving precise electric field profiles, analogous to the 0.125 µm pillars utilized in the LPSJ design.
Custom Wafer Size	Plates/wafers available up to 125 mm (PCD).	Supports scale-up and fabrication of large-area super junction devices.
Electrode & Contact Integration	In-house custom metalization services (Au, Pt, Pd, Ti, W, Cu).	Enables the deposition of required ohmic and Schottky contacts (p⁺, n⁺ connections) directly onto SCD or BDD layers for complex vertical/lateral device integration.
Surface Quality	Precision polishing services: Ra < 1 nm (SCD), Ra < 5 nm (Inch-size PCD).	Ensures the surface planarity critical for subsequent epitaxial regrowth and high-yield device fabrication in complex multi-layer architectures.

Engineering Support

The challenges of optimizing the R_on,sp and BV trade-off are inherent to all WBGs. While GaN polarization engineering offers a strong solution, diamond provides the highest intrinsic performance ceiling. 6CCVD’s in-house PhD team specializes in CVD diamond material properties, including doping, thermal management, and contact physics, crucial for advanced power electronics applications.

We offer expert consultation to transition high-field research from GaN to diamond platforms or to integrate diamond thermal solutions into existing GaN power modules, maximizing the overall system Figure-of-Merit (V_br²/R_on,sp).

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

View Original Abstract

Owing to the large breakdown electric field, wide bandgap semiconductors such as SiC, GaN, Ga2O3 and diamond based power devices are the focus for next generation power switching applications. The unipolar trade-off relationship between the area specific-on resistance and breakdown voltage is often employed to compare the performance limitation among various materials. The GaN material system has a unique advantage due to its prominent spontaneous and piezoelectric polarization effects in GaN, AlN, InN, AlxInyGaN alloys and flexibility in inserting appropriate heterojunctions thus dramatically broaden the device design space.

Tech Support

Original Source

DOI: https://doi.org/10.1109/drc.2015.7175549