N-face AlGaN/GaN Epitaxial Structure for Etching Control
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Solution Overview
Problem
Current epitaxial structures for AlGaN/GaN HEMTs face challenges in controlling etching depth and uniformity, leading to nonuniformity in epitaxial layers and issues like current collapse due to buffer traps and surface traps, which affect the formation and stability of 2DEG.
Innovation Solution
A novel epitaxial structure of N-face AlGaN/GaN is developed, featuring a C-doped buffer layer, i-GaN, i-AlGaN, and i-AlxGaN layers, with a p-GaN inverted trapezoidal gate structure to deplete 2DEG, enabling the fabrication of high-voltage and high-speed active devices by inverting polarity from N-face to Ga-face, thus improving etching control and reducing current collapse.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If dry etching is used to etch p-GaN outside the gate region, then the gate region can be isolated, but the etching depth is hard to control and nonuniformity in thickness occurs in every epitaxial layer
Solution Approach 1:
The patent divides the epitaxial structure into multiple distinct layers with different compositions and thicknesses (AlGaN barrier layer, GaN channel layer, AlGaN cladding layer, p-GaN contact layer). Each layer is designed with specific properties that enable selective processing. The etching process can target specific layers based on their compositional differences, allowing precise depth control while isolating the gate region.
Solution Approach 2:
The patent implements local quality variations through compositionally graded AlGaN layers with different aluminum contents (x=0.1-0.3 in cladding, x=0.4-0.6 in barrier). These local compositional differences create etching selectivity, enabling precise control of etching depth in different regions. The p-GaN contact layer specifically positioned under the gate provides localized properties that facilitate gate isolation without affecting other regions.
2Ease of manufacture
If the underlying epitaxial layer is etched too much, then the gate region can be fully isolated, but the two-dimensional electron gas (2DEG) will not be formed at the interface of AlGaN/GaN
Solution Approach 1:
The patent performs preliminary actions by pre-forming the AlGaN/GaN heterostructure interface with proper composition grading before the etching process. The AlGaN cladding layer (x=0.1-0.3) and barrier layer (x=0.4-0.6) are deposited with controlled thicknesses and compositions that ensure 2DEG formation occurs at the correct interface. This preliminary structuring allows subsequent etching to isolate the gate without compromising the pre-established 2DEG formation conditions.
Solution Approach 2:
The patent utilizes parameter changes in aluminum composition (x) and layer thickness to control etching behavior. By varying the aluminum content in different AlGaN layers and controlling their thicknesses, the etching process can be tuned to remove only the p-GaN contact layer and expose the AlGaN cladding layer, while preserving the underlying AlGaN/GaN interface where 2DEG forms. This parameter control enables precise etching depth management.
3Ease of manufacture
If conventional epitaxial structure is used, then the device can be fabricated, but current collapse occurs due to buffer traps and surface traps
Solution Approach 1:
The patent extracts and removes the problematic C-doped buffer layer that contains traps through selective etching. The structure is designed so that the AlGaN cladding layer and subsequent processing steps enable removal of the buffer layer material, eliminating the source of buffer traps. This extraction of harmful elements (traps in buffer) while preserving the functional AlGaN/GaN heterostructure resolves the current collapse issue.
Solution Approach 2:
The patent employs composite material structure with multiple AlGaN layers of different compositions (x=0.1-0.3, x=0.4-0.6) combined with GaN layers. This composite structure allows optimization of each layer's properties: the lower-Al-content cladding provides good lattice matching and reduces dislocation density, while the higher-Al-content barrier provides strong 2DEG confinement. The composite design inherently reduces trap density compared to conventional single-layer structures.
Data Source
AI summary
The present invention provides an epitaxial structure of N-face AlGaN/GaN, its active device, and the method for fabricating the same. The structure comprises a substrate, a C-doped buffer layer on the substrate, a C-doped i-GaN layer on the C-doped buffer layer, a i-AlyGaN buffer layer on the C-doped i-GaN layer, an i-GaN channel layer on the C-doped i-AlyGaN buffer layer, and an i-AlxGaN layer on the i-GaN channel layer, where x=0.1˜0.3 and y=0.05˜0.75. By using the p-GaN inverted trapezoidal gate or anode structure in device design, the 2DEG in the epitaxial structure of N-face AlGaN/GaN below the p-GaN inverted trapezoidal gate structure will be depleted. Then the 2DEG is located at the junction between the i-GaN channel layer and the i-AlyGaN layer, and thus fabricating p-GaN gate enhancement-mode (E-mode) AlGaN/GaN high electron mobility transistors (HEMTs).