GaN HEMT Cap Layer AlGaN Buffer for Leakage Reduction
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Solution Overview
Problem
GaN-based HEMTs often experience gate leakage current and lowered withstand voltage due to dielectric breakdown and lattice defects caused by strain relaxation at the interface between the i-AlN and n-GaN layers, leading to current collapse.
Innovation Solution
A cap layer structure is introduced with an AlGaN-containing layer between the n-GaN and i-AlN layers to reduce piezoelectric effects and an AlGaN-containing layer between the i-AlN and n-GaN layers to alleviate elastic strain, preventing the formation of two-dimensional electron gas and lattice defects, thereby suppressing gate leakage current and current collapse.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If an i-AlN layer is formed directly over an n-GaN layer to achieve normally-off operation, then the device can supply large current, but gate leakage current occurs and withstand voltage is lowered due to dielectric breakdown and lattice defects
Solution Approach 1:
The i-AlN layer is segmented into two separate layers: a first i-AlN layer formed over the n-GaN layer, and a second i-AlN layer formed over the first i-AlN layer. This segmentation allows each layer to have optimized thickness and composition, reducing strain accumulation and preventing lattice defects while maintaining the normally-off operation capability and current supply.
Solution Approach 2:
An AlGaN layer is introduced as an intermediary layer between the n-GaN layer and the first i-AlN layer. This AlGaN layer acts as a buffer that gradually transitions the lattice structure, reducing the strain at the interface and preventing piezoelectric effects that would otherwise cause gate leakage current and dielectric breakdown.
2Reliability
If the AlN layer is placed close to the n-GaN layer to suppress two-dimensional electron gas formation, then normally-off operation is achieved, but piezoelectric effects and elastic strain cause lattice defects
Solution Approach 1:
The AlGaN layer serves as an intermediary between the n-GaN layer and the first i-AlN layer, providing a gradual lattice transition that reduces elastic strain and piezoelectric effects. This intermediary structure maintains the normally-off operation by preventing 2DEG formation while preserving lattice stability through compositional grading.
Solution Approach 2:
The AlGaN layer has a composition gradient with varying Al content, creating a gradual change in lattice constant and piezoelectric properties. This parameter change approach reduces the abrupt strain at interfaces, preventing lattice defects while maintaining the electric field necessary for normally-off operation.
3Device complexity
If a simple AlN layer structure is used to reduce device complexity, then manufacturing is easier, but gate leakage current and current collapse occur due to interface defects
Solution Approach 1:
The AlN layer is divided into two separate i-AlN layers with an AlGaN layer in between, creating a segmented structure that reduces interface strain and prevents lattice defects. This segmentation approach maintains manufacturing simplicity while significantly improving reliability by eliminating the root causes of gate leakage current and current collapse.
Solution Approach 2:
The cap layer uses a composite structure combining AlN and AlGaN layers, where each material contributes specific properties: AlN provides high breakdown field strength and normally-off operation, while AlGaN provides strain buffering and lattice matching. This composite approach enhances reliability without significantly increasing manufacturing complexity.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The proposed structure effectively reduces gate leakage current and enhances withstand voltage by maintaining the conduction band energy further from the Fermi level and minimizing lattice defects, resulting in improved reliability of GaN-based HEMTs.
Implementation Method 1
an AlGaN-containing layer between the n-GaN layer and the i-AlN layer to reduce piezoelectric effects
Implementation Method 2
an AlGaN-containing layer between the i-AlN layer and the n-GaN layer to alleviate elastic strain
Implementation Method 3
preventing the formation of two-dimensional electron gas and lattice defects, thereby suppressing gate leakage current
Data Source
AI summary
A compound semiconductor device includes: an electron transport layer formed over a substrate; an electron supply layer formed over the electron transport layer; and a cap layer formed over the electron supply layer; the cap layer includes a first compound semiconductor layer containing GaN; a second compound semiconductor layer containing AlN, which is formed over the first compound semiconductor layer; a third compound semiconductor layer containing GaN, which is formed over the second compound semiconductor layer; and at least one of a first AlGaN-containing layer and a second AlGaN-containing layer, with the first AlGaN-containing layer formed between the first compound semiconductor layer and the second compound semiconductor layer and the Al content increases toward the second compound semiconductor layer, and the second AlGaN-containing layer formed between the second compound semiconductor layer and the third compound semiconductor layer and the Al content increases toward the second compound semiconductor layer.