GaN MOSFET Recessed Gate Structure for Normally-Off Operation
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Solution Overview
Problem
Conventional AlGaN/GaN hetero junction field effect transistors (HFETs) are typically normally-on devices, which are not suitable for high power switching applications due to high on-resistance and vulnerability to current collapse, and achieving a normally-off type device with low resistance and high breakdown voltage is challenging.
Innovation Solution
A GaN-based field effect transistor structure comprising a p-type channel layer, an electron supply layer with a higher band gap, a surface layer with a lower band gap, and an insulating film formed on the recessed channel layer, where the gate electrode is positioned between source and drain electrodes, reducing surface trap density and preventing current collapse.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If n-type layer is formed by ion implantation to achieve normally-off type device, then device can be turned off, but resistance of n-type layer cannot be reduced sufficiently
Solution Approach 1:
The patent changes the formation method parameter from ion implantation to epitaxial growth, and adjusts the aluminum composition ratio parameter in the electron supply layer to optimize both resistance and breakdown voltage characteristics
Solution Approach 2:
The patent uses a composite structure combining p-type GaN channel layer with AlGaN electron supply layer having different band gaps, creating a heterostructure that achieves both low resistance and high breakdown voltage
2Manufacturing precision
If AlGaN/GaN hetero junction structure is used to reduce on-resistance, then electron mobility improves, but current collapse phenomenon occurs
Solution Approach 1:
The patent applies different material properties to different regions: p-type doping in the channel layer region, aluminum-containing electron supply layer with higher band gap, and aluminum-free or low-aluminum surface layer, creating localized quality variations that prevent current collapse while maintaining low on-resistance
Solution Approach 2:
The patent changes the aluminum composition ratio parameter across different layers and regions, with higher aluminum content in the electron supply layer near the gate and lower or zero aluminum content in the surface layer, optimizing both electrical performance and stability
3Reliability
If MOS structure is adopted to achieve normally-off type device, then device can be turned off, but breakdown voltage decreases
Solution Approach 1:
The patent employs a composite heterostructure of p-type GaN channel layer and AlGaN electron supply layer with higher band gap, utilizing the material property differences to achieve both normally-off operation and high breakdown voltage through the hetero junction boundary effects
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 structure achieves low on-resistance, high breakdown voltage, and reduces the influence of current collapse, enabling a normally-off operation with improved switching speed and reliability.
Implementation Method 1
AlGaN/GaN hetero junction structure involves generation of two-dimensional electron gas at the boundary due to the spontaneous polarization
Implementation Method 2
AlGaN/GaN hetero junction structure involves generation of two-dimensional electron gas at the boundary due to the spontaneous polarization and the Piezo effect
Implementation Method 3
a GaN-based field effect transistor with high voltage and low resistance and with lesser influence of current collapse by reducing surface traps between a semiconductor layer and an insulating film
Data Source
AI summary
A GaN-based field effect transistor (MOSFET) is comprised of a channel layer comprised of p-type GaN, an electron supply layer, a surface layer having band gap energy smaller than that of the electron supply layer, sequentially laminated on a substrate, and recess section is formed by removing a part of the drift layer, the electron supply layer, and the surface layer down to a depth that reaches to the channel layer. A source electrode and a drain electrode are formed so that the recess section positions between them, a gate insulation film is formed on the surface layer and on inner-surface of the recess section including the channel layer, and a gate electrode is formed on the gate insulating film in the recess section.