Group-III-Nitride Layer Structure for Normally-Off Transistors
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Solution Overview
Problem
Existing group-III-nitride heterojunction field effect transistors (HEMTs) are primarily normally-on, which is not suitable for power switching applications requiring normally-off operation, and struggle to achieve low on-state resistance and high positive gate voltage driving due to limitations in reducing two-dimensional electron gas density and threshold voltage.
Innovation Solution
A group-III-nitride layer structure comprising a p-type back-barrier layer and an electron-supply layer with a channel layer in between, where the back-barrier layer has a wider band-gap energy than the electron-supply layer, creating an intrinsic hole-generated electric field to lift the conduction-band edge above the Fermi level, enabling normally-off operation with high on-state resistance and current-driving capability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If fluoride-based plasma treatment or recess etching is used to achieve normally-off operation, then positive threshold voltage is achieved, but device stability degrades under high voltage and high temperature operations
Solution Approach 1:
The patent changes the material composition parameter by introducing a p-type back-barrier layer with specific doping concentration and a thin InGaN cap layer with controlled thickness and composition. This structural parameter change achieves normally-off operation through intrinsic material properties rather than post-processing treatments, thereby maintaining device stability under high voltage and temperature operations while achieving positive threshold voltage.
2Loss of energy
If recess etching is performed to lower on-state resistance, then current conduction improves, but manufacturing complexity and reliability concerns increase
Solution Approach 1:
The patent extracts the complex recess etching process from the manufacturing sequence by designing a layer structure where the thin InGaN cap layer (5-10 nm) naturally provides the necessary barrier function. This eliminates the need for additional recess etching steps to achieve low on-state resistance, reducing manufacturing complexity while maintaining electrical performance.
3Ease of operation
If thin InGaN cap layer is used to raise conduction band for normally-off operation, then positive threshold voltage is achieved, but reliability of recess etching becomes a major issue
Solution Approach 1:
The patent implements a self-service mechanism where the thin InGaN cap layer inherently provides the conduction band offset necessary for normally-off operation. The layer structure itself serves the function of threshold voltage control without requiring additional processing steps like recess etching, thereby eliminating the reliability issues associated with those processes.
4Ease of operation
If normally-on HEMT is used in cascaded configuration with Si FET to achieve normally-off operation, then threshold voltage control is achieved, but additional chip integration and slower internal body are required
Solution Approach 1:
The patent segments the transistor structure into distinct functional layers: AlGaN electron supply layer, GaN channel layer, and p-type AlGaN back-barrier layer with InGaN cap. This segmentation allows each layer to perform its specific function independently, achieving normally-off operation through the intrinsic properties of the layered structure rather than through complex cascaded device integration.
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 layer structure achieves a positive threshold voltage shift of up to 6V, allowing for high drain current and low gate current, while preventing punch-through effects and eliminating the need for recess etching or ion treatment, resulting in a cost-effective and reliable normally-off transistor design.
Implementation Method 1
creating an intrinsic hole-generated electric field to lift the conduction-band edge above the Fermi level
Data Source
AI summary
A layer structure for a normally-off transistor has an electron-supply layer made of a group-III-nitride material, a back-barrier layer made of a group-III-nitride material, a channel layer between the electron-supply layer and the back-barrier layer, made of a group-III-nitride material having a band-gap energy that is lower than the band-gap energies of the other layer mentioned. The material of the back-barrier layer is of p-type conductivity, while the material of the electron-supply layer and the material of the channel layer are not of p-type conductivity, the band-gap energy of the electron-supply layer is smaller than the band-gap energy of the back-barrier layer. In absence of an external voltage a lower conduction-band-edge of the third group-III-nitride material in the channel layer is higher in energy than a Fermi level of the material in the channel layer.