GaN HEMT Normally-Off Operation via AlGaN Composition Gradient
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Solution Overview
Problem
Existing GaN based HEMTs face challenges in achieving normally-off operation due to the difficulty in activating p-type impurities and the damage caused by high-temperature heat treatment and etching processes, leading to increased sheet resistance and leak current.
Innovation Solution
A compound semiconductor device structure is developed with a substrate, a first AlGaN layer, a second AlGaN layer, an electron transit layer, and an electron supply layer, where the composition of the AlGaN layers (Alx1Ga1-x1N and Alx2Ga1-x2N) and surface charge distribution are optimized to reduce stress and increase negative charges, enabling a higher threshold voltage and reliable normally-off operation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a p-type semiconductor layer is provided between gate electrode and active region to enable normally-off operation, then normally-off operation is achieved, but high-temperature heat treatment is required which damages the interface between electron transit layer and electron supply layer, lowering electron mobility
Solution Approach 1:
The invention changes the material composition parameter by introducing a second AlGaN layer with higher Al composition (x2) than the first AlGaN layer (x1), where 0<x1<x2≤0.5. This compositional gradient creates piezoelectric and spontaneous polarization effects that generate positive fixed charges at the interface, enabling normally-off operation without requiring high-temperature heat treatment or p-type doping, thus preserving interface quality and electron mobility
Solution Approach 2:
The invention uses a composite structure of multiple AlGaN layers with different Al compositions (x1 and x2) stacked sequentially. This composite material approach creates a polarization-induced charge layer at the interface between layers, which provides the necessary positive charges to deplete electrons in the channel, achieving normally-off operation without damaging high-temperature processing
2Reliability
If etching is performed to divide the 2DEG and create structure for normally-off operation, then normally-off operation is achieved, but significant damage occurs in vicinity of electron transit layer, increasing sheet resistance and leak current
Solution Approach 1:
The invention changes the structural parameter by creating a compositional gradient in the AlGaN layers (x1<x2) that induces polarization charges at the interface. This eliminates the need for etching the electron supply layer to divide 2DEG, as the polarization-induced positive charges naturally deplete the channel electrons, achieving normally-off operation without damaging the electron transit layer and maintaining low sheet resistance
Solution Approach 2:
The invention extracts the function of creating positive charges from the etching process and relocates it to the interface between AlGaN layers through polarization effects. This removes the harmful etching step entirely, preventing damage to the electron transit layer and avoiding increases in sheet resistance and leak current
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 optimized structure ensures a large threshold voltage and reliable normally-off operation by reducing electric charge generation and interface damage, thus improving the performance and applicability of GaN based HEMTs for high-power and high-voltage applications.
Implementation Method 1
a second AlGaN layer which is different from the first AlGaN layer in terms of composition and is formed over the first AlGaN layer... Positive charges exist at a lower surface of the second AlGaN layer
Implementation Method 2
a composition of the first AlGaN layer is represented by Alx1Ga1-x1N... a composition of the second AlGaN layer is represented by Alx2Ga1-x2N... Negative charges exist at an upper surface of the second AlGaN layer more than positive charges existing at a lower surface of the second AlGaN layer
Data Source
AI summary
A first AlGaN layer formed over a substrate, a second AlGaN layer formed over the first AlGaN layer, an electron transit layer formed over the second AlGaN layer, and an electron supply layer formed over the electron transit layer are provided. A relationship of “0≦x1<x2≦1” is found when a composition of the first AlGaN layer is represented by Alx1Ga1-x1N, and a composition of the second AlGaN layer is represented by Alx2Ga1-x2N. Negative charges exist at an upper surface of the AlGaN layer more than positive charges existing at a lower surface of the AlGaN layer.