GaN MOSFET Dislocation Density and Impurity Control
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Solution Overview
Problem
Conventional MOSFETs formed with gallium nitride (GaN) on sapphire substrates face issues with actual threshold voltage and carrier mobility being lower than set values due to high dislocation density and p-type impurity clumping, leading to suboptimal device performance.
Innovation Solution
A MOSFET design featuring a gallium nitride substrate with a dislocation density less than or equal to 1E+6 cm−2 and an epitaxial layer with a p-type impurity concentration of magnesium (Mg) less than or equal to 5E+17 cm−3, using silicon dioxide or aluminum oxide as the gate insulating film, and a trench gate structure to prevent impurity clumping and control threshold voltage and carrier mobility.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If gallium nitride is used to form the channel formation region, then high electron mobility and wide bandgap properties are achieved, but the actual threshold voltage becomes lower than the setting value and actual carrier mobility becomes lower than the setting value due to high dislocation density and p-type impurity clumping
Solution Approach 1:
The patent changes the substrate dislocation density parameter from conventional high levels to less than or equal to 1×10^6 cm^-2, and adjusts the p-type impurity concentration parameter to less than or equal to 5×10^17 cm^-3. These parameter changes directly resolve the contradiction by enabling precise control of threshold voltage and carrier mobility while maintaining GaN's high electron mobility and wide bandgap properties
Solution Approach 2:
The patent performs preliminary action by selecting and preparing a gallium nitride substrate with predetermined low dislocation density (≤1×10^6 cm^-2) before forming the epitaxial layer. This preliminary substrate preparation prevents dislocation propagation and impurity clumping in subsequent processing, thereby ensuring accurate control of threshold voltage and carrier mobility from the outset
2Ease of manufacture
If conventional epitaxial growth on sapphire substrate is used, then device fabrication is achieved, but high dislocation density causes p-type impurity clumping and deviates electrical characteristics from setting values
Solution Approach 1:
The patent changes the substrate material parameter from sapphire to gallium nitride with controlled dislocation density (≤1×10^6 cm^-2), and adjusts the p-type impurity concentration parameter to ≤5×10^17 cm^-3. This parameter change maintains ease of epitaxial layer formation while eliminating the harmful effects of high dislocation density and impurity clumping, achieving reliable electrical characteristic control
Solution Approach 2:
The patent introduces a carefully controlled gallium nitride substrate as an intermediary between the sapphire base and the active device region. This intermediary substrate with low dislocation density acts as a buffer that prevents dislocation propagation and impurity clumping, enabling both easy epitaxial growth and reliable electrical characteristic control
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
This design effectively controls threshold voltage between 3V and 18V and carrier mobility between 10 cm2/Vs and 2000 cm2/Vs, preventing them from being lower than set values, thereby enhancing the performance and reliability of the MOSFET.
Implementation Method 1
a gate insulating film, and a gate electrode. The gate insulating film may be provided in direct contact with the epitaxial layer. The gate electrode may be provided in contact with the gate insulating film.
Implementation Method 2
The gallium nitride substrate may have a dislocation density less than or equal to 1E+6 cm−2. The epitaxial layer has a region with a p-type impurity concentration less than or equal to 5E+17 cm−3.
Data Source
AI summary
When a channel formation region is formed of GaN in a MOSFET, there are cases where the actual threshold voltage (Vth) is lower than the setting value thereof and the actual carrier mobility (μ) during the ON state is lower than the setting value thereof. The reason for threshold voltage (Vth) and the carrier mobility (μ) being lower than the setting values is unknown. A MOSFET including a gallium nitride substrate, an epitaxial layer made of gallium nitride provided on top of the gallium nitride substrate, a gate insulating film provided in direct contact with the epitaxial layer, and a gate electrode provided in contact with the gate insulating film. The gallium nitride substrate has a dislocation density less than or equal to 1E+6 cm−2, and the epitaxial layer has a region with a p-type impurity concentration less than or equal to 5E+17 cm−3.