SiC MOSFET Conductive Layer Prevents Bipolar Current
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Solution Overview
Problem
Silicon carbide semiconductor devices face reliability issues due to bipolar current flow during free-wheeling operations, leading to stacking fault extension and insulation breakdown, which are not effectively addressed by existing methods that either increase chip size or reduce resistance, resulting in tradeoffs that do not simultaneously reduce bipolar current and prevent element destruction during switching.
Innovation Solution
A silicon carbide semiconductor device structure is designed with a Schottky diode and conductive layers of lower sheet resistance than the well region, preventing bipolar current flow by ensuring no ohmic connection between the conductive layer and the second well region, and reducing displacement current-induced voltage through a capacitor structure, thereby enhancing reliability and preventing insulation breakdown.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a unipolar diode (Schottky barrier diode) is provided in the active region to prevent bipolar current flow, then reliability is improved, but device complexity increases
Solution Approach 1:
The patent merges the Schottky barrier diode structure with the existing MOSFET body diode structure by forming the Schottky barrier at the interface between the n-type drift layer and the p-type well region. This integration allows the same structural elements to serve dual functions: the body diode for standard operation and the Schottky barrier for preventing bipolar current during free-wheeling operations, thereby improving reliability without proportionally increasing device complexity
Solution Approach 2:
The p-type well region serves multiple functions: it forms the body diode cathode, creates the Schottky barrier with the n-type drift layer to prevent bipolar current, and provides structural support. This multi-functionality allows a single structural element to address multiple reliability concerns, reducing the need for additional separate components
2Ease of operation
If the body diode is used as a free-wheeling diode, then ease of operation is improved, but reliability deteriorates due to bipolar current flow
Solution Approach 1:
The Schottky barrier is preliminarily formed at the interface between the n-type drift layer and p-type well region to prevent bipolar current flow before it can occur during free-wheeling operations. By establishing this barrier in advance, the device automatically blocks harmful bipolar current when the body diode conducts, allowing easy free-wheeling operation without reliability degradation
3Reliability
If stress testing is applied to screen out elements with large forward voltage change, then reliability is improved, but productivity decreases due to extended testing time
Solution Approach 1:
The Schottky barrier structure is preliminarily designed and formed during manufacturing to inherently prevent bipolar current flow. This preliminary structural design eliminates the need for extended stress testing to screen out defective elements, as the structure itself ensures reliability by design rather than requiring lengthy operational screening processes
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 bipolar current during free-wheeling operations, improves reliability by preventing element destruction, and maintains a reduced chip size, addressing the tradeoff issues of existing solutions.
Implementation Method 1
an n-type channel epitaxial layer is formed on a p-type well region forming an active region, the channel epitaxial layer operates as a unipolar diode
Implementation Method 2
reducing displacement current-induced voltage through a capacitor structure
Implementation Method 3
a conductive layer formed above the bottom surface of the second well region so as not to be in ohmic connection with the second well region, the conductive layer being lower in sheet resistance than the second well region
Data Source
AI summary
In an SiC-MOSFET with a built-in Schottky diode, a bipolar current may be passed in a second well region formed at a terminal part to reduce a breakdown voltage. In the SiC-MOSFET with the built-in Schottky diode, a conductive layer in Schottky connection with the second well region is provided on the second well region in the terminal part, and the conductive layer is electrically connected with a source electrode of the MOSFET. A conductive layer contact hole is provided for connecting only the conductive layer and the source electrode.