Controllable Surge MOSFET With Parallel BJT For High Current Handling
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Solution Overview
Problem
High power silicon carbide MOSFETs face challenges in handling surge currents due to defects in substrates and gate insulation layers, leading to low manufacturing yields and potential damage from high current levels during surge conditions, and existing devices lack efficient surge current handling capabilities.
Innovation Solution
A controllable surge MOSFET (CST) configuration that includes a power MOSFET, a MOS-gated bipolar junction transistor (BJT) in parallel, and a driver MOSFET, where the BJT carries surge currents and naturally saturates them, protecting the device and downstream equipment from damage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If power MOSFETs are designed to handle peak current levels, then surge current capability is improved, but device complexity and manufacturing cost increase
Solution Approach 1:
The power MOSFET is segmented into multiple parallel current paths: a first current path through the drift region for normal operation, and a second current path through a conductive type region for surge current handling. This segmentation allows the device to handle surge currents without requiring the entire structure to be oversized for peak conditions.
Solution Approach 2:
A conductive type region with higher carrier concentration is locally formed in the drift region to create a low-resistance path specifically for surge current. This local modification allows surge current handling capability to be enhanced without changing the overall device structure or increasing manufacturing complexity.
2Power
If power MOSFETs are designed for high current density, then forward current capability is improved, but on-resistance increases due to drift region characteristics
Solution Approach 1:
A conductive type region with higher carrier concentration is locally formed in the drift region to create a low-resistance path specifically for surge current. This local modification allows surge current handling capability to be enhanced without changing the overall device structure or increasing manufacturing complexity.
Solution Approach 2:
The drift region is formed as a composite structure with a first conductive type region (lower carrier concentration) and a second conductive type region (higher carrier concentration). This composite structure optimizes the balance between on-resistance and surge current capability by having different regions serve different functions.
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 CST effectively handles surge currents up to 100 amps with fast switching speeds during normal operation and reduced switching speeds during surge conditions, providing a compact, cost-effective solution with high current density support and low incremental fabrication costs.
Implementation Method 1
MOSFETs are unipolar devices in which current conduction occurs solely through majority carrier transport, MOSFETs may exhibit very high switching speeds
Implementation Method 2
The gate forms a capacitor with the channel region. Thus, only minimal charging and discharging current ('displacement current') is required during switching
Implementation Method 3
the BJT carries surge currents and naturally saturates them, protecting the device and downstream equipment from damage
Data Source
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AI summary
Semiconductor switching devices include a wide band-gap power transistor, a wide band-gap surge current transistor that is coupled in parallel to the power transistor, and a wide band-gap driver transistor that is configured to drive the surge current transistor. Substantially all of the on-state output current of the semiconductor switching device flows through the channel of the power transistor when a drain-source voltage of the power transistor is within a first voltage range, which range may correspond, for example, to the drain-source voltages expected during normal operation. In contrast, the semiconductor switching device is further configured so that in the on-state the output current flows through both the surge current transistor and the channel of the power transistor when the drain-source voltage of the power transistor is within a second, higher voltage range.