MOSFET Stripe Cells with Varying Thresholds for Thermal Runaway Prevention

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Solution Overview

Problem

Modern power MOSFETs, especially trench and vertical double diffused MOSFETs, are susceptible to thermal runaway when operated in the linear regime due to a critical current density effect, where increased temperature leads to higher current density, causing device failure.

Innovation Solution

The transistor cells are divided into regions with varying threshold voltages, with low threshold voltage regions distributed along stripes to preferentially conduct current, reducing the risk of thermal runaway by increasing current density above or closer to the critical current density, and using a voltage clamp circuit to manage voltage spikes.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Adaptability or versatility

If modern power MOSFETs are operated in the linear regime, then they can self-protect from over-voltage spikes and limit current, but they become susceptible to thermal runaway due to operating below the critical current density

Engineering Contradiction:

Improvelinear mode operation capabilityVSAvoidthermal runaway susceptibility

Core Design Contradiction:

Adaptability or versatilityVSReliability

Solution Approach 1:

The patent applies local quality by creating regions with different threshold voltages within the MOSFET structure. Specifically, it forms a body region with a first threshold voltage and a channel region with a second, higher threshold voltage. This spatial differentiation of electrical properties ensures that when the device operates in the linear regime, the current density is concentrated in the lower threshold voltage body region, pushing it above the critical current density and preventing thermal runaway while maintaining linear mode functionality.

Inventive Principle:

Principle #3Local quality

2Ease of operation

If the MOSFET is operated below the critical current density to maintain normal operation, then the device can function properly, but thermal runaway occurs due to positive feedback between temperature and current density

Engineering Contradiction:

Improvenormal operationVSAvoidthermal runaway

Core Design Contradiction:

Ease of operationVSObject-affected harmful factors

Solution Approach 1:

The patent changes the electrical parameter distribution within the device by creating a body region with a first threshold voltage and a channel region with a second, higher threshold voltage. This parameter differentiation modifies the current conduction characteristics so that during normal operation, the body region operates at higher current density (above critical) while the channel region operates at lower current density, thereby preventing thermal runaway feedback while maintaining normal device functionality.

Inventive Principle:

Principle #35Parameter changes

3Reliability

If asymmetric cells with different threshold voltages are used to prevent thermal runaway, then current density distribution improves, but current crowding occurs in continuous areas

Engineering Contradiction:

Improvethermal runaway preventionVSAvoidcurrent crowding

Core Design Contradiction:

ReliabilityVSObject-generated harmful factors

Solution Approach 1:

The patent segments the device into distinct body and channel regions with different threshold voltages. The body region, having the lower threshold voltage, is further divided into multiple cells that are electrically connected in parallel. This segmentation distributes the high current density across multiple discrete regions rather than allowing it to concentrate in continuous areas, thereby preventing current crowding while maintaining thermal runaway prevention through the lower threshold voltage regions.

Inventive Principle:

Principle #1Segmentation

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 solution effectively mitigates thermal runaway and current crowding, allowing for safer operation in both linear and switching applications by ensuring even current distribution and managing voltage spikes, thereby preventing device failure.

Implementation Method 1

By dividing at least some of the cells into a plurality of regions with varying threshold voltage, when the transistor is just turned on conduction will occur preferentially in regions of low threshold voltage. The current density in these regions will accordingly be higher, either above the critical current density Jc or less far below it, reducing the risk of thermal runaway.

Methodology Applied

Scientific EffectElectrical conduction: Conduction (electrical)

Implementation Method 2

using a voltage clamp circuit to manage voltage spikes

Methodology Applied

Scientific EffectVoltage clamping:

Data Source

PatentUS7737507B2Insulated gate field effect transistors

Publication Date: 2010.06.15 NEXPERIA BV

AI summary

The invention relates to FETs with stripe cells (6). Some of the cells have alternating low and high threshold regions (10, 8) along their length. In a linear operations regime, the low threshold regions conduct preferentially and increase the current density, thereby reducing the risk of thermal runaway. By distributing the low threshold regions (10) along the length of the cells (6), the risk of current crowding is reduced.