Angled RESURF Layers in Lateral Power MOSFETs
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Solution Overview
Problem
High power lateral MOSFETs face challenges in achieving high breakdown voltage and low on-resistance due to the limitations of conventional drift regions, and the RESURF structure complexity, particularly in controlling threshold voltage and connecting P layers to the source electrode.
Innovation Solution
A RESURF structure with alternating P and N-type layers angled upward under the lateral gate, eliminating the need for a deep trenched gate and body, and using a novel process to form the MOSFET with epitaxially grown layers, allowing for a shallow P-body channel region inversion and effective long drift region without increasing the top surface area.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional drift region is used in a lateral MOSFET, then the device structure is simple, but the breakdown voltage is limited and on-resistance is high
Solution Approach 1:
The drift region is segmented into alternating P-type and N-type layers forming a RESURF structure. This segmentation allows the electric field to be distributed across multiple PN junctions, increasing breakdown voltage while maintaining a manageable device structure through periodic layering rather than a single continuous drift region
Solution Approach 2:
The drift region uses a composite structure of alternating P-type and N-type semiconductor layers. This composite material approach enables simultaneous achievement of high breakdown voltage through field distribution and low on-resistance through higher doping levels in the N-type layers, overcoming the limitations of conventional single-material drift regions
2Reliability
If deep P-body and trenched gate structures are used, then breakdown voltage increases, but device complexity and manufacturing difficulty increase
Solution Approach 1:
The invention extracts and eliminates the deep trenched gate structure from the device architecture. Instead of using a trenched gate extending deep into the substrate, a planar gate structure is employed that works in conjunction with the RESURF layers to achieve the same breakdown voltage enhancement, significantly simplifying manufacturing
Solution Approach 2:
Instead of using a deep trenched gate to control the channel, the invention inverts the approach by using a planar gate with a shallow P-body that relies on the RESURF layer configuration to achieve voltage control. This inversion of the traditional deep-trench architecture simplifies the manufacturing process while maintaining device performance
3Ease of operation
If P layers in RESURF structure are connected to source electrode, then current flow is enabled, but current flow limitation becomes difficult
Solution Approach 1:
The P-body region is configured with local quality variations - it is shallow and positioned specifically to connect the P-type RESURF layers to the source electrode while incorporating a depletion region that locally limits current flow. This localized structural differentiation enables simultaneous current conduction and current limitation functions
4Reliability
If RESURF layers are configured horizontally, then manufacturing is simplified, but electric field concentration limits breakdown voltage
Solution Approach 1:
The RESURF layers are configured with a vertical component in addition to their horizontal extension. The layers angle upward from the horizontal drift region toward the gate, creating a three-dimensional structure that distributes the electric field more effectively across multiple dimensions, increasing breakdown voltage without significantly complicating the manufacturing process
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 enhances breakdown voltage and reduces on-resistance by evenly depleting the RESURF layers, allowing higher doping levels and improved current handling while simplifying the device structure and facilitating easier connection of P layers to the source.
Implementation Method 1
portions of the P-type layers under the lateral gate are inverted by the gate and form a conductive channel between the N+ source region and the N-type layers of the RESURF structure
Implementation Method 2
When the MOSFET is off, the P and N-type layers become entirely depleted more evenly along their length so the electric field is not concentrated near the channel region
Data Source
AI summary
In one embodiment, a RESURF structure between a source and a drain in a lateral MOSFET is formed in a trench having a flat bottom surface and angled sidewalls toward the source. Alternating P and N-type layers are epitaxially grown in the trench, and their charges balanced to achieve a high breakdown voltage. In the area of the source, the ends of the P and N-layers angle upward to the surface under the lateral gate and contact the body region. Thus, for an N-channel MOSFET, a positive gate voltage above the threshold forms a channel between the source and the N-layers in the RESURF structure as well as creates an inversion of the ends of the P-layers near the surface for low on-resistance. In another embodiment, the RESURF structure is vertically corrugated by being formed around trenches, thus extending the length of the RESURF structure for a higher breakdown voltage.