Power MOSFET Stepwise Drift Region for On-Resistance and HCI
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Solution Overview
Problem
Power MOSFETs face challenges with high on-resistance (Rdson) and hot carrier injection (HCI) issues, which affect their performance and reliability.
Innovation Solution
The formation of a power MOSFET structure with a first drift region of one conductivity type and a second drift region of opposite conductivity type, where the second drift region is shallower than the first, forming a stepwise shape, reduces on-resistance and device capacitance while improving HCI capability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a single drift region is used in power MOSFET, then the device structure is simple, but the on-resistance is high
Solution Approach 1:
The drift region is segmented into multiple regions with different conductivity types (first drift region with first conductivity type, second drift region with second conductivity type). This segmentation allows each region to contribute differently to the overall electrical characteristics, reducing the total on-resistance while maintaining a manageable structural complexity through systematic doping profiles.
Solution Approach 2:
Different regions of the drift structure are assigned different local properties through selective doping. The first drift region has a first doping concentration and conductivity type, while the second drift region has a different doping concentration and conductivity type. This local quality variation optimizes the electrical field distribution and reduces on-resistance in specific areas without uniformly increasing complexity throughout the entire device.
2Reliability
If current flow is near the silicon surface, then device capacitance is reduced, but hot carrier injection problems increase
Solution Approach 1:
The patent extends the drift region depth into the third dimension (vertical dimension into the substrate). By forming drift regions that extend deeper into the substrate rather than confining current flow strictly near the surface, the design trades some capacitance for significantly reduced hot carrier injection, as the extended depth provides a more favorable electric field distribution that protects against hot carrier effects.
Solution Approach 2:
The second drift region with opposite conductivity type acts as an intermediary structure between the surface region and the substrate. This intermediate region modifies the electric field distribution and carrier transport characteristics, mediating between the conflicting requirements of low capacitance (surface proximity) and low hot carrier injection (deeper penetration), thereby resolving the contradiction through a transitional structural element.
Data Source
AI summary
Power Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs) and methods of forming the same are provided. A power MOSFET may comprise a first drift region formed at a side of a gate electrode, and a second drift region beneath the gate electrode, adjacent to the first drift region, with a depth less than a depth of the first drift region so that the first drift region and the second drift region together form a stepwise shape. A sum of a depth of the second drift region, a depth of the gate dielectric, and a depth of the gate electrode may be of substantially a same value as a depth of the first drift region. The first drift region and the second drift region may be formed at the same time, using the gate electrode as a part of the implanting mask.