Coupled Polysilicon Guard Rings for Breakdown Voltage Stability
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Solution Overview
Problem
Power field effect transistors, particularly high-voltage JFETs and LDMOS devices, face challenges in maintaining breakdown voltage stability due to mobile and fixed surface charges, which can lead to reliability issues and voltage drift during long-term high-temperature reverse bias testing, especially when using standard CMOS processes with long drift regions.
Innovation Solution
The implementation of coupled polysilicon guard rings disposed above the drift region, electrically coupled to power device regions, which form PN junctions to spread electric fields and operate in reverse bias, reducing leakage current and mitigating the effects of surface charges without increasing process complexity or affecting device performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the drift region length is increased to improve breakdown voltage, then breakdown voltage is improved, but device area increases and specific on-resistance deteriorates
Solution Approach 1:
The drift region is segmented into multiple sections with different doping concentrations. High-voltage regions have lower doping concentrations to support higher breakdown voltages, while low-voltage regions have higher doping concentrations to reduce on-resistance. This segmentation allows the device to achieve high breakdown voltage without proportionally increasing the overall drift region length and device area.
2Ease of manufacture
If standard CMOS processes are used with long drift regions, then manufacturing simplicity is maintained, but breakdown voltage stability deteriorates due to mobile and fixed surface charges
Solution Approach 1:
Guard rings are formed around the drift region before final device operation. These guard rings are doped with high concentration phosphorus to create a protective barrier that repels mobile and fixed surface charges from the drift region boundaries. This preliminary protective structure prevents charge accumulation that would otherwise cause breakdown voltage drift during long-term high-temperature reverse bias testing, while still using standard CMOS processes.
3Strength
If doping concentrations are adjusted according to RESURF techniques, then breakdown voltage is improved, but device complexity increases
Solution Approach 1:
Different doping concentrations are applied to different spatial regions of the drift region. High-voltage areas receive lower doping concentrations to maximize breakdown voltage, while areas near guard rings or in low-voltage sections receive higher doping concentrations to control electric field distribution and reduce on-resistance. This local quality approach optimizes breakdown voltage without requiring complex multi-layer doping profiles throughout the entire device.
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 enhances breakdown voltage stability by effectively spreading electric fields and reducing voltage drift, thereby improving the reliability and performance of power semiconductor devices without introducing additional complexity.
Implementation Method 1
form PN junctions to spread electric fields and operate in reverse bias, reducing leakage current
Data Source
AI summary
Coupled polysilicon guard rings for enhancing breakdown voltage in a power semiconductor device are presented herein. Polysilicon guard rings are disposed above the power device drift region and electrically coupled to power device regions (e.g., device diffusions) so as to spread electric fields associated with an operating voltage. Additionally, PN junctions (i.e., p-type and n-type junctions) are formed within the polysilicon guard rings to operate in reverse bias with a low leakage current between the power device regions (e.g., device diffusions). Low leakage current may advantageously enhance the electric field spreading without deleteriously affecting existing (i.e., normal) power device performance; and enhanced electric field spreading may in turn reduce breakdown-voltage drift.