P-Channel LDMOS NBL Structure for Breakdown Voltage and On-State Resistance Trade-off
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Solution Overview
Problem
P-channel LDMOS devices face challenges in achieving a good on-state resistance (Rdson) and breakdown voltage (BV) trade-off due to the inhibition of the RESURF principle, leading to increased Rdson when using thin film SOI substrates, where the small active silicon area limits the definition of an n-type floating region.
Innovation Solution
A p-channel LDMOS device with a controlled n-type buried layer (NBL) is introduced, which provides an evacuation path for electrons and enhances space-charge depletion, improving RESURF effectiveness and allowing for increased p-well doping to maintain charge balance, thus reducing Rdson and increasing BV.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If an n-type floating region is defined in thin film SOI substrate to achieve RESURF effect, then breakdown voltage is improved, but the small active silicon area increases Rdson
Solution Approach 1:
The patent introduces a vertically-oriented n-type buried layer (NBL) extending from the substrate up to the drift region, adding a vertical dimension to the RESURF effect. This vertical NBL structure enables the RESURF effect to occur in the vertical direction rather than requiring a lateral n-type floating region, thereby achieving high breakdown voltage without sacrificing active silicon area and maintaining low on-state resistance.
2Reliability
If p-well doping is increased to reduce Rdson, then on-state resistance is improved, but charge balance is disrupted and BV decreases
Solution Approach 1:
The n-type buried layer acts as a feedback mechanism that compensates for the charge imbalance caused by increased p-well doping. The NBL provides additional positive charge that balances the increased negative charge from higher p-well doping concentrations, allowing the device to maintain both low on-state resistance and high breakdown voltage simultaneously through dynamic charge balance adjustment.
3Ease of operation
If drain and handle wafer are biased to the same potential in p-channel LDMOS, then device operation is simplified, but RESURF principle is inhibited
Solution Approach 1:
The n-type buried layer serves as an intermediary structure that enables the RESURF effect to function even when the drain and handle wafer are at the same potential. By introducing this intermediate n-type layer between the p-channel drift region and the substrate, the patent creates the necessary electric field conditions for RESURF without requiring complex biasing arrangements, thus maintaining ease of operation while achieving high breakdown voltage.
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 NBL layer significantly improves the BV versus Rdson trade-off and provides high immunity to the Kirk effect, resulting in lower cost transistors with enhanced safe-operating-area and reduced on-state resistance.
Implementation Method 1
providing an evacuation path for electrons generated by impact ionization
Implementation Method 2
The addition of the NBL deep inside the drift region supports a space-charge depletion region which increases the RESURF effectiveness, thus improving BV
Implementation Method 3
An optimum NBL implanted dose can be set to ensure fully compensated charge balance among n and p doping in the drift region (charge balance conditions)
Data Source
AI summary
A p-channel LDMOS device with a controlled n-type buried layer (NBL) is disclosed. A Shallow Trench Isolation (STI) oxidation is defined, partially or totally covering the drift region length. The NBL layer, which can be defined with the p-well mask, connects to the n-well diffusion, thus providing an evacuation path for electrons generated by impact ionization. High immunity to the Kirk effect is also achieved, resulting in a significantly improved safe-operating-area (SOA). The addition of the NBL deep inside the drift region supports a space-charge depletion region which increases the RESURF effectiveness, thus improving BV. An optimum NBL implanted dose can be set to ensure fully compensated charge balance among n and p doping in the drift region (charge balance conditions). The p-well implanted dose can be further increased to maintain a charge balance, which leads to an Rdson reduction.