MOS Varactor RESURF Structure for Wider Tuning Range
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Solution Overview
Problem
Metal-oxide-semiconductor (MOS) varactors face a tradeoff between Q factor and tuning range due to the doping concentration of the N-type well, which affects the capacitance and depletion depth, limiting their performance in LC-tank voltage-controlled oscillators.
Innovation Solution
Incorporating a reduced surface field (RESURF) region with opposite doping type below the drift region enhances depletion, increasing the maximum depletion depth and thus reducing the minimum capacitance, thereby expanding the tuning range while maintaining a high Q factor.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If doping concentration is increased to reduce resistance, then electrical conductivity is improved, but maximum depletion depth and tuning range decrease
Solution Approach 1:
The patent applies local quality by creating a RESURF region with a specific doping concentration that differs from other regions of the substrate. This localized doping structure allows the area under the gate to achieve full depletion while maintaining acceptable conductivity, resolving the contradiction between conductivity and tuning range.
Solution Approach 2:
The patent changes the doping concentration parameter spatially across the substrate. By implementing a graded or non-uniform doping profile with a RESURF region, the maximum depletion depth is increased without sacrificing overall electrical conductivity, thereby expanding the tuning range while maintaining reliability.
2Reliability
If doping concentration is increased to reduce resistance, then Q factor is improved, but tuning flexibility decreases
Solution Approach 1:
The RESURF region creates a localized area with optimized doping characteristics that enables full depletion under the gate structure. This local modification allows the varactor to achieve high Q factor through reduced series resistance while simultaneously providing enhanced tuning flexibility through increased maximum depletion depth.
Solution Approach 2:
By modifying the doping concentration parameter in the RESURF region, the patent achieves a balance between Q factor and tuning flexibility. The changed doping profile allows for greater depletion depth variation during tuning while maintaining low resistance for high Q factor performance.
3Adaptability or versatility
If maximum depletion depth is increased to expand tuning range, then capacitance variation is improved, but series resistance increases
Solution Approach 1:
The RESURF region addresses this contradiction by creating a localized doping structure that optimizes the balance between depletion depth and resistance. The specific doping concentration in this region allows maximum depletion depth to increase for expanded tuning range while the overall structure maintains acceptable series resistance levels.
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 RESURF region allows for increased Q factor and tuning range of the MOS varactor, enabling better flexibility for circuit designers by optimizing the capacitance and depletion characteristics.
Implementation Method 1
enhances depletion under the gate structure, allowing for increased maximum depletion depth
Implementation Method 2
reduced surface field (RESURF) region in the substrate of MOS varactors, which enhances depletion under the gate structure
Data Source
AI summary
Various embodiments of the present disclosure are directed towards an integrated chip including a first doped region in a substrate and comprising a first doping type. A gate structure is over the first doped region. A pair of contact regions are in the substrate on opposing sides of the gate structure and comprising the first doping type. The first doped region continuously laterally extends between the pair of contact regions and contacts the pair of contact regions. A second doped region is in the substrate and along a bottom of the first doped region. The second doped region comprises a second doping type opposite the first doping type.