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 increased doping concentration, 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 in the varactor design, with a doping type opposite to the drift region, enhances depletion and increases the maximum depletion depth, thereby reducing the minimum capacitance and increasing the tuning range while maintaining a high Q factor.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If doping concentration is increased to improve Q factor, then Q factor is improved, but tuning range is reduced

Engineering Contradiction:

ImproveQ factorVSAvoidtuning range

Core Design Contradiction:

ReliabilityVSAdaptability or versatility

Solution Approach 1:

The patent applies local quality by creating a RESURF region with opposite doping type adjacent to the drift region. This localized doped region modifies the electric field distribution specifically in the depletion area, allowing high doping concentration in the drift region for high Q factor while the RESURF region enables full depletion for extended tuning range.

Inventive Principle:

Principle #3Local quality

Solution Approach 2:

The patent changes the doping parameters by introducing a RESURF region with opposite doping type and specific doping concentration. This parameter modification allows the drift region to maintain high doping for high Q factor while the RESURF region enables complete depletion of the drift region, achieving full tuning range without compromising Q factor.

Inventive Principle:

Principle #35Parameter changes

2Quantity of substance

If doping concentration is increased to reduce minimum capacitance, then minimum capacitance is reduced, but depletion depth is limited

Engineering Contradiction:

ImprovecapacitanceVSAvoiddepletion depth

Core Design Contradiction:

Quantity of substanceVSLength of stationary object

Solution Approach 1:

The RESURF region is introduced as a localized doped region with opposite doping type adjacent to the drift region. This local modification creates an extended depletion region that increases depletion depth without requiring increased doping concentration in the drift region, thereby reducing minimum capacitance while achieving full depletion.

Inventive Principle:

Principle #3Local quality

Solution Approach 2:

The patent extends the depletion region in the vertical dimension by introducing the RESURF region adjacent to the drift region. This dimensional extension allows the depletion region to penetrate deeper into the substrate, increasing depletion depth and reducing minimum capacitance without compromising the drift region's doping concentration.

Inventive Principle:

Principle #17Another dimension (Dimensionality change)

3Adaptability or versatility

If RESURF region is added to increase tuning range, then tuning range is increased, but device complexity is increased

Engineering Contradiction:

Improvetuning rangeVSAvoidstructure complexity

Core Design Contradiction:

Adaptability or versatilityVSDevice complexity

Solution Approach 1:

The RESURF region is merged with the drift region in a seamless integration where the RESURF region serves as an adjacent doped region. This merging approach extends the depletion region and increases tuning range without requiring separate complex structures or additional processing steps beyond standard doping techniques.

Inventive Principle:

Principle #5Merging (Combining)

Solution Approach 2:

The RESURF region serves multiple functions: it extends the depletion region depth, enables full depletion of the drift region, and maintains compatibility with standard CMOS fabrication processes. This multi-functionality achieves extended tuning range without proportionally increasing device complexity.

Inventive Principle:

Principle #6Universality (Multi-functionality)

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 full depletion and improved Q factor while expanding the tuning range, providing greater flexibility for circuit designers by optimizing the capacitance and resistance characteristics of the MOS varactor.

Implementation Method 1

Incorporating a reduced surface field (RESURF) region in the varactor design, with a doping type opposite to the drift region, enhances depletion and increases the maximum depletion depth

Methodology Applied

Scientific EffectDepletion region extension: Electric Field

Data Source

PatentUS11978810B2Method for forming an IC including a varactor with reduced surface field region

Publication Date: 2024.05.07 TAIWAN SEMICONDUCTOR MANUFACTURING CO LTD

AI summary

Various embodiments of the present disclosure are directed towards a method for forming a varactor comprising a reduced surface field (RESURF) region. The method includes forming a drift region having a first doping type within a substrate. A RESURF region having a second doping type is formed within the substrate such that the RESURF region is below the drift region. A gate structure is formed on the substrate. A pair of contact regions is formed within the substrate on opposing sides of the gate structure. The contact regions respectively abut the drift region and have the first doping type, and wherein the first doping type is opposite the second doping type.