RF Switch Capacitance Cancellation Using Varactor Linearization

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Solution Overview

Problem

Bulk CMOS high-power RF switches suffer from non-linear capacitances due to drain-body and source-body diodes, which degrade linearity and create design challenges, while SOI switches, though better, still have residual capacitances that need reduction for improved performance.

Innovation Solution

Implementing a voltage variable capacitor, such as a varactor, in parallel or series with the NFET device to counteract the non-linear capacitance by connecting its terminals to the drain and source terminals, thereby achieving a substantially constant capacitance region across a voltage range.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Ease of manufacture

If bulk CMOS process is used for high power RF switch design, then cost advantage and better scalability are achieved, but non-linear capacitances from drain-body and source-body diodes degrade linearity

Engineering Contradiction:

Improvecost advantage and scalabilityVSAvoidlinearity

Core Design Contradiction:

Ease of manufactureVSManufacturing precision

Solution Approach 1:

The patent converts the harmful non-linear capacitance effect into a beneficial cancellation mechanism by introducing varactors with opposite non-linearity characteristics. The varactors' non-linear capacitance-voltage response is specifically designed to counteract and cancel the non-linear drain-body and source-body capacitances, transforming the linearity degradation problem into a solution where the same non-linear effect is used for cancellation.

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

Solution Approach 2:

The patent changes the capacitance parameters by introducing voltage-variable capacitors (varactors) whose capacitance values can be dynamically adjusted through bias voltage control. By tuning the varactor capacitance parameters to match and oppose the non-linear characteristics of the switch capacitances, the overall linearity is improved while maintaining the bulk CMOS manufacturing advantages.

Inventive Principle:

Principle #35Parameter changes

2Loss of energy

If RF floating is applied to body, N-well and substrate, then insertion loss is improved, but drain-body and source-body capacitances remain and cause design challenges

Engineering Contradiction:

Improveinsertion lossVSAvoidlinearity

Core Design Contradiction:

Loss of energyVSManufacturing precision

Solution Approach 1:

The patent applies the same cancellation principle to address the remaining drain-body and source-body capacitances after RF floating. The varactors are configured to specifically target and cancel these residual non-linear capacitances, converting what would be harmful linearity-degrading effects into beneficial cancellation that further improves overall switch performance.

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

Solution Approach 2:

The patent introduces dynamic control of capacitance through voltage-variable varactors that can be tuned in real-time. This dynamic adjustment capability allows the system to optimize both insertion loss and linearity performance by adapting the varactor capacitance values to compensate for the residual drain-body and source-body capacitances that remain even after RF floating is applied.

Inventive Principle:

Principle #15Dynamics

3Manufacturing precision

If SOI process is used for high power switches, then linearity is improved due to minimal diode capacitances, but residual drain-body and source-body capacitances still need reduction

Engineering Contradiction:

ImprovelinearityVSAvoidresidual capacitance performance

Core Design Contradiction:

Manufacturing precisionVSReliability

Solution Approach 1:

The patent applies the varactor cancellation technique to SOI switches to further reduce the already minimal residual drain-body and source-body capacitances. By introducing varactors with opposite non-linearity characteristics, the patent converts the remaining harmful residual capacitances into beneficial cancellation, achieving even higher linearity performance in SOI-based high power switches.

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

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 method significantly improves linearity by flattening the capacitance-voltage response, reducing harmonics, and enhancing the performance of high-power RF switches in both bulk CMOS and SOI technologies.

Implementation Method 1

the larger parasitic capacitances present a design challenge

Methodology Applied

Scientific EffectCapacitance: Capacitance

Implementation Method 2

Implementing a voltage variable capacitor, such as a varactor, in parallel or series with the NFET device to counteract the non-linear capacitance

Methodology Applied

Scientific EffectVaractor effect:

Data Source

PatentUS12519469B2Switch capacitance cancellation circuit

Publication Date: 2026.01.06 MURATA MFG CO LTD

AI summary

Methods and devices used to cancel non-linear capacitances in high power radio frequency (RF) switches manufactured in bulk complementary metal-oxide-semiconductor (CMOS) processes are disclosed. The methods and devices are also applicable to stacked switches and RF switches fabricated in silicon-on-insulator (SOI) technology.