Floating Diffusion Reset Level Boost in CMOS Pixel Cell

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Solution Overview

Problem

Conventional CMOS image sensors face challenges in achieving higher floating diffusion reset levels without adding additional circuitry, which increases costs and chip real estate usage, affecting image quality and lag performance.

Innovation Solution

The floating diffusion reset level is boosted by utilizing the capacitance between the input and output terminals of an amplifier transistor in the pixel cell, specifically through the gate-source capacitance of a source follower coupled transistor, allowing the select transistor to be switched on before the reset transistor is turned off during the reset operation, temporarily pulling down and then restoring the output terminal voltage to boost the floating diffusion voltage.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If additional circuitry is added to pixel cells to increase reset levels, then floating diffusion reset level is improved, but device complexity and chip real estate increase

Engineering Contradiction:

Improvefloating diffusion reset levelVSAvoidadditional circuitry

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The amplifier transistor's own gate-source capacitance is utilized to boost the reset level, allowing the existing component to serve dual purposes: signal amplification and reset level enhancement, eliminating the need for separate boosting circuitry

Inventive Principle:

Principle #25Self-service

Solution Approach 2:

The gate-source capacitance of the amplifier transistor is used for both its primary amplification function and as a reset level boosting mechanism, making the same component serve multiple functions within the pixel cell

Inventive Principle:

Principle #6Universality (Multi-functionality)

2Reliability

If additional circuitry is added to pixel cells to increase reset levels, then floating diffusion reset level is improved, but manufacturing cost increases

Engineering Contradiction:

Improvefloating diffusion reset levelVSAvoidmanufacturing cost

Core Design Contradiction:

ReliabilityVSEase of manufacture

Solution Approach 1:

Inventive Principle:

Principle #25Self-service

Solution Approach 2:

The solution uses existing standard transistor components with their inherent capacitance properties rather than adding expensive specialized circuitry, leveraging readily available component characteristics for the boosting function

Inventive Principle:

Principle #27Cheap short-living objects (Disposable)

3Ease of operation

If conventional reset operation is used without timing optimization, then circuit operation is simple, but image lag performance deteriorates

Engineering Contradiction:

Improvereset operation simplicityVSAvoidimage lag performance

Core Design Contradiction:

Ease of operationVSReliability

Solution Approach 1:

The select transistor is turned on before the reset transistor is turned off, preparing the output terminal to receive and stabilize the boosted voltage, which prevents image lag by ensuring proper charge transfer timing

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The reset operation uses dynamic timing control where the select and reset transistors are switched at optimized times relative to each other, allowing the system to adapt the reset sequence for improved image lag performance while maintaining operational simplicity

Inventive Principle:

Principle #15Dynamics

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 method enhances the floating diffusion reset level without significant side effects on conversion gain, improving image quality and lag performance while minimizing additional circuitry requirements.

Implementation Method 1

utilizing the capacitance between the input and output terminals of an amplifier transistor in the pixel cell, specifically through the gate-source capacitance of a source follower coupled transistor

Methodology Applied

Scientific EffectCapacitance: Capacitance

Data Source

PatentUS9491386B2Floating diffusion reset level boost in pixel cell

Publication Date: 2016.11.08 OMNIVISION TECHNOLOGIES INC

AI summary

A reset level in a pixel cell is boosted by switching ON a reset transistor of the pixel cell to charge the floating diffusion to a first reset level during a reset operation. A select transistor is switched from OFF to ON during the floating diffusion reset operation to discharge an output terminal of an amplifier transistor. The reset transistor is switched OFF after the output terminal of the amplifier transistor has been discharged in response to the switching ON of the select transistor. The output terminal of the amplifier transistor charges to a static level after being discharged. The floating diffusion coupled to the input terminal of the amplifier transistor follows the output terminal of the amplifier transistor across an amplifier capacitance coupled between the input terminal and the output terminal of the amplifier transistor to boost the reset level of the floating diffusion.