Multi-mode Pixel Circuit Design for Conversion Gain and Electron Management

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Solution Overview

Problem

CMOS imager circuits face limitations in pixel performance due to inefficiencies in charge transfer, amplification, and signal output, particularly in modes like blooming control, linear-logarithmic, and global shutter, where existing pixel circuits struggle with conversion gain and electron management.

Innovation Solution

A multi-mode pixel circuit design featuring a readout circuit with a floating diffusion node, switch circuits, and a virtual capacitor, allowing for different conversion gains and modes of operation such as dual conversion gain, blooming control, linear-logarithmic, and global shutter, by controlling switches with specific signals to manage electron transfer and storage effectively.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If a conventional pixel circuit with single floating diffusion node is used, then the circuit structure is simple, but the conversion gain is limited and electron management efficiency is poor

Engineering Contradiction:

Improveconversion gainVSAvoidcircuit structure

Core Design Contradiction:

Measurement precisionVSDevice complexity

Solution Approach 1:

The pixel circuit is divided into multiple pixel units, each with its own floating diffusion node. This segmentation allows independent charge storage and readout operations, enabling higher conversion gain through optimized charge transfer paths while maintaining manageable circuit complexity through modular design

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

A tail node is introduced as an intermediary element coupled to the floating diffusion node of another pixel unit. This tail node facilitates efficient electron transfer and management between pixels, improving conversion gain and electron management efficiency without requiring complete circuit redesign

Inventive Principle:

Principle #24Intermediary (Mediator)

2Adaptability or versatility

If multi-mode operation is implemented for different illumination environments, then the adaptability is improved, but the device complexity increases

Engineering Contradiction:

Improveoperational modesVSAvoidswitch circuit control

Core Design Contradiction:

Adaptability or versatilityVSDevice complexity

Solution Approach 1:

The pixel circuit is designed with multi-functional capability to operate in different modes (photodiode mode, photogate mode, blooming control mode, logarithmic mode, global shutter mode) using the same basic circuit structure. Switches are controlled by different signal combinations to achieve mode switching, providing universal operation across diverse illumination environments without requiring separate circuits for each mode

Inventive Principle:

Principle #6Universality (Multi-functionality)

Solution Approach 2:

The circuit employs dynamic switch control mechanisms where the operational mode is determined by real-time control signals. This allows the pixel to adapt its behavior dynamically based on illumination conditions, maintaining versatility while managing complexity through software-controlled switch states rather than hardwired configurations

Inventive Principle:

Principle #15Dynamics

3Productivity

If charge transfer efficiency is improved through optimized readout circuit, then the signal output quality is enhanced, but the manufacturing precision requirements increase

Engineering Contradiction:

Improvecharge transfer efficiencyVSAvoidtransistor formation

Core Design Contradiction:

ProductivityVSManufacturing precision

Solution Approach 1:

Multiple functional elements are merged into a unified readout circuit structure where the floating diffusion node, output transistor, and switch circuit work together as an integrated system. This merging optimizes charge transfer efficiency through coordinated operation while using standard CMOS fabrication processes to avoid excessive manufacturing precision requirements

Inventive Principle:

Principle #5Merging (Combining)

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 pixel circuit achieves improved conversion gain and efficient electron management across various modes, enhancing pixel performance and dynamic range, while preventing information loss and extending the operational capabilities in high and low illumination environments.

Implementation Method 1

Each pixel cell has a readout circuit that includes at least an output field effect transistor formed in the substrate and a charge-storage region formed on the substrate connected to the gate of an output transistor. The charge-storage region may be constructed as a floating diffusion node.

Methodology Applied

Scientific EffectPhotoelectric effect: Photoelectric Effect

Implementation Method 2

a floating diffusion node for storing data of the photosensor

Methodology Applied

Scientific EffectCapacitance: Capacitance

Data Source

PatentUS9204074B2Multi-mode operation pixel design for pixel performance improvement

Publication Date: 2015.12.01 HIMAX IMAGING LIMITED

AI summary

A pixel circuit includes a plurality of pixel units, and one of the pixel units includes a photosensor, a readout circuit, and a switch circuit. The readout circuit is coupled to a supply voltage and the photosensor, which includes a floating diffusion node for storing data of the photosensor and an output node for outputting data of the floating diffusion node. The switch circuit is coupled between the photosensor and a tail node, wherein the tail node is coupled to the floating diffusion node of another pixel unit.