Pixel Circuit Surface Doped Region Dark Current Prevention

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Solution Overview

Problem

CMOS image sensors experience increased noise and picture-quality deterioration during global shutter operations due to the inability to sample the reset signal immediately before the image signal, leading to dark current issues in the standby state of photodiodes.

Innovation Solution

A pixel circuit with an additional charge storing unit and a surface-doped region is introduced, allowing for the generation of a reset signal before the image signal, and the surface-doped region prevents dark current, ensuring charge preservation during long standby times.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Productivity

If a global shutter operation is implemented in a conventional pixel circuit, then all photodiodes can transfer charges simultaneously to floating diffusion nodes, but the reset signal cannot be sampled immediately before the image signal, resulting in increased noise

Engineering Contradiction:

Improveglobal shutter operation capabilityVSAvoidsignal sampling timing

Core Design Contradiction:

ProductivityVSMeasurement precision

Solution Approach 1:

The pixel circuit is segmented into multiple floating diffusion nodes (first FD node and second FD node) with dedicated transfer transistors. The first FD node receives reset signals while the second FD node receives image signals, allowing independent timing control and immediate sampling of reset signal before image signal for each pixel column

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

An additional charge storing unit (capacitor) is introduced as an intermediary between the photodiode and floating diffusion nodes. This capacitor temporarily stores charges from photodiodes, enabling the reset signal to be transferred and sampled before the image signal while maintaining charge integrity

Inventive Principle:

Principle #24Intermediary (Mediator)

2Productivity

If photodiodes remain in standby state for long periods in global shutter operation, then all rows can capture images simultaneously, but dark current accumulates causing picture-quality deterioration

Engineering Contradiction:

Improvesimultaneous charge transferVSAvoiddark current

Core Design Contradiction:

ProductivityVSObject-generated harmful factors

Solution Approach 1:

The harmful dark current is extracted and isolated by directing it to a dedicated first floating diffusion node that is separately read out from the second floating diffusion node containing image signals. This separation allows dark current to be removed from the image signal path, preventing picture-quality deterioration

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

Reset signals are transferred to the first floating diffusion node immediately after photodiode reset, before the image signal transfer. This preliminary action allows early sampling and subtraction of reset-level dark current through correlated double sampling, compensating for dark current accumulation during standby

Inventive Principle:

Principle #10Preliminary action

3Reliability

If an additional charge storing unit and surface-doped region are added to the pixel circuit, then reset signal can be generated before image signal and dark current prevented, but device complexity increases

Engineering Contradiction:

Improvenoise reduction and dark current preventionVSAvoidpixel circuit structure

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The charge storing unit is implemented by merging the capacitor structure with existing pixel circuit elements, sharing nodes and transistors where possible. The surface-doped region is integrated into the substrate structure, combining multiple functions (charge storage, dark current blocking, signal transfer) into a unified circuit architecture

Inventive Principle:

Principle #5Merging (Combining)

Solution Approach 2:

The first and second floating diffusion nodes serve multiple functions: the first FD node handles reset signal sampling and dark current collection, while the second FD node handles image signal sampling. Transfer transistors are configured to perform both charge transfer and signal routing functions, reducing the need for additional dedicated components

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

This solution reduces noise in correlated double sampling and maintains picture quality even in global shutter operations with a large number of rows by generating the reset signal immediately before the image signal and preventing dark current loss.

Implementation Method 1

Each pixel in the APS array 110 has a respective photodiode PD that photo-electrically transforms incident light into a charge

Methodology Applied

Scientific EffectPhotoelectric effect: Photoelectric Effect

Implementation Method 2

a surface-doped region is formed in the charge storing unit for preventing dark current in the charge storing unit

Methodology Applied

Scientific EffectDoping: Dopants

Data Source

PatentUS7414233B2Pixel circuit with surface doped region between multiple transfer transistors and image sensor including the same

Publication Date: 2008.08.19 SAMSUNG ELECTRONICS CO LTD

AI summary

A pixel circuit of an image sensor includes a photo-converting unit such as a photo-diode for generating charge from incident light. The pixel circuit also includes a charge storing capacitor for storing the charge generated by the photo-converting unit. The pixel circuit further includes a floating diffusion node that receives the charge from the charge storing unit after being reset. Thus, an image signal VSIG is generated after a reset signal VRES is generated from the pixel circuit.