CMOS Pixel Transfer Gate Pulsing for Dark Current Reduction

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Solution Overview

Problem

Conventional CMOS image sensor pixel cells face issues with dark current and fixed pattern noise due to inefficient charge transfer, which is exacerbated by the location of the transfer transistor gate near the photodiode, leading to increased thermally generated charge carriers and incomplete photo-generated charge transfer.

Innovation Solution

The pixel cell operates with a transfer gate that is activated by pulses during or at the end of the charge integration period, maintaining the reset gate at a constant voltage and pulsing the transfer gate to minimize dark current and ensure complete charge transfer, while also allowing for high dynamic range operation by fluctuating the reset gate voltage and pulsing the transfer gate multiple times.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Productivity

If the transfer transistor gate is located near the photodiode to facilitate charge transfer, then charge transfer efficiency is improved, but dark current increases due to thermally generated charge carriers

Engineering Contradiction:

Improvecharge transfer efficiencyVSAvoiddark current

Core Design Contradiction:

ProductivityVSObject-generated harmful factors

Solution Approach 1:

The transfer gate is activated by periodic pulses during or at the end of the charge integration period rather than being continuously on. This pulsed operation allows the transfer gate to be off during most of the integration time, minimizing thermally generated charge carrier accumulation, while still enabling complete charge transfer when pulsed at the appropriate time

Inventive Principle:

Principle #19Periodic action

Solution Approach 2:

The transfer gate voltage is dynamically controlled through pulsing during the integration period rather than being statically maintained at a high level. This dynamic control allows the system to optimize between minimizing dark current (gate off) and ensuring complete charge transfer (gate on during pulse)

Inventive Principle:

Principle #15Dynamics

2Reliability

If the transfer gate is continuously activated to ensure complete charge transfer, then charge transfer completeness is improved, but dark current and fixed pattern noise increase

Engineering Contradiction:

Improvecharge transfer completenessVSAvoidfixed pattern noise

Core Design Contradiction:

ReliabilityVSObject-generated harmful factors

Solution Approach 1:

The transfer gate is pulsed periodically during the integration period rather than being continuously activated. This periodic pulsing ensures that charge transfer occurs at specific moments while minimizing the overall time the gate is conductive, thereby reducing dark current accumulation and fixed pattern noise

Inventive Principle:

Principle #19Periodic action

Solution Approach 2:

The pulsed transfer gate operation maintains continuous charge transfer capability throughout the integration period by strategically timing pulses, ensuring that photo-generated charges are continuously transferred to the floating diffusion region without requiring the gate to be continuously on

Inventive Principle:

Principle #20Continuity of useful action

3Adaptability or versatility

If multiple pulses are applied to the transfer gate during integration, then dynamic range is improved, but device complexity increases

Engineering Contradiction:

Improvedynamic rangeVSAvoidcontrol signal complexity

Core Design Contradiction:

Adaptability or versatilityVSDevice complexity

Solution Approach 1:

Multiple periodic pulses are applied to the transfer gate during the integration period to achieve high dynamic range operation. These pulses are generated through controlled fluctuation of the reset gate voltage, enabling the pixel to respond to a wide range of light intensities while maintaining a relatively simple overall device structure

Inventive Principle:

Principle #19Periodic action

Solution Approach 2:

The transfer gate serves multiple functions through its pulsed operation: it transfers charge during integration, enables high dynamic range operation through multiple pulses, and can be controlled through the reset gate voltage fluctuation mechanism, reducing the need for separate dedicated control circuits

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 approach reduces dark current and fixed pattern noise, achieving efficient charge transfer with minimal lag and increased dynamic range, resulting in improved image quality and reduced noise.

Implementation Method 1

a photosensor, such as e.g., a photogate, photoconductor, or photosensor having an associated charge accumulation region within a substrate for accumulating photo-generated charge

Methodology Applied

Scientific EffectPhotoelectric effect: Photoelectric Effect

Implementation Method 2

a transfer transistor having a gate for transferring photoelectric charges from the photodiode to a sensing node

Methodology Applied

Scientific EffectCharge transfer: Conduction (electrical)

Data Source

PatentUS7829832B2Method for operating a pixel cell using multiple pulses to a transistor transfer gate

Publication Date: 2010.11.09 APTINA IMAGING CORP

AI summary

Methods for operating a pixel cell include efficient transferring of photo-charges using multiple pulses to a transistor transfer gate during a charge integration period for an associated photosensor. The pixel cell can be operated with efficient transfer characteristics in either normal or high dynamic range (HDR) mode. The high dynamic range can be realized by either operating an optional HDR transistor or by fluctuating the voltage applied to a reset gate.