CMOS Pixel Overflow Control for Charge Leakage Suppression
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Solution Overview
Problem
In image pickup devices, especially CMOS image sensors, global electronic shutter operations can lead to color mixing due to charge leakage from the photoelectric conversion unit to the charge holding unit, resulting in a decline in image quality.
Innovation Solution
The implementation of an image pickup device with a pixel configuration that includes a photoelectric conversion unit, a charge holding unit, a transfer unit, and an overflow control unit, which has two potential barrier modes to prevent charge leakage, allowing both units to hold charges of different frames without mixing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the transfer gate is switched off to prevent charge transfer during global electronic shutter operation, then charge leakage from photoelectric conversion unit to charge holding unit is reduced, but color mixing still occurs due to charges climbing over the potential barrier
Solution Approach 1:
The patent introduces a third potential barrier structure as an intermediary element between the photoelectric conversion unit and the charge holding unit. This intermediate potential barrier acts as an additional obstacle that prevents charges from climbing over the transfer gate's potential barrier, thereby blocking the charge leakage path that causes color mixing while maintaining the transfer gate's charge transfer function.
Solution Approach 2:
The patent creates an asymmetric potential barrier configuration where the transfer gate's potential barrier and the third potential barrier have different heights and positions. The third potential barrier is specifically designed with a lower height than the transfer gate's barrier, creating an asymmetric structure that allows controlled charge transfer when needed while providing enhanced blocking capability during the global electronic shutter period to prevent color mixing.
2Reliability
If a high potential barrier is formed by the transfer gate to prevent charge leakage, then charge transfer control is improved, but charge leakage still occurs due to saturated charges climbing over the barrier
Solution Approach 1:
The patent segments the single potential barrier function into two separate potential barriers: the transfer gate's potential barrier and the third potential barrier. This segmentation divides the charge blocking function into two distinct barriers, where the third potential barrier specifically handles the prevention of charge leakage from saturated photoelectric conversion units, while the transfer gate's barrier maintains primary charge transfer control.
Solution Approach 2:
The third potential barrier serves as an intermediary blocking structure that intercepts charges before they can climb over the transfer gate's potential barrier. This intermediate barrier captures or redirects the leaking charges, preventing them from reaching the charge holding unit and causing color mixing, thereby reducing charge loss without compromising the transfer gate's control function.
3Object-affected harmful factors
If the third potential barrier is set to a lower height than the transfer gate's barrier, then charge leakage is suppressed, but the structure becomes more complex
Solution Approach 1:
The third potential barrier structure is designed to serve multiple functions simultaneously: it acts as a charge blocking barrier during the global electronic shutter period, provides an additional overflow path for excess charges, and works in conjunction with the transfer gate to maintain overall charge transfer control. This multi-functionality justifies the added structural element by providing multiple benefits from a single addition.
Solution Approach 2:
The third potential barrier is strategically positioned and dimensioned with specific local characteristics - it has a lower height than the transfer gate's barrier and is located at a specific position relative to the photoelectric conversion unit and charge holding unit. This localized optimization allows the barrier to effectively suppress charge leakage in the critical region where color mixing occurs, while maintaining overall system 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 configuration effectively suppresses color mixing, ensuring improved image quality by appropriately controlling the overflow control unit's potential barriers to prevent charge leakage during global electronic shutter operations.
Implementation Method 1
a photoelectric conversion unit which performs photoelectric conversion
Implementation Method 2
a transfer unit which transfers a charge of the photoelectric conversion unit to the charge holding unit when being in an on-state
Implementation Method 3
an overflow control unit which discharges a charge of the photoelectric conversion unit when being in an on-state, and has a first mode in which a first potential barrier is formed
Data Source
AI summary
A pixel includes: a photoelectric conversion unit; a charge holding unit; a transfer unit which transfers a charge of the photoelectric conversion unit to the charge holding unit in an on-state; and an overflow control unit which discharges a charge of the photoelectric conversion unit in an on-state. There is a period in which both the transfer unit and the overflow control unit are in an off-state and the photoelectric conversion unit and the charge holding unit respectively hold a charge of a different frame. In a first mode, a first potential barrier is formed, the first potential barrier being lower than a potential barrier formed between the photoelectric conversion unit and the charge holding unit by the transfer unit in an off-state. In a second mode, a second potential barrier that is even lower than the first potential barrier is formed.