CMOS Image Sensor Pixel Charge Discharge via Transfer Transistor
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Solution Overview
Problem
The addition of a charge discharging transistor in CMOS image sensors reduces layout efficiency and sensitivity due to increased signal interconnections, which can lead to reduced light incidence and image distortion, especially when strong light sources are encountered.
Innovation Solution
The charge discharging transistor is driven using a signal for reading signals from each pixel, eliminating the need for a dedicated signal line, thereby maintaining layout efficiency and sensitivity by sharing the driving signal with existing pixel structure components.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a charge discharging transistor is added to the pixel to prevent charge accumulation and saturation, then reliability is improved, but device complexity increases due to additional signal interconnections
Solution Approach 1:
The patent applies the universality principle by enabling the transfer transistor to perform dual functions: transferring signal charges to the floating diffusion region during normal operation and discharging accumulated charges during the electronic shutter period. This eliminates the need for a separate charge discharging transistor, thereby maintaining reliability while reducing device complexity and the number of signal interconnections.
2Reliability
If a charge discharging transistor is added to the pixel, then charge discharge function is improved, but layout efficiency deteriorates due to increased interconnections
Solution Approach 1:
The transfer transistor is designed to serve multiple purposes: it transfers signal charges during photoelectric conversion and also discharges accumulated charges during the electronic shutter period when the shutter is closed. This multi-functional design eliminates the need for additional transistors and interconnections, improving layout efficiency while maintaining charge discharge capability.
Solution Approach 2:
The patent merges the charge transfer function and charge discharge function into a single transfer transistor. By controlling the timing of the transfer transistor's operation through the electronic shutter control signal, the same transistor performs both charge transfer to the floating diffusion region and charge discharge to the substrate, thereby simplifying the pixel structure and improving layout efficiency.
3Reliability
If upper layer interconnections are added to drive the charge discharging transistor, then charge discharge function is improved, but sensitivity deteriorates due to reduced light incidence
Solution Approach 1:
The transfer transistor performs dual functions of charge transfer and charge discharge without requiring additional upper layer interconnections. Since the same transistor and its existing interconnections are used for both functions, no additional light-blocking structures are needed, thereby maintaining photoelectric conversion sensitivity while achieving reliable charge discharge control during the electronic shutter period.
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 allows for effective discharge of charges without reducing layout efficiency or sensitivity, preventing image distortion and saturation issues, even under strong light conditions.
Implementation Method 1
a photodiode PD that photoelectrically converts incident light into signal charges and accumulates the same
Data Source
Figure 1
Figure 2~3
Figure 4
AI summary
A solid-state image capturing apparatus includes: a pixel array unit including two-dimensionally arranged pixels each including a photoelectric conversion unit, a transfer transistor that transfers charges accumulated in the photoelectric conversion unit, and a charge discharging transistor that selectively discharges the charges accumulated in the photoelectric conversion unit; and a driving unit that performs driving for reading signals from each pixel of the pixel array unit, and drives the charge discharging transistor by using a signal for the driving.