Solid-State Image Sensor Charge Discharge for After-Image Reduction
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Solution Overview
Problem
Existing solid-state image sensors suffer from after-image artifacts due to charge overflow, particularly when capturing moving subjects, as charge is not completely transferred from the memory part to the floating diffusion, leading to noise and distortion in images.
Innovation Solution
A solid-state image sensor design that includes a photoelectric conversion part, a charge holding part, a first transfer gate, a second transfer gate, and a charge discharging gate, where the charge accumulated in the charge holding part is discharged before the next frame's charge accumulation, and the voltage settings for the transfer gates are optimized to enhance the potential difference between the PD-MEM barrier and the MEM electric-potential, reducing charge overflow.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If charge is transferred from the memory part to the floating diffusion in the electrical global exposure method, then the photocharge is read out for image capture, but charge overflow occurs causing after-image artifacts and noise
Solution Approach 1:
The patent applies preliminary action by discharging accumulated charge from the memory part before the next exposure cycle begins. This prevents charge overflow into the floating diffusion that causes after-image artifacts, while maintaining efficient charge transfer during the actual readout phase. The preliminary discharge operation clears residual charge that would otherwise contaminate subsequent images.
Solution Approach 2:
The patent segments the charge handling process into distinct phases: charge accumulation during exposure, charge transfer to floating diffusion for readout, and charge discharge from memory part for the next cycle. This segmentation allows optimization of each phase independently, ensuring complete charge transfer during readout while preventing overflow during the transition to the next exposure cycle.
2Stability of the object's composition
If the mechanical shutter method is used to control exposure time, then all pixels can have the same exposure time period, but the device size increases and the mechanism speed is limited
Solution Approach 1:
The patent replaces the mechanical shutter system with an electrical control mechanism. Instead of using a physical shutter to control exposure timing, the invention uses electrical signals to control the transfer and discharge of charge between the photodiode, memory part, and floating diffusion. This substitution eliminates moving parts, reducing device complexity and size while maintaining uniform exposure timing across all pixels through synchronized electrical control.
3Reliability
If the transfer gate is closed to hold photocharge in the floating diffusion, then the charge is preserved for readout, but kTC noise cannot be removed and image quality deteriorates
Solution Approach 1:
The patent applies preliminary action by discharging charge from the memory part before the next exposure cycle begins. This preliminary discharge operation removes accumulated charge that would cause after-image artifacts, ensuring clean signal acquisition for the subsequent exposure while maintaining stable charge holding capability during the actual readout phase through proper transfer gate control.
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 design effectively reduces after-image artifacts by ensuring complete charge transfer and minimizing noise, resulting in improved image quality and reduced distortion when capturing moving subjects.
Implementation Method 1
a photodiode, a transfer gate, a floating diffusion (FD)... The CMOS image sensor reads out, through MOS transistors, a photocharge accumulated in a pn junction capacitor of a photodiode which is a photoelectric transducer
Data Source
AI summary
There is provided a solid-state image sensor including a photoelectric conversion part which generates a charge corresponding to received light and accumulates the charge therein, a charge holding part in which before the charge accumulated in the photoelectric conversion part is transferred to a floating diffusion region, the charge is held for a predetermined time, a first transfer gate which transfers the charge accumulated in the photoelectric conversion part to the charge holding part, a second transfer gate which transfers the charge held in the charge holding part to the floating diffusion region, and a charge discharging gate which discharges the charge in the photoelectric conversion part. Before charge accumulation in the photoelectric conversion part for the next frame is started, part of the charge accumulated in the charge holding part is discharged.