CMOS Image Sensor Two-Stage Charge Transfer for S/N Ratio
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Solution Overview
Problem
Conventional CMOS image sensors face a trade-off where reducing the capacitance of the floating diffusion to improve the signal-to-noise ratio (S/N) results in decreased electron accumulation in the photodiode, leading to a drop in S/N ratio, as the capacitance of the photodiode must also be reduced to maintain charge transfer efficiency.
Innovation Solution
The proposed imaging device employs a two-stage charge transfer method, where the first charge is transferred to a capacitance element with a smaller capacitance than the photodiode, and the second charge is transferred in a second mode, allowing for increased signal voltage and gain while maintaining a fixed charge accumulation in the photodiode, thereby improving S/N ratio and sensitivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the capacitance of the floating diffusion is reduced to improve the signal-to-noise ratio, then the gain increases, but the photodiode capacitance must also be reduced which decreases the charge accumulation and lowers the S/N ratio
Solution Approach 1:
The patent divides the charge transfer process into two distinct stages: a first charge transfer in the first sub-period and a second charge transfer in the second sub-period. This segmentation allows the system to transfer different portions of accumulated charge at different times, enabling the floating diffusion capacitance to be reduced for higher gain while the photodiode maintains sufficient capacitance for adequate charge accumulation. The dual-stage approach resolves the contradiction by separating the charge transfer function into multiple temporal phases.
Solution Approach 2:
The patent introduces dynamic control of the transfer transistor gate voltage with two different voltage levels corresponding to the first and second sub-periods. This dynamic adjustment allows optimization of charge transfer efficiency in each stage while maintaining the ability to accumulate sufficient charge in the photodiode. The dynamic gate voltage control enables the system to adapt the transfer characteristics to the specific requirements of each transfer stage, resolving the static contradiction between gain and charge accumulation.
2Productivity
If the capacitance of the photodiode is reduced to match the reduced floating diffusion capacitance, then charge transfer efficiency is maintained, but the amount of accumulated charge decreases leading to lower S/N ratio
Solution Approach 1:
The patent segments the charge transfer operation into two sub-periods with distinct transfer characteristics. During the first sub-period, charge is transferred with specific efficiency requirements, and during the second sub-period, additional charge transfer occurs with different optimization goals. This temporal segmentation allows the photodiode to maintain larger capacitance for sufficient charge accumulation while still achieving high transfer efficiency through the two-stage process.
Solution Approach 2:
The patent changes the operating parameters of the transfer transistor by applying two different gate voltage levels corresponding to the first and second sub-periods. This parameter change enables optimization of charge transfer efficiency in each stage independently, allowing the photodiode to maintain larger capacitance without sacrificing overall transfer efficiency. The parameter changes resolve the contradiction by enabling efficient transfer from a larger-capacitance photodiode through controlled voltage adjustment.
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 enhances the S/N ratio and sensitivity by increasing the signal voltage while preventing a reduction in the charge accumulated in the photodiode, effectively addressing the trade-off issue between capacitance and noise suppression.
Implementation Method 1
a photodiode that receives light to produce charge
Data Source
AI summary
The problem of the invention is to improve S/N and provide a high-sensitivity imaging device. The CMOS image sensor includes multiple pixels arranged in a two-dimensional array, where each pixel includes a photodiode PD that receives light to produce charge, a capacitance element FD, and a transfer transistor M1 connected between photodiode PD and capacitance element FD, where the capacitance of capacitance element FD is less than the capacitance of photodiode PD. With the drive method, transfer transistor M1 turns on during a predetermined period in a first charge transfer mode after the charge accumulation period is completed; first charge Q1 accumulated on photodiode PD is transferred to capacitance element FD; the charge on capacitance element FD is then reset; transfer transistor M1 turns on during a predetermined period in a second charge transfer mode after reset is completed; and second charge Q2 accumulated on photodiode PD is transferred to transfer element FD.