CMOS Image Sensor Shared Sensing Node Capacitance
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Solution Overview
Problem
Conventional CMOS image sensors face challenges in achieving sufficient storage capacitance at the sensing node, despite enhanced fill-factor and photodiode area, which limits photosensitivity and image quality.
Innovation Solution
The CMOS image sensor employs a sharing scheme where the sensing node of an adjacent non-selected pixel is connected to the selected pixel during operation, utilizing an NMOS transistor switching device controlled by a line or row select signal to increase storage capacitance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the photodiode area is increased to enhance fill-factor, then photosensitivity is improved, but the storage capacitance at the sensing node remains insufficient
Solution Approach 1:
The patent merges the sensing node of a selected pixel with the sensing node of an adjacent non-selected pixel by connecting them through a switching device. This combination increases the total storage capacitance available at the sensing node, allowing it to receive and store the increased photocharges generated by the enhanced fill-factor photodiode area.
Solution Approach 2:
The patent introduces a switching device as an intermediary element that selectively connects the sensing node of the current pixel to the sensing node of the adjacent pixel. This switching device is controlled by the line select signal and enables dynamic connection only when needed, thereby increasing storage capacitance without permanently affecting pixel independence.
2Reliability
If the sensing node capacitance is increased to store more photocharges, then image quality is enhanced, but the pixel structure becomes more complex
Solution Approach 1:
The patent makes the sensing node multi-functional by enabling it to serve both as the storage node for the current selected pixel and as an extended storage node for adjacent non-selected pixels. The same sensing node structure performs multiple functions depending on the switching state, avoiding the need for separate additional capacitance structures.
Solution Approach 2:
The patent utilizes existing sensing node structures from adjacent pixels to provide the additional storage capacitance needed. Instead of adding external capacitance elements, the system serves itself by repurposing the inherent sensing node capacitance of neighboring pixels through the switching connection.
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 increased storage capacitance, enabling the reception of more photocharges and enhancing image quality by leveraging the enhanced fill-factor of the photodiode area.
Implementation Method 1
a photocharge generation means for generating photocharges by absorbing an external light
Implementation Method 2
utilizing an NMOS transistor switching device controlled by a line or row select signal to increase storage capacitance
Data Source
AI summary
A CMOS image sensor has a pixel array provided with a plurality of unit pixels arranged in a matrix shape of rows and columns. Each of the unit pixel includes a photocharge generation means for generating photocharges by absorbing an external light; and a sensing node for receiving the photocharges transferred from the photocharge generation means, wherein the sensing node of the unit pixel in a previous scan line is shared with a sensing node of a unit pixel in a current scan line in response to a line select signal of the current line.