SPAD Image Sensor Pixel Circuit With Shared Readout and Quenching
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Solution Overview
Problem
Conventional single photon avalanche diode (SPAD) image sensors face limitations in minimum pixel size and fill factor due to the requirement for separate quenching and readout circuits, which can decrease the fill factor and restrict pixel size.
Innovation Solution
The image sensor incorporates a quenching and readout circuit within each pixel circuit, utilizing only N-type or P-type transistors, allowing for a single Nwell per pixel and eliminating the need for separate quenching circuits, thus minimizing pixel size limitations and maintaining a high fill factor.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of stationary object
If an independent 3D quenching circuit is formed outside the pixel circuit, then the fill factor is improved, but the device complexity increases and manufacturing precision requirements increase
Solution Approach 1:
The quenching circuit is merged with the pixel circuit by integrating it into the same planar structure. The quenching circuit shares the pixel region with the SPAD and other pixel components, eliminating the need for separate 3D stacking. This integration maintains high fill factor while reducing overall device complexity through unified circuit design.
Solution Approach 2:
The quenching function is extracted from a separate 3D circuit and implemented as a planar circuit within the pixel region. By taking out the quenching operation and implementing it through transistors and capacitors in the same layer as the pixel circuit, the design avoids complex 3D integration while maintaining performance.
2Reliability
If a logic circuit with p-type and N-type transistors is used for quenching, then the quenching function is improved, but the minimum pixel size increases due to multiple Nwells requirement
Solution Approach 1:
Instead of using both p-type and N-type transistors throughout the pixel circuit, the invention uses local quality by employing only N-type transistors (or only P-type) in specific regions. This allows the pixel to have a single Nwell (or single Pwell) structure, reducing the minimum pixel size while maintaining reliable quenching function through localized circuit design.
Solution Approach 2:
The invention changes the transistor type parameter from a mixed p-type and N-type configuration to a single-type configuration. This parameter change eliminates the need for multiple wells with different potentials, thereby reducing the minimum pixel size while preserving the quenching function through adjusted circuit topology and operating parameters.
3Reliability
If separate quenching and readout circuits are used, then the quenching reliability is improved, but the fill factor decreases and pixel density is reduced
Solution Approach 1:
The quenching circuit and readout circuit are merged into a single integrated pixel circuit. The same transistors and nodes serve dual functions: quenching the SPAD avalanche and reading out the detected signal. This merging maintains quenching reliability while maximizing the fill factor by eliminating redundant circuit elements.
Solution Approach 2:
Circuit elements are designed with multi-functionality, where the same components perform both quenching and readout operations. The pixel circuit nodes and transistors are universally used for multiple purposes, ensuring reliable quenching while maximizing the active area available for photon detection.
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 enables efficient detection of weak light and high-frequency signals with improved pixel density and fill factor, allowing for effective photon event counting and image processing without compromising the sensor's performance.
Implementation Method 1
When each photon is received by the SPAD, an avalanche current is triggered to respond that one photon is detected
Data Source
AI summary
There is provided an image sensor employing an avalanche diode. The image sensor includes a plurality of pixel circuits arranged in a matrix, a plurality of pulling circuits, a plurality of output circuits and a global current source circuit. Each of the plurality of pixel circuits includes a single photon avalanche diode and a P-type or N-type select switch transistor. Each of the plurality of pulling circuits is arranged corresponding to one pixel circuit column. The global current source circuit is used to form a current mirror with each of the plurality of pulling circuits. Each of the plurality of output circuits is shared by at least two pixel circuits.