SPAD Pixel Circuit Layout for Smaller Image Sensor Pixels
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Solution Overview
Problem
Existing image sensors with single photon avalanche diodes (SPADs) face challenges in achieving a low limitation on minimum pixel size and high fill factor due to the requirements of quenching circuits.
Innovation Solution
The image sensor incorporates an SPAD within each pixel circuit, utilizing a quenching and readout circuit that only includes N-type or P-type transistors, thereby eliminating the need for Nwells with different potentials and reducing the pixel size limitation while maintaining a high fill factor.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional quenching circuit with p-type and N-type transistors is used, then the quenching function is achieved, but the minimum pixel size is limited due to the need for multiple Nwells with different potentials
Solution Approach 1:
The patent applies homogeneity by using only N-type transistors (or only P-type transistors) in the quenching circuit, eliminating the need for multiple Nwells with different potentials. This uniform transistor type approach reduces the complexity of well structures and allows smaller pixel dimensions while maintaining the quenching function.
Solution Approach 2:
The patent extracts the requirement for multiple Nwells with different potentials from the quenching circuit design. By removing this constraint and using a single type of transistor, the design achieves smaller pixel size without compromising the quenching capability.
2Area of moving object
If an independent 3D quenching circuit is formed outside the pixel circuit, then the fill factor is improved, but the pixel circuit complexity increases
Solution Approach 1:
The patent merges the quenching circuit with the pixel circuit by integrating the quenching transistors directly within the pixel structure. This combination allows the quenching function to be performed without requiring separate external circuitry, maintaining high fill factor while managing complexity through unified design.
3Reliability
If two Nwells with different potentials are formed within a signal pixel unit, then the quenching circuit can be implemented, but the minimum distance between Nwells causes limitation on minimum pixel size
Solution Approach 1:
The patent uses only N-type transistors throughout the pixel circuit, including the quenching circuit. This eliminates the need for multiple Nwells with different potentials, removing the minimum distance constraint between Nwells and enabling smaller pixel sizes while maintaining reliable quenching circuit operation.
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 allows for efficient detection of weak light and high frequency signals with minimal pixel size constraints and high fill factor, enhancing the sensor's performance in photon detection applications.
Implementation Method 1
a single photon avalanche diode (SPAD) can be used as a detector for weak light, and has the benefits of high avalanche gain, fast response and low power consumption. 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 and a global current source circuit. Each of the plurality of pixel circuits includes a single photon avalanche diode (SPAD) and a floating diffusion. 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. The floating diffusion is used to record a voltage of one photon event detected by the SPAD in an exposure period.