SPAD Image Sensor Shared Output Circuit for Smaller Pixels
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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 of separate quenching and readout circuits, which can decrease the fill factor and restrict the minimum pixel unit size.
Innovation Solution
The image sensor incorporates a quenching and readout circuit within each pixel circuit, utilizing only N-type or P-type transistors, eliminating the need for separate Nwells and allowing for a higher fill factor and reduced minimum pixel size by integrating the pulling circuit outside each pixel, thus maintaining a high fill factor without increasing the pixel size limitations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional quenching circuit using logic circuits with p-type and N-type transistors is used, then the quenching function is achieved, but the minimum pixel size is increased due to the need for multiple Nwells with different potentials
Solution Approach 1:
The patent extracts the quenching function from the complex logic circuit structure and implements it using a simplified single-type transistor configuration. By removing the need for multiple Nwells with different potentials and using only N-type or only P-type transistors, the circuit area is significantly reduced while maintaining the quenching capability.
Solution Approach 2:
The patent creates a universal quenching circuit structure that can be implemented using a single transistor type (either N-type or P-type). This multi-functional approach allows the same circuit topology to achieve quenching without requiring different potential Nwells, thereby reducing the minimum pixel size requirement.
2Area of moving 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 integration is reduced
Solution Approach 1:
The patent merges the quenching circuit with the pixel circuit by integrating them into a single unified structure. Instead of forming an independent 3D circuit outside the pixel, the quenching functionality is combined with the pixel circuit using shared capacitors and transistors, thereby improving integration while maintaining fill factor.
Solution Approach 2:
The patent creates a universal circuit structure where the same transistors and capacitors serve multiple functions - both as part of the pixel circuit and as the quenching circuit. This multi-functionality eliminates the need for separate independent circuits, reducing overall device complexity while maintaining high fill factor.
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 pixel size or fill factor.
Implementation Method 1
When each photon is received by the SPAD, an avalanche current is triggered to respond that one photon is detected
Implementation Method 2
a quenching circuit is required in operation so as to quickly pull down a bias voltage of the SPAD to be lower than a breakdown voltage
Implementation Method 3
the bias voltage is quickly pulled up to be higher than the breakdown voltage to cause the SPAD to return to a photon detecting state
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.