Avalanche Diode Pixel Circuit With Shared Nwell Quenching
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Solution Overview
Problem
Conventional single photon avalanche diode (SPAD) quenching circuits face limitations in minimizing pixel size while maintaining a high fill factor, as they often require separate chip arrangements or complex transistor combinations that restrict pixel size and impact fill factor.
Innovation Solution
The proposed image sensor incorporates a quenching and readout circuit within each pixel circuit, utilizing only N-type or P-type transistors, which eliminates the need for separate chip arrangements and allows for a simpler circuit structure, reducing pixel size limitations and maintaining a high fill factor.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area 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 minimum pixel size limitation worsens
Solution Approach 1:
The patent merges the quenching circuit and readout circuit into the pixel circuit by sharing the same Nwell structure. The quenching circuit uses the Nwell as a shared node for both the SPAD cathode and the quenching transistor, eliminating the need for separate chip arrangements and reducing device complexity while maintaining high fill factor.
2Reliability
If a logic circuit with p-type and N-type transistors is used for quenching, then the quenching function is achieved, but the minimum pixel size increases due to multiple Nwells requirement
Solution Approach 1:
The patent applies local quality by using only N-type transistors in the pixel circuit, which allows all transistors to share the same Nwell structure. This local modification to the circuit topology (using unipolar transistors instead of bipolar) enables space optimization while maintaining the quenching function through the shared Nwell node.
Solution Approach 2:
The Nwell structure serves multiple functions simultaneously: it acts as the cathode for the SPAD, the drain for the quenching transistor, and the shared well for all N-type transistors in the pixel circuit. This multi-functionality eliminates the need for separate Nwells and reduces minimum pixel size.
3Area of moving object
If separate chip arrangements are used for pixel circuit and quenching circuit, then the fill factor is improved, but the device complexity and manufacturing complexity increase
Solution Approach 1:
The patent combines the pixel circuit and quenching circuit into a single integrated structure on the same chip. The shared Nwell architecture allows both circuits to coexist in the same pixel unit without requiring separate chip arrangements, simplifying manufacturing 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 reduced pixel size constraints and improved fill factor, enhancing the performance of SPAD-based image sensors.
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 and a global current source circuit. Each of the plurality of pixel circuits includes a single photon avalanche diode and four P-type or N-type transistors. 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.