Solid-State Image Sensor Pixel Cell for High Integration
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Solution Overview
Problem
Traditional solid-state image sensors with photocount functions are not suitable for high integration due to large pixel cell sizes, which limits the number of pixels and introduces errors in photon counting.
Innovation Solution
A solid-state image sensor design featuring a pixel array with an avalanche photodiode in two operational modes, a floating diffusion, transfer and reset transistors, an amplification transistor, and a memory with a capacitor structure, allowing for smaller pixel cells and reduced circuit elements, enabling higher integration and accurate photon counting.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If traditional solid-state image sensors with photocount function are used, then photon counting capability is achieved, but pixel cell size becomes large which reduces integration density
Solution Approach 1:
The pixel cell is segmented into distinct functional regions: an avalanche photodiode region for photon detection, a floating diffusion region for charge accumulation, and a memory region for charge storage. This segmentation allows each region to be optimized independently, reducing the overall pixel cell size while maintaining photocount functionality.
Solution Approach 2:
The patent implements a nested structure where the floating diffusion is positioned within or adjacent to the avalanche photodiode, and the memory capacitor is integrated within the same pixel cell footprint. This nesting approach allows multiple functional elements to share space, significantly reducing the pixel cell area compared to traditional layouts.
2Area of stationary object
If pixel cell size is reduced for high integration, then integration density increases, but photon counting accuracy may deteriorate
Solution Approach 1:
The patent applies local quality by creating a specialized micro-region within the pixel cell dedicated to photon counting. The avalanche photodiode is configured with specific doping profiles and geometric characteristics optimized for single-photon detection, while the floating diffusion and memory are sized appropriately for charge accumulation and storage. This localized optimization ensures high photon counting accuracy even in compact pixel cells.
Solution Approach 2:
The patent utilizes vertical dimensionality by stacking functional layers: the avalanche photodiode is positioned in a first vertical level, the floating diffusion in a second level, and the memory capacitor in a third level. This three-dimensional arrangement allows sufficient functional area for accurate photon counting while maintaining a small planar footprint for high integration density.
3Quantity of substance
If more pixels are integrated, then pixel density increases, but circuit complexity and manufacturing difficulty increase
Solution Approach 1:
The patent implements a universal pixel cell design where the same structural elements serve multiple functions: the avalanche photodiode performs both photon detection and charge generation, the floating diffusion serves as both charge accumulation node and transfer interface, and the memory capacitor provides both charge storage and signal level stabilization. This multi-functionality reduces the number of dedicated circuit elements needed, simplifying the overall design and reducing manufacturing complexity.
Solution Approach 2:
The patent merges several functions into shared components: the transfer transistor simultaneously controls charge transfer from the avalanche photodiode to the floating diffusion and also acts as a reset switch. The memory capacitor is integrated directly within the pixel cell rather than being a separate external component. This merging of functions and components reduces the total circuit element count and interconnect complexity, facilitating high-pixel-density integration.
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
The design achieves a higher integration density and accurate photon counting with reduced pixel size and error, facilitating real-time imaging and increased pixel density.
Implementation Method 1
an avalanche photodiode having a first mode in which a charge amount of charges approximately proportional to the number of photons which cause photoelectric conversion is collected in a cathode of the avalanche photodiode when photons enter the avalanche photodiode
Implementation Method 2
a second mode in which a saturated amount of charges is collected in the cathode when photons enter the avalanche photodiode under application of a second bias having a larger potential difference
Data Source
AI summary
A solid-state image sensor includes a pixel array including pixel cells arranged in a matrix. Each of the pixel cells includes an avalanche photodiode, a floating diffusion which accumulates charges, a transfer transistor which connects a cathode of the avalanche photodiode to the floating diffusion, a first reset transistor for resetting charges collected in the cathode of the avalanche photodiode, a second reset transistor for resetting charges accumulated in the floating diffusion, an amplification transistor for converting a charge amount of charges accumulated in the floating diffusion into a voltage, a memory which accumulates charges, and a count transistor which connects the floating diffusion to the memory.