Digital Pixel Image Sensor Parasitic Charge Discharge
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Solution Overview
Problem
Conventional image sensors using analog pixels are vulnerable to noise and coupling, which affects their performance in processing high-resolution image signals.
Innovation Solution
A digital pixel design that includes a photo diode, storage diode, floating diffusion node, and transistors to generate, store, and transmit optical signals digitally, preventing parasitic charge transmission and enhancing operating properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If analog pixels are used to process image signals, then the device complexity is reduced, but the noise vulnerability and coupling increase
Solution Approach 1:
The pixel is divided into multiple functional regions: a photo diode region for optical signal generation, a storage node region for charge accumulation, a floating diffusion node region for signal conversion, and multiple transistor control regions. This segmentation isolates different functions to prevent coupling and reduce noise interference between regions.
Solution Approach 2:
A storage node is introduced as an intermediary between the photo diode and the floating diffusion node. This storage node temporarily holds the generated charge and allows controlled transfer to the floating diffusion node, preventing direct coupling and reducing noise transmission during the conversion process.
2Ease of manufacture
If analog pixels are used, then the manufacturing process is simpler, but the signal processing reliability deteriorates
Solution Approach 1:
The storage node performs preliminary charge accumulation and holding before the final conversion to voltage at the floating diffusion node. This preliminary action allows the system to prepare the signal in a controlled manner, improving reliability by ensuring complete charge collection and preventing signal loss during processing.
Solution Approach 2:
The pixel circuit maintains continuous control over the charge signal through multiple transistor switches that can sequentially transfer charges between nodes. This continuous control ensures the signal remains intact and can be processed reliably without interruption or degradation throughout the conversion process.
3Ease of operation
If a simple pixel structure is used, then the ease of operation is improved, but the parasitic charge transmission increases
Solution Approach 1:
The discharge transistor is specifically designed to extract and remove parasitic charges from the photo diode region. By providing a dedicated discharge path controlled by the discharge transistor, parasitic charges are separated from the useful signal charges, preventing their transmission to subsequent stages and improving signal purity.
Solution Approach 2:
The pixel circuit operates in periodic cycles: during the integration period, charges are accumulated in the storage node; during the transfer period, charges are moved to the floating diffusion node; and during the discharge period, parasitic charges are removed. This periodic operation allows systematic handling of both signal and parasitic charges, improving operation control while reducing parasitic transmission.
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 digital pixel design improves image sensor performance by reducing noise and enhancing parasitic light sensitivity and shutter efficiency, allowing for more efficient data storage, reading, and processing.
Implementation Method 1
a photo diode connected to a first node and configured to generate an optical signal from an incident light
Data Source
AI summary
A digital pixel includes a photo diode connected to a first node and configured to generate an optical signal from an incident light, a storage diode configured to store the optical signal in a second node, a floating diffusion node configured to output a detection signal based on the optical signal, a first transmission transistor connected between the first and second nodes, and configured to transmit the optical signal from the first node to the second node, a second transmission transistor connected between the second node and the floating diffusion node, and configured to transmit the optical signal from the second node to the floating diffusion node, and a discharge transistor connected to the first node and configured to be turned on in a section in which the second transmission transistor is turned on to discharge a parasitic charge generated in the first node.