Image Sensor Precharge Boost for White Dot Noise and Lag
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Solution Overview
Problem
Image sensors suffer from white dot noise and precharge lag due to electron retention in photodiodes, which affects full well capacity and image quality.
Innovation Solution
The implementation of a precharge boost mechanism using a boost capacitor to increase the voltage of the floating diffusion node, preventing voltage loss during readout and reducing lag by coupling a boost voltage via a boost capacitor between the floating diffusion and a voltage reference source.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If precharge operation is performed to clear electrons from photodiodes, then white dot noise is reduced, but precharge lag increases and full well capacity decreases
Solution Approach 1:
The patent applies preliminary action by performing a precharge operation before the main image capture to clear residual electrons from photodiodes. This preliminary electron clearing prevents white dot noise during actual imaging, though it introduces time lag. The boost capacitor mechanism is then used to mitigate the capacity loss caused by this preliminary operation.
Solution Approach 2:
The patent changes the voltage parameter of the floating diffusion node by coupling a boost voltage through a boost capacitor. This voltage enhancement compensates for the voltage loss that occurs during precharge operation, thereby maintaining full well capacity despite the preliminary electron clearing operation.
2Object-affected harmful factors
If precharge operation is performed to clear electrons from photodiodes, then white dot noise is reduced, but full well capacity decreases
Solution Approach 1:
The patent applies the counterweight principle by introducing a boost capacitor that provides a compensating voltage boost to the floating diffusion node. This boost counteracts the voltage loss from precharge operation, effectively restoring and maintaining the full well capacity that would otherwise be reduced by the electron clearing operation.
Solution Approach 2:
The patent changes the voltage parameter of the floating diffusion node by coupling a boost voltage through a boost capacitor. This voltage enhancement compensates for the voltage loss that occurs during precharge operation, thereby maintaining full well capacity despite the preliminary electron clearing operation.
3Object-affected harmful factors
If voltage is increased during precharge to improve electron clearing, then white dot noise is reduced, but voltage loss during readout increases
Solution Approach 1:
The patent introduces a boost capacitor as an intermediary energy storage element between the voltage reference source and the floating diffusion node. This mediator stores energy during the precharge phase and releases it during readout, preventing voltage loss without requiring continuous high voltage application that would increase energy consumption.
Solution Approach 2:
The patent employs periodic action by applying the boost voltage through the boost capacitor at specific phases of the operation cycle. The boost is applied during precharge to enhance electron clearing, then maintained or adjusted during readout to compensate for voltage loss, creating a periodic voltage enhancement pattern that optimizes both noise reduction and energy efficiency.
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 solution effectively reduces white dot noise and precharge lag, enhancing the full well capacity and image quality by ensuring stable voltage levels during image acquisition.
Implementation Method 1
coupling a boost voltage via a boost capacitor between the floating diffusion and a voltage reference source
Data Source
AI summary
Image sensors with precharge boost are disclosed herein. An example image sensor may include pixels that each include a photodiode to receive image light and produce image charge in response, a floating diffusion to receive the image charge, a transfer gate to couple the photodiode to the floating diffusion in response to a transfer control signal, a reset gate to couple a reset voltage to the floating diffusion in response to a reset control signal, and a boost capacitor coupled between the floating diffusion and a boost voltage source, wherein, during a precharge operation, the boost voltage is provided to the boost capacitor for a portion of time the transfer gate is enabled and while the reset gate is disabled.