CMOS Image Sensor Transfer Gate Boost for Black Dot Reduction
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Solution Overview
Problem
Increased full well capacity (FWC) in CMOS image sensors leads to higher dynamic range and signal-to-noise ratio but results in increased occurrence of 'black dots' due to non-linear operation of pixel transistors, especially in low light conditions.
Innovation Solution
The method involves increasing the capacitance of the charge-to-voltage conversion mechanism by partially enabling a second transfer transistor during readout, adding parasitic capacitance without additional circuitry, allowing for higher reset voltage and FWC without significant increases in black dot occurrence.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the full well capacity (FWC) of photodiodes is increased to achieve higher dynamic range and signal-to-noise ratio, then image quality is improved, but the occurrence of black dots increases due to non-linear operation of pixel transistors
Solution Approach 1:
The patent changes the capacitance parameter of the charge-to-voltage conversion mechanism by utilizing the parasitic capacitance of transfer transistors. This parameter change allows the system to accommodate higher FWC while maintaining linear transistor operation, thus preventing black dot formation while preserving signal-to-noise ratio improvements
Solution Approach 2:
The patent introduces the parasitic capacitance of transfer transistors as an intermediary element to the charge-to-voltage conversion mechanism. This intermediary capacitance acts as a buffer that allows higher reset voltages without causing non-linear operation of the source follower transistor, thereby enabling higher FWC without black dots
2Illumination intensity
If the full well capacity (FWC) is increased to allow larger voltage swing for higher dynamic range, then dynamic range is improved, but pixel transistors operate outside linear region causing signal compression
Solution Approach 1:
The patent modifies the capacitance parameter of the charge-to-voltage conversion mechanism by adding the parasitic capacitance of transfer transistors. This parameter modification enables the system to handle larger voltage swings corresponding to higher dynamic range while keeping the source follower transistor within its linear operation region, thus maintaining signal linearity
Solution Approach 2:
The patent dynamically utilizes the parasitic capacitance of transfer transistors during the readout process. By controlling the timing and state of transfer transistors, the system dynamically adjusts the effective capacitance to maintain optimal operating conditions for linear transistor operation across varying signal levels
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 approach enables increased FWC while minimizing the occurrence of black dots, thereby improving image quality without additional components or circuitry.
Implementation Method 1
A pixel generally includes a photodiode (i.e., a photo-sensitive region responsible for collecting electromagnetic energy and converting the collected electromagnetic energy into electrons)
Implementation Method 2
Photo-generated charge accumulated in the photosensitive region of the photodiode is ultimately converted into a voltage by a charge-to-voltage mechanism (also referred to as a floating diffusion) included in the pixel
Implementation Method 3
enabling a first transfer transistor to transfer photo-generated charge from a first photosensitive region to the shared charge-to-voltage mechanism
Data Source
AI summary
An image sensor includes photosensitive regions, transfer transistors, and one or more shared charge-to-voltage mechanism. A method for reading out the image sensor includes enabling a first transfer transistor to transfer photo-generated charge from a first photosensitive region to a shared charge-to-voltage mechanism. The method also includes no more than partially enabling a second transfer transistor to partially turn on the second transfer transistor to increase a capacitance of the shared charge-to-voltage mechanism while the photo-generated charge is transferred from the first photosensitive region to the shared charge-to-voltage mechanism.