MOS Capacitor Image Sensor Negative Bias Pinning
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Solution Overview
Problem
CMOS image sensors using global shutters face challenges in maintaining image quality due to sensitivity deterioration and noise issues caused by reduced light receiving and storage areas, as well as difficulties in achieving simultaneity in image capture, especially with moving subjects.
Innovation Solution
A solid-state image sensor design incorporating a MOS capacitor structure with a transfer transistor and depletion transistor to manage charge storage and transfer, allowing for simultaneous exposure across all pixels and reducing noise through Correlated Double Sampling (CDS) processing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a floating diffusion (FD) is used as a storage region, then the light receiving area and storage area can be maintained, but pinning cannot be achieved causing image quality deterioration due to dark current and crystal defects
Solution Approach 1:
The patent changes the electrical parameter of the storage region by applying a negative bias voltage to the MOS capacitor gate electrode. This parameter change enables pinning effect that suppresses dark current while maintaining the floating diffusion structure for charge storage, thereby resolving the contradiction between maintaining light receiving area and eliminating dark current.
2Quantity of substance
If a long time period is used from FD resetting to signal reading, then charge storage is sufficient, but noise elimination using CDS becomes difficult
Solution Approach 1:
The patent applies a negative bias to the MOS capacitor gate electrode in advance, before signal reading begins. This preliminary action creates the pinning effect that suppresses dark current and enables effective CDS noise elimination, while still allowing sufficient charge storage time in the floating diffusion region.
3Area of stationary object
If the light receiving area and storage area are reduced for pixel microminiaturization, then device integration is improved, but sensitivity and dynamic range deteriorate
Solution Approach 1:
By applying a negative bias to the MOS capacitor, the patent enhances the pinning effect that suppresses dark current and noise. This allows smaller pixel areas to maintain or improve sensitivity, as the electrical parameter change compensates for the reduced physical area by reducing harmful currents more effectively.
4Stability of the object's composition
If a global shutter is employed to achieve simultaneity in image capture, then distortion in moving subjects is eliminated, but the device structure becomes more complex requiring additional charge storage sections
Solution Approach 1:
The MOS capacitor structure serves multiple functions: it acts as both the charge storage region for the global shutter operation and the element for applying negative bias to enable pinning. This multi-functionality eliminates the need for separate dark current suppression structures, thereby reducing overall device complexity while maintaining image simultaneity.
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
Enhances sensitivity and reduces noise in image sensors, enabling high-quality image capture without distortion, even with moving subjects, by effectively managing charge storage and transfer within the MOS capacitor structure.
Implementation Method 1
a light receiving section which stores a charge generated by photoelectric conversion
Data Source
AI summary
There is provided a solid-state image sensor including a plurality of unit pixels arranged thereon, the plurality of unit pixels each including a light receiving section which stores a charge generated by photoelectric conversion, a signal storage section which is connected to the light receiving section and has a structure of a MOS capacitor, and a signal output section to which a gate electrode of the MOS capacitor is connected.