Image Sensor Settling Time via Localized Gate Oxide Thickness
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Solution Overview
Problem
Conventional image sensors face challenges in reducing settling time for output voltage, which limits their operating speed due to the uniform thickness of gate oxide films in all pixel transistors, resulting in increased RC delay and settling time.
Innovation Solution
The implementation of image sensors with a dual or triple gate oxide film scheme, where the source follower transistor's channel oxide film is thinner than the gate oxide films of other transistors, reducing output resistance and RC delay, thereby shortening the settling time of the output voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of time
If uniform thickness gate oxide films are used in all pixel transistors, then manufacturing simplicity is maintained, but settling time and RC delay increase
Solution Approach 1:
The patent applies different gate oxide film thicknesses to different transistor types within the pixel circuit. Specifically, the source follower transistor uses a thinner gate oxide film (first thickness) while other transistors use a thicker gate oxide film (second thickness). This local differentiation optimizes the settling time by reducing RC delay in the source follower without compromising the overall circuit performance or manufacturing feasibility.
2Speed
If thinner gate oxide films are used to reduce RC delay, then settling time decreases, but manufacturing precision requirements increase
Solution Approach 1:
The patent implements a dual-thickness gate oxide film structure where only specific transistors (source follower) use thinner films for high-speed operation, while other transistors use thicker films that are easier to manufacture with standard precision. This localized approach achieves high operating speed without requiring the entire pixel circuit to meet stringent thickness control requirements.
Solution Approach 2:
The gate oxide film structure is segmented into at least two distinct thickness regions corresponding to different transistor functions. The first gate oxide film (thinner) is formed for the source follower transistor, and the second gate oxide film (thicker) is formed for other transistors. This segmentation allows each region to be optimized independently for its specific function while maintaining overall manufacturing feasibility.
Data Source
AI summary
Image sensors are provided including a structure capable of settling an output voltage within a very short time for implementing a high-speed image sensor. The image sensor includes a pixel area, in which a photo-diode (PD) and a transfer transistor (Tr) configured to transmit charges accumulated in the PD to a floating diffusion (FD) area are disposed; and a Tr area, which is disposed adjacent to the pixel area and includes a first Tr, a second Tr, and a third Tr, wherein a first gate oxide film disposed below a first gate electrode of the first Tr and a second gate oxide film disposed below a second gate electrode of the second Tr include channel oxide films thinner than a gate oxide film of the transfer Tr.