CMOS Image Sensor Current Mirror Biasing
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Solution Overview
Problem
Conventional CMOS image sensors suffer from noise issues due to NMOSFET reset transistors causing incomplete reset of the floating diffusion node and generation of hot electrons, leading to reduced image quality and increased 1/f noise.
Innovation Solution
The use of PMOSFET reset transistors and a current mirror unit with mirroring transistors for biasing, along with butting contacts to enhance complete resetting and reduce noise, results in improved gain maintenance and reduced 1/f noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of moving object
If NMOSFET reset transistors are used, then the pixel area is reduced, but the floating diffusion node is not fully reset and hot electrons are generated causing noise
Solution Approach 1:
The patent changes the key parameter of transistor type from NMOSFET to PMOSFET for the reset transistor. This parameter change enables complete reset of the floating diffusion node and eliminates hot electron generation, resolving the noise issue while maintaining compact pixel design through the current mirror structure.
Solution Approach 2:
The patent introduces a current mirror unit as an intermediary mechanism that couples the reset transistor to the floating diffusion node. This current mirror structure enables controlled charging and complete resetting of the floating diffusion node, solving the incomplete reset problem caused by direct NMOSFET connection.
2Reliability
If current mirror unit is added for biasing, then gain maintenance is improved, but device complexity increases
Solution Approach 1:
The current mirror unit performs multiple functions simultaneously: it provides biasing for the drive transistor to maintain gain, controls the reset operation through mirrored current, and enables complete charging of the floating diffusion node. This multi-functionality achieves gain maintenance without proportionally increasing complexity.
3Reliability
If PMOSFET reset transistors are used, then complete reset and noise reduction are achieved, but manufacturing complexity increases
Solution Approach 1:
The patent changes the transistor type parameter from NMOSFET to PMOSFET in the reset transistor position. While this improves reset completeness and reduces noise, it does require separate well formation (P-well for PMOSFET) which adds manufacturing steps compared to standard NMOSFET processes.
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 solution achieves full reset of the floating diffusion node, minimizes 1/f noise, and reduces hot electron generation, resulting in improved image quality and reduced area of the pixel.
Implementation Method 1
The photosensitive device accumulates charge in response to received light
Data Source
AI summary
An image sensor includes a pixel with a drive transistor and a select transistor. The drive transistor is driven according to a voltage at a floating diffusion node. The select transistor is coupled in series with the drive transistor for being turned on when the pixel is selected. The image sensor also includes a current mirror unit having first and second branches conducting mirrored currents. The first branch is coupled to the drive transistor, and the second branch is coupled to the select transistor at an output node of the pixel. With such biasing by the current mirror, gain drop in the drive transistor is minimized.