Square-Gate Source-Follower Layout for Low-Noise CMOS Pixels

Solution Overview

Problem

Conventional CMOS image sensor pixel designs face challenges in maintaining acceptable noise performance while optimizing full-well capacitance as pixel dimensions decrease, leading to a trade-off between image performance and noise performance due to the shared footprint of photodiodes and source-follower transistors.

Innovation Solution

The implementation of a square-gate source-follower transistor with parallel current channels and a square-gate structure that effectively doubles the active region width without increasing physical dimensions, enhancing transconductance, noise performance, and reducing power consumption.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Use of energy by moving object

If the size of the photodiode area is increased to improve full-well capacitance and image performance, then the dynamic range and contrast are improved, but the active area available for the source-follower transistor is reduced, worsening noise performance

Engineering Contradiction:

Improvefull-well capacitanceVSAvoidnoise performance

Core Design Contradiction:

Use of energy by moving objectVSReliability

Solution Approach 1:

The source-follower transistor is segmented into multiple parallel current channels (first current channel and second current channel), each with its own source region and shared drain region. This segmentation allows the transistor to achieve higher effective width and improved noise performance within the same physical footprint, resolving the trade-off between photodiode area and transistor active area.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The patent transitions from a conventional single-channel linear layout to a multi-channel parallel architecture with a square-gate configuration. By utilizing vertical stacking and parallel current paths, the design effectively doubles the transistor width without increasing the horizontal footprint, thereby improving noise performance while maintaining compatibility with the photodiode area requirements.

Inventive Principle:

Principle #17Another dimension (Dimensionality change)

2Reliability

If the active area of the source-follower transistor is increased to improve noise performance and signal-to-noise ratio, then the noise performance is improved, but the photodiode area is reduced, worsening image performance

Engineering Contradiction:

Improvesignal-to-noise ratioVSAvoidphotodiode area

Core Design Contradiction:

ReliabilityVSUse of energy by moving object

Solution Approach 1:

The source-follower transistor is segmented into multiple parallel current channels (first current channel and second current channel), each with its own source region and shared drain region. This segmentation allows the transistor to achieve higher effective width and improved noise performance within the same physical footprint, resolving the trade-off between photodiode area and transistor active area.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The patent transitions from a conventional single-channel linear layout to a multi-channel parallel architecture with a square-gate configuration. By utilizing vertical stacking and parallel current paths, the design effectively doubles the transistor width without increasing the horizontal footprint, thereby improving noise performance while maintaining compatibility with the photodiode area requirements.

Inventive Principle:

Principle #17Another dimension (Dimensionality change)

3Ease of manufacture

If conventional linear source-follower designs are used with fixed dimensions, then the manufacturing is simple, but the transconductance and frame rate are limited

Engineering Contradiction:

Improvemanufacturing simplicityVSAvoidframe rate

Core Design Contradiction:

Ease of manufactureVSProductivity

Solution Approach 1:

The source-follower transistor is segmented into multiple parallel current channels (first current channel and second current channel), each with its own source region and shared drain region. This segmentation allows the transistor to achieve higher effective width and improved noise performance within the same physical footprint, resolving the trade-off between photodiode area and transistor active area.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The patent changes the geometric parameters of the transistor by implementing a square-gate configuration with multiple parallel current channels. This parameter change effectively doubles the transistor width (W) while maintaining the same physical footprint, thereby increasing transconductance (gm ∝ W/L) and enabling higher frame rates without complicating the manufacturing process.

Inventive Principle:

Principle #35Parameter changes

Data Source

PatentUS11830897B2Square-gate source-follower for CMOS image sensor pixel

Publication Date: 2023.11.28 SHENZHEN GOODIX TECH CO LTD

AI summary

Techniques are described for implementing a square-gate source-follower (SGSF) transistor for integration with complementary metal-oxide semiconductor (CMOS) image sensor (CIS) pixels. The SGSF transistor can have an active layer with active regions, including a drain region separated from each of two source regions to form parallel current channels. A square-gate structure layer includes main-gate regions, each disposed above a corresponding one of the current channels, and a side-gate region to couple the main-gate regions. At a particular physical width (W) and current channel length (L), the parallel current channels can act similarly to a conventional linear source-follower having dimensions of 2W and the same L. SGSF implementations can provide a number of features, including higher frame rate, lower power consumption, and lower noise, as compared to those of a conventional source-follower transistor of comparable W and L dimensions.