Tri-Level Biased Transfer Gate for Global Shutter CMOS Pixels
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Solution Overview
Problem
CMOS image sensors face challenges in achieving global shutter mode without exposure time skew, leading to picture distortion, and suffer from kTC noise and high dark current issues due to the use of additional charge storage sites and pinned diodes, which increase pixel size and cost.
Innovation Solution
A CMOS image sensor pixel design incorporating a transfer gate that operates among three biasing conditions to transfer, store, and read out charge efficiently, utilizing a p+ type barrier implant and pinned charge transfer barrier to manage electron flow and confinement, enabling both global and rolling shutter modes while minimizing dark current and noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If additional charge storage sites and pinned diodes are used to achieve global shutter mode, then exposure time skew is eliminated and picture distortion is prevented, but pixel size increases and manufacturing cost increases
Solution Approach 1:
The patent combines the charge storage function and the transfer gate function into a single integrated structure. The transfer gate serves dual purposes: transferring charge from the photodiode to the floating diffusion, and simultaneously acting as the charge storage site during the integration period. This merging eliminates the need for separate additional charge storage sites and pinned diodes, thereby preventing pixel size increase while maintaining global shutter mode operation.
Solution Approach 2:
The transfer gate is designed to perform multiple functions: it acts as a transfer mechanism for charge, serves as a storage site during integration, and enables global shutter operation. This multi-functionality allows the system to achieve global shutter mode without adding extra components that would increase pixel size and manufacturing cost.
2Reliability
If additional charge storage sites and pinned diodes are used to achieve global shutter mode, then exposure time skew is eliminated and picture distortion is prevented, but manufacturing cost increases
Solution Approach 1:
The patent merges the transfer gate and charge storage site into a single structure, eliminating the need for additional pinned diodes and charge storage sites. This reduction in component count directly lowers manufacturing complexity and cost while maintaining global shutter mode functionality.
Solution Approach 2:
The transfer gate is designed to perform multiple functions including charge transfer, charge storage, and enabling global shutter operation. This multi-functionality reduces the total number of components required, thereby simplifying the manufacturing process and reducing production costs.
3Productivity
If standard transfer gates are used for charge transfer, then charge transfer is achieved, but dark current increases due to interface effects between silicon and silicon-dioxide
Solution Approach 1:
The patent modifies the biasing parameters of the transfer gate by implementing tri-level biasing instead of standard single-level biasing. By applying different voltage levels at different stages (higher voltage for transfer initiation, intermediate voltage during transfer, and lower voltage to minimize dark current), the system achieves efficient charge transfer while minimizing dark current generation at the silicon-silicon dioxide interface.
Solution Approach 2:
The transfer gate employs periodic tri-level biasing sequences to control charge transfer. The bias voltage is periodically adjusted through three distinct levels: a first level to initiate transfer, a second level to complete transfer, and a third level to minimize dark current. This periodic modulation of bias levels enables efficient charge transfer while reducing harmful dark current effects.
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 design allows for high-performance image capture with reduced noise and dark current, efficient charge handling, and compact pixel size, supporting fast scanning applications with low power consumption.
Implementation Method 1
a first biasing condition in which electrons are transferable from a photodiode to a potential well under the transfer gate
Implementation Method 2
a second biasing condition in which the electrons are confined in the potential well under the transfer gate
Implementation Method 3
utilizing a p+ type barrier implant and pinned charge transfer barrier to manage electron flow and confinement
Implementation Method 4
Image sensors typically sense light by converting impinging photons into electrons that are integrated (collected) in sensor pixels
Data Source
AI summary
An image sensor includes at least one pixel with a transfer gate that is controllable among at least three biasing conditions, including a first biasing condition in which electrons are transferable from a photodiode to a potential well under the transfer gate, a second biasing condition in which the electrons are confined in the potential well under the transfer gate, and a third biasing condition in which the electrons are transferable out of the potential well under the transfer gate. The pixel includes a p+ type doped barrier implant located at least partially under a portion of the transfer gate, and a pinned charge transfer barrier located on the opposite side of the transfer gate from the photodiode that includes a p+ type doped region and an n-type doped region. The image sensor can operate in a global shutter mode and/or a rolling shutter mode.