L-Shaped Transfer Transistor Gate for CMOS Image Sensor Signal Transfer
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Solution Overview
Problem
CMOS image sensors face sensitivity degradation due to increased distance between the control electrode of the transfer transistor and the carrier accumulation region, leading to incomplete signal transfer and reduced light conversion efficiency near the center of the photodiode.
Innovation Solution
A photoelectric conversion device with a control electrode having an L-shape configuration and additional bypass regions of opposite conductivity type, extending along both channel width and length directions, facilitates complete depletion of the carrier accumulation region and improves signal transfer characteristics by reducing potential barriers and enhancing light conversion efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If a bypass region is provided between the carrier accumulation region and the transfer transistor, then the threshold voltage of the transfer transistor is lowered, but the distance from the control electrode to the carrier accumulation region increases, causing incomplete signal transfer
Solution Approach 1:
The control electrode is extended in the channel width direction (lateral dimension) to overlap with the carrier accumulation region, adding a new spatial dimension to the electrode structure. This lateral extension allows the electrode to reach closer to the carrier accumulation region without increasing the channel length, thereby improving signal transfer completeness while maintaining low threshold voltage through the bypass region
2Reliability
If the gate length of the control electrode is increased toward the center of the photodiode, then signal transfer is improved, but light conversion efficiency near the center of the photodiode degrades
Solution Approach 1:
The control electrode structure is made non-uniform with different sections having different functions: one section extends toward the carrier accumulation region for signal transfer, while another section is positioned to avoid the center of the photodiode to preserve light conversion efficiency. This local differentiation allows the electrode to optimize for signal transfer in critical areas while maintaining photodiode performance in light-sensitive areas
3Reliability
If the control electrode overlaps the carrier accumulation region, then signal transfer is improved, but the structure complexity increases
Solution Approach 1:
The control electrode is merged with the bypass region structure, where the bypass region serves dual purposes: lowering the threshold voltage of the transfer transistor and facilitating signal transfer to the carrier accumulation region. This merging of functions reduces the need for separate structural elements, thereby improving signal transfer while minimizing increases in device complexity
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 effectively suppresses sensitivity degradation and achieves excellent transfer characteristics by ensuring complete depletion of the carrier accumulation region and efficient light conversion, even near the center of the photodiode, thereby improving overall image sensing performance.
Implementation Method 1
a photoelectric conversion unit having a carrier accumulation region
Implementation Method 2
a floating diffusion region which converts electric carriers into a voltage
Data Source
AI summary
A photoelectric conversion device comprises a photoelectric conversion unit, a floating diffusion region, a transfer transistor, and an output unit. A control electrode of the transfer transistor includes a first portion which extends along a channel width direction and overlaps a first boundary side when seen through from a direction perpendicular to a light receiving surface of the photoelectric conversion unit, and a second portion which extends along a channel length direction from one end of the first portion and overlaps a second boundary side when seen through from the direction perpendicular to the light receiving surface, and the control electrode of the transfer transistor has an L shape when viewed from the direction perpendicular to the light receiving surface.