Vertical Transfer Gate Structure to Prevent Charge Pumping
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Solution Overview
Problem
In CMOS image sensors, the long vertical gate electrode in transfer transistors leads to a long charge transfer path, causing charge to easily return to the photodiode side, thereby deteriorating the charge transfer characteristic.
Innovation Solution
A vertical transistor with a first gate electrode inside a hole in the semiconductor substrate and a second gate electrode outside the hole, connected to the first, where the first gate electrode has different conductivity parts to vary the potential gradient of the charge transfer path, preventing charge from being pumped back to the photodiode.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If a vertical transistor with a long vertical gate electrode is used to increase the saturation signal amount of the photodiode, then the saturation signal amount is improved, but the charge transfer characteristic is deteriorated due to charge pumping back to the photodiode
Solution Approach 1:
The gate electrode is divided into multiple sections along the depth direction, with each section having different conductivity characteristics. This segmentation allows different portions of the gate electrode to perform different functions: the lower section maintains strong electric field for charge transfer, while the upper section reduces electric field to prevent charge pumping back to the photodiode.
Solution Approach 2:
Different sections of the gate electrode are assigned different conductivity properties to create localized electric field characteristics. The lower section near the photodiode has lower conductivity to prevent charge pumping, while the middle section has higher conductivity to ensure efficient charge transfer, achieving optimal performance in different regions.
2Ease of operation
If the gate electrode is made long in the depth direction to cover the entire transfer path, then the transfer transistor can control the entire charge transfer process, but charge in the middle of transfer easily returns to the photodiode side
Solution Approach 1:
The gate electrode structure is designed to create a dynamic electric field distribution along the depth direction. By varying the conductivity in different sections, the electric field strength is dynamically adjusted to be strong where needed for charge transfer and weak where needed to prevent charge pumping, optimizing the transfer process at different locations.
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
This configuration improves the charge transfer characteristic by curbing charge pumping and enhancing the performance of the imaging device even with long transfer paths.
Implementation Method 1
it is possible to vary the potential gradient of a channel region (i.e., charge transfer path) formed in the semiconductor substrate
Data Source
AI summary
Provided are an imaging device and an electronic device capable of improving the charge transfer characteristic. The imaging device includes a semiconductor substrate and a vertical transistor provided on the semiconductor substrate. The semiconductor substrate is provided with a hole opened to the first main surface side. The vertical transistor includes a first gate electrode provided inside the hole, and a second gate electrode provided outside the hole and connected to the first gate electrode. The first gate electrode includes a first part and a second part including a material having conductivity different from conductivity of the first part.