Image Sensor Transfer Gate Overlap for Carrier Efficiency
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Solution Overview
Problem
In image sensors, particularly CMOS image sensors, the transfer efficiency of carriers from photodiodes to floating diffusion regions is low due to carriers being distant from the vertical transfer transistor, leading to slow lateral diffusion and reduced overall transfer efficiency, which affects image quality and limits circuit design flexibility.
Innovation Solution
The design includes a transfer transistor with a gate that extends from the semiconductor substrate downward to a carrier-accumulation region, overlapping more than half of its width, creating a fast transfer channel that facilitates quicker carrier transfer from the accumulation region to the floating diffusion region, enhancing transfer efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If photodiode lateral size is increased to facilitate light absorption, then light absorption capability is improved, but carrier transfer efficiency deteriorates due to increased distance from vertical transfer transistor
Solution Approach 1:
The transfer gate is extended in the lateral dimension to overlap more than half the width of the carrier-accumulation region, creating a fast transfer channel that reduces carrier transfer distance and time, thereby resolving the contradiction between large photodiode size for light absorption and efficient carrier transfer
2Adaptability or versatility
If vertical transfer transistor is positioned away from photodiode, then circuit design flexibility is improved, but carrier transfer time increases due to slow lateral diffusion
Solution Approach 1:
An extended transfer gate structure acts as an intermediary, creating a fast transfer channel that mediates between the photodiode and floating diffusion region, enabling quick carrier transfer even when components are positioned apart for design flexibility
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 design significantly reduces the time carriers take to reach the floating diffusion region, improving overall carrier transfer efficiency, allowing for better image quality and increased flexibility in circuit design.
Implementation Method 1
each having a light-sensing element, typically a photodiode (PD), which receives incident light and produces carriers in an amount proportional to the incident energy
Implementation Method 2
a gate of the transfer transistor extends from over the semiconductor substrate downward to the carrier-accumulation region in the semiconductor substrate, and wherein the gate extends away from the floating diffusion region and overlaps more than half a width of the carrier-accumulation region
Data Source
AI summary
An image sensor and an electronic device are disclosed. At least one pixel in the image sensor includes a photodiode, a floating diffusion region and a transfer transistor located between the photodiode and the floating diffusion region. The photodiode includes a carrier-accumulation region, and a gate of the transfer transistor extends up to the carrier-accumulation region. The gate extends away from the floating diffusion region and overlaps over half of a width of the carrier-accumulation region. Since carriers move at a higher speed in a fast transfer channel in the semiconductor substrate around such a gate, increasing the length of the transfer transistor's gate extending away from the floating diffusion region and overlapping range with the carrier-accumulation region can facilitate fast movement of carriers from the carrier-accumulation region through such fast transfer channels to the floating diffusion region, thereby improving overall carrier transfer efficiency and optimizing performance thereof.