MOS Transistor Channel Segmentation for Image Sensor Noise and Linearity
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Solution Overview
Problem
In solid state image pickup elements, the reduction of threshold voltage due to channel doped layers in buried channel type transistors can lead to worsened linearity of source follower circuits, as they operate outside the saturation region, affecting noise reduction and signal amplification.
Innovation Solution
The channel region of the transistor is divided into a source-side region with a higher concentration of first conductivity type impurity and a drain-side region with a higher concentration of second conductivity type impurity, improving embedability and reducing noise while suppressing threshold voltage reduction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a channel doped layer is provided in the transistor to shape it into a buried channel type transistor, then 1/f noise is reduced and SN ratio is improved, but threshold voltage is reduced causing the amplification transistor to operate out of the saturation region and worsening linearity of the source follower circuit
Solution Approach 1:
The channel doped layer is divided into a first channel doped layer (near source) and a second channel doped layer (near drain) with different impurity concentrations. The first channel doped layer has higher impurity concentration to reduce 1/f noise, while the second channel doped layer has lower impurity concentration to maintain threshold voltage and ensure saturation region operation. This segmentation allows simultaneous optimization of noise performance and linearity.
Solution Approach 2:
Different regions of the channel are given different impurity concentrations tailored to their specific functional requirements. The source-side region receives higher impurity concentration for noise reduction, while the drain-side region receives lower impurity concentration for threshold voltage control. This local quality differentiation resolves the contradiction between noise performance and linearity.
2Ease of manufacture
If a channel doped layer is provided to improve embedability of the transistor, then embedability is improved, but threshold voltage is reduced under the influence of n-type impurity
Solution Approach 1:
The channel doped layer is segmented into two distinct layers with different impurity concentrations. The first channel doped layer provides sufficient impurity concentration for good embedability, while the second channel doped layer with lower concentration prevents excessive threshold voltage reduction. This segmentation enables simultaneous achievement of manufacturability and electrical performance.
Solution Approach 2:
The impurity concentration parameter is varied across different regions of the channel. By changing the impurity concentration from high near the source to low near the drain, the patent achieves both good embedability (through sufficient overall doping) and maintained threshold voltage (through reduced doping in critical regions).
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 approach allows for simultaneous reduction in noise and suppression of threshold voltage reduction in the amplification transistor, enhancing the linearity and performance of the source follower circuit.
Implementation Method 1
a 1/f noise which is generated in the channel region of the amplification transistor is reduced
Implementation Method 2
a threshold voltage of the transistor is reduced under the influence of an n-type impurity
Data Source
AI summary
Provided is a solid state image pickup element including a MOS type transistor which amplifies a signal which is based on electric charges generated in a photoelectric conversion unit of a pixel. A channel region of the transistor is divided into a source-side region and a drain-side region. When a conductivity type of the transistor is defined as a first conductivity type and a conductivity type which is opposite to the first conductivity type is defined as a second conductivity type, a concentration of a first conductivity type impurity in the source-side region is higher than a concentration of the first conductivity type impurity in the drain-side region or a concentration of a second conductivity type impurity in the drain-side region is higher than a concentration of the second conductivity type impurity in the source-side region.