N-channel Source Follower With P-doped Polysilicon Gate
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Solution Overview
Problem
CMOS image sensor arrays face noise issues due to low power-supply rejection ratio in the source-follower stage, particularly when the reset charge level is near the power voltage and the source-follower has a low threshold, leading to poor photodiode sensitivity and increased noise in images.
Innovation Solution
The use of an N-channel transistor with a P-doped polysilicon gate in the source-follower stage of the tiling unit, which increases the threshold voltage and improves the power supply rejection ratio, reducing noise by increasing the source-to-drain voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If an N-channel transistor with N-doped polysilicon gate is used in the source-follower stage, then the threshold voltage is low which allows good photodiode sensitivity, but the power-supply rejection ratio is low causing noise in pictures
Solution Approach 1:
The patent changes the doping type of the polysilicon gate from N-type to P-type in the source-follower transistor. This parameter change increases the threshold voltage from typical low values to approximately 0.7V, which improves the power-supply rejection ratio and reduces noise, while still maintaining adequate photodiode sensitivity through optimized reset charge levels.
2Measurement precision
If the reset charge level is near the power voltage to the source follower, then good photodiode sensitivity is achieved, but the source-follower stage has low drop which reduces power-supply rejection ratio
Solution Approach 1:
The patent modifies the threshold voltage parameter of the source-follower transistor by using P-doped polysilicon gate instead of N-doped. This increases the threshold voltage to approximately 0.7V, which improves the power-supply rejection ratio by about 10dB while allowing the reset charge level to remain near the power voltage for good photodiode sensitivity.
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 enhances the power supply rejection ratio and reduces noise at the column sense line, improving image quality by maintaining higher sensitivity of the photodiode.
Implementation Method 1
N-channel transistor having a P-doped polysilicon gate
Data Source
AI summary
An image sensor array has a tiling unit comprising a source follower stage coupled to buffer signals from a photodiode when the unit is read onto a sense line, the source follower stage differs from conventional sensor arrays because it uses an N-channel transistor having a P-doped polysilicon gate. In embodiments, other transistors of the array have conventional N-channel transistors with N-doped polysilicon gates.