Nonplanar Transistor Channels With Uniform Threshold Turn-On
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Solution Overview
Problem
In image sensors, increasing pixel count leads to longer bitline setting times due to higher bitline loading, which can be mitigated by increasing transconductance of source follower and row select transistors, but this results in short channel effects and noise issues, such as Random Telegraph Signal (RTS), and undesirable increases in pixel size.
Innovation Solution
The implementation of transistors with nonplanar electron channels having uniform threshold voltages across all portions, achieved through structures like trenches and voltage modulation layers, ensures that all parts of the electron channel turn on simultaneously, reducing power consumption and improving operation speed.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If the source follower channel length is shortened to increase transconductance, then the transconductance increases, but short channel effects and noise (RTS) worsen
Solution Approach 1:
The patent transitions from a planar channel structure to a non-planar channel structure by forming channels on the sidewalls of recesses in the semiconductor substrate. This dimensional change allows the channel to extend vertically into the substrate, increasing the effective channel length without increasing the planar footprint, thereby maintaining high transconductance while avoiding short channel effects.
Solution Approach 2:
The patent embeds the channel-forming recesses within the semiconductor substrate, creating nested structures where the channel is formed inside the substrate volume. This nesting approach allows the channel to utilize the third dimension (depth into substrate) to achieve longer effective length while maintaining a compact planar layout.
2Power
If the source follower channel width is increased to increase transconductance, then the transconductance increases, but the pixel size increases
Solution Approach 1:
The patent moves the channel structure from a two-dimensional planar configuration to a three-dimensional non-planar configuration by forming channels on the vertical sidewalls of recesses. This allows the channel width to be effectively increased through the vertical dimension without increasing the horizontal pixel footprint.
Solution Approach 2:
The patent employs curved or angled sidewall structures in the recesses to form the channels, utilizing non-linear geometries to maximize the channel width within the available vertical space while maintaining a compact planar footprint.
3Ease of manufacture
If a planar transistor structure is used, then the manufacturing is simpler, but different parts of the electron channel have different threshold voltages causing non-uniform turn-on
Solution Approach 1:
The patent divides the channel into multiple discrete segments formed on separate sidewalls of individual recesses. Each segment can be independently controlled, and the segmentation allows for uniform electric field distribution across all channel portions, ensuring consistent threshold voltage throughout the channel structure.
Solution Approach 2:
The patent creates locally optimized channel structures by forming recesses with specific geometries and doping profiles tailored to each channel segment. This local quality control ensures that each portion of the non-planar channel has uniform electrical characteristics and consistent threshold voltage.
Data Source
AI summary
Transistors having nonplanar electron channels in the channel width plane have one or more features that cause the different parts of the nonplanar electron channel to turn on at substantially the same threshold voltage. Advantageously, such transistors have substantially uniform threshold voltage across the nonplanar electron channel. Devices, image sensors, and pixels incorporating such transistors are also provided, in addition to methods of manufacturing the same.