Uniform Threshold Voltage Non-Planar Transistors for Image Sensors
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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 development of transistors with nonplanar electron channels that have uniform threshold voltages across all portions, achieved through novel structures such as doped voltage modulation layers and multi-material gates, ensuring 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) occur
Solution Approach 1:
The patent transitions from a planar channel structure to a non-planar channel structure by forming the channel on the sidewalls of a recessed region. This dimensional change allows the channel to extend vertically along the sidewalls, increasing the effective channel width without proportionally increasing the pixel area, while maintaining sufficient channel length to avoid short channel effects
Solution Approach 2:
The non-planar channel is formed by creating a recessed region in the semiconductor substrate and forming the channel on its sidewalls. This nested structure allows the channel to utilize the vertical depth of the recess, effectively increasing the channel width while maintaining a controlled channel length that avoids short channel effects
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 channel is formed on the sidewalls of a recessed region, utilizing the vertical dimension to increase the effective channel width. This allows the channel to have a larger width without proportionally increasing the horizontal footprint of the pixel, thereby increasing transconductance while controlling pixel size
3Power
If a nonplanar channel structure is used to increase transconductance, then the transconductance increases, but different parts of the channel have different threshold voltages
Solution Approach 1:
The patent applies different doping concentrations to different regions: a first dopant is introduced at a first concentration in the recessed region, and a second dopant is introduced at a second concentration in the channel region. This local variation in doping quality compensates for the non-uniform electric field distribution in the non-planar channel, ensuring uniform threshold voltage across all parts of the channel
Solution Approach 2:
The patent changes the doping concentration parameter across different regions of the channel. By introducing dopants at different concentrations in different regions, the threshold voltage is adjusted locally to compensate for the non-planar geometry, ensuring uniform turn-on characteristics across the entire channel
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 solution reduces power consumption, speeds up device operation, and enhances performance by ensuring all parts of the nonplanar electron channel turn on at the same threshold voltage, thereby improving the overall efficiency and speed of image sensor readout.
Implementation Method 1
A doped voltage modulation layer is introduced beneath a nonplanar electron channel of a transistor. The doped voltage modulation layer is configured to modulate a threshold voltage of different portions of the nonplanar electron channel such that all portions of the nonplanar electron channel have a uniform threshold voltage.
Implementation Method 2
In another embodiment, a multi-material gate is configured to modulate a threshold voltage of different portions of the nonplanar electron channel such that all portions of the nonplanar electron channel have a uniform 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.