Junction Gate Photodiode Pixels for Charge Storage
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Solution Overview
Problem
Conventional back-side illuminated image sensors face challenges in maximizing pixel area for charge storage due to the large area occupied by transistor gate structures, leading to reduced charge storage capacity and increased noise, particularly at high temperatures and from ionizing radiation.
Innovation Solution
Integration of pixel circuit components into single structures in a vertical direction, using junction gate photodiodes that eliminate metal-oxide-semiconductor transistors, allowing for shared circuitry among pixels and improved noise reduction through self-aligned processing steps, resulting in increased radiation hardness and reduced dark current effects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of moving object
If metal-oxide-semiconductor transistors are used in pixel circuits, then circuit functionality is achieved, but pixel area for charge storage is reduced
Solution Approach 1:
The patent transitions from planar 2D transistor layouts to vertical 3D junction gate structures. The photodiode and transistor components are stacked vertically with the photodiode at the bottom, charge storage node in the middle, and transistor gate at the top, separated by insulating layers. This vertical integration eliminates the need for large lateral gate surface area while maintaining circuit functionality, thereby maximizing the pixel area available for charge storage.
2Reliability
If conventional pixel circuits with multiple transistors are used, then signal processing capability is provided, but noise is increased
Solution Approach 1:
The patent merges the photodiode and transistor into a single integrated structure where the photodiode anode is directly connected to the transistor gate, and the photodiode cathode forms the charge storage node. This integration reduces the number of discrete components and interconnections, thereby minimizing noise sources while preserving signal processing capability through the transistor's source follower configuration.
3Reliability
If larger transistor gate area is used, then better electrical control is achieved, but pixel charge storage capacity is reduced
Solution Approach 1:
The patent employs vertical junction gate structures where the gate extends in the depth direction rather than laterally. This allows sufficient gate area for electrical control to be achieved through vertical extension into the substrate, while the lateral pixel area remains available for charge storage. The insulating layers enable this vertical stacking without electrical interference.
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 maximizes pixel area for charge storage, enhances sensor reliability, reduces noise, and improves radiation hardness, particularly at high temperatures, by integrating source-follower and floating diffusion transistors into a single structure without the need for metal-oxide-semiconductor transistors.
Implementation Method 1
Typical image sensors sense light by converting impinging photons into electrons that are integrated (collected) in sensor pixels
Data Source
AI summary
Image sensor pixels are provided having junction gate photodiodes. A group of pixels may have a shared floating diffusion region and a shared source-follower transistor. The source-follower transistor may be a JFET source-follower with a gate that forms the floating diffusion region. The JFET source-follower may be a vertical or lateral JFET. A reset diode may be forward-biased to reset the floating diffusion region. Each pixel may have a JFET that serves as a charge transfer barrier between the junction gate photodiode and the floating diffusion region. The charge transfer barrier JFET may be a lateral JFET. The image sensor pixels may be formed without any metal-oxide-semiconductor devices.