Image Sensor Pixel Circuit Body Contact Optimization
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Solution Overview
Problem
The existing pixel circuits in image sensors suffer from reduced sensitivity due to the body effect of the source follower transistor, which limits the charge to column voltage conversion factor, thereby affecting image quality.
Innovation Solution
The body contact of the source follower transistor is connected to the output line, allowing the source follower and read transistors to be formed in the same well, surrounded by shallow or deep trench isolation, and the reset transistor is also connected to the output line, enhancing the charge to column voltage conversion factor.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If the body contact of the source follower transistor is connected to ground, then the circuit structure is simple, but the sensitivity and gain of the transistor are reduced due to the body effect
Solution Approach 1:
The patent changes the connection parameter of the body contact from ground to output line, which modifies the electrical potential reference point. This parameter change eliminates the body effect by ensuring the body and source are at the same potential, thereby increasing the transistor gain and sensitivity without adding complex circuit structures.
Solution Approach 2:
The patent applies the equipotentiality principle by connecting the body contact to the output line, which ensures that the body and source of the source follower transistor are at the same electrical potential. This eliminates potential differences that cause the body effect, thereby maximizing the transistor gain while maintaining a simple circuit structure.
2Ease of manufacture
If the source follower and read transistors are formed in separate wells, then the isolation is easier, but the area occupied increases and sensitivity is reduced
Solution Approach 1:
The patent merges the source follower transistor and read transistor into the same well structure. This consolidation reduces the total area occupied by these transistors while the body contact connection to the output line ensures proper electrical isolation and prevents interference, thereby achieving both area reduction and ease of manufacture.
Solution Approach 2:
The shared well structure serves multiple functions: it provides a common substrate for both transistors, reduces area, and when combined with the body contact connection to the output line, ensures proper electrical isolation. This multi-functional approach simplifies the manufacturing process while maintaining performance.
3Stability of the object's composition
If the body contact is connected to ground, then the reference potential is stable, but the charge to column voltage conversion factor is limited
Solution Approach 1:
The patent changes the reference potential connection from ground to output line. This parameter change allows the body contact to follow the output voltage level, eliminating the body effect and increasing the charge to column voltage conversion factor. The stability is maintained because the body and source remain at the same potential throughout the signal swing.
Data Source
AI summary
A pixel circuit of an image sensor includes a sense node for storing a charge transferred from one or more photodiodes, a source follower transistor having its gate coupled to the sense node and its source node coupled to an output line of the pixel circuit via a read transistor, wherein a body contact of the source follower transistor is connected to the output line.