Segmented Photodiode Structure for Fast Charge Readout
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Solution Overview
Problem
The large size of photodiodes in existing solid state imaging devices hinders the formation of an effective electric field for charge reading and increases the distance charge needs to travel, making it difficult to read charge quickly enough for time-of-flight distance measurement applications.
Innovation Solution
Dividing the photodiode into smaller units with memories in between to reduce the distance charge needs to travel, allowing for faster charge reading through separate readout gates driven at different timings.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of moving object
If a large photodiode is used, then the photoelectric conversion area is increased, but the charge reading speed decreases due to longer charge travel distance and ineffective electric field formation
Solution Approach 1:
The photodiode is divided into multiple smaller photodiodes (first photodiode and second photodiode) arranged in a specific pattern. This segmentation reduces the charge travel distance within each photodiode while maintaining overall photoelectric conversion area, thereby improving charge reading speed without sacrificing light detection capability.
2Quantity of substance
If a large photodiode is used, then more charge is generated, but the distance charge must travel increases making rapid reading difficult
Solution Approach 1:
The photodiode structure is segmented into multiple smaller units with memory elements positioned between them. This allows charge to be collected over a large total area while each individual charge carrier travels only a short distance to its nearest memory element, resolving the contradiction between total charge quantity and individual charge travel distance.
Solution Approach 2:
Memory elements are introduced as intermediary structures between the photodiodes. These memory elements serve as intermediate charge collection points, reducing the distance charge must travel from the photodiode to the readout circuit while maintaining efficient charge collection from the extended photodiode area.
3Productivity
If a large photodiode is used, then photoelectric conversion efficiency is improved, but electric field formation for charge reading becomes ineffective
Solution Approach 1:
Dividing the photodiode into smaller units creates multiple regions where electric fields can be effectively established. Each small photodiode-memory pair forms a compact unit with effective electric field formation, while the overall structure maintains high photoelectric conversion efficiency through the cumulative area of all photodiodes.
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 enhances the speed of charge reading from the photodiode, improving the efficiency of distance measurement in solid state imaging devices and increasing area efficiency for better signal handling.
Implementation Method 1
two photodiodes PD1 and PD2 which generate charge by photoelectric conversion
Data Source
AI summary
A plurality of pixels are two-dimensionally arranged on a semiconductor substrate. Each of the pixels includes: two photodiodes each generating charge by photoelectric conversion; first and second memories spaced apart from each other between the two photodiodes as viewed in cross section; a first readout gate reading charge from the two photodiodes to the first memory; and a second readout gate reading charge from the two photodiodes to the second memory.