CMOS Imager Pixel with Switchable Infrared Photodiode
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Solution Overview
Problem
Current CMOS imagers experience significant read noise when detecting low light levels, particularly in infrared spectral ranges, due to the use of three-transistor read circuits which are inefficient for visible CMOS imagers.
Innovation Solution
A CMOS imager pixel design that includes an infrared photodiode capable of generating current from radiation above 950 nm, a conversion circuit, and a switchable configuration with an electron reservoir and diffusion nodes to manage electron transfer, reducing read noise by separating the reading and conversion processes in time.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a three-transistor read circuit is used for infrared photodiode detection, then the circuit can read and convert the current produced by the infrared photodiode, but the read noise is significant
Solution Approach 1:
The patent applies the dynamics principle by making the connection between the infrared photodiode and conversion circuit switchable rather than fixed. The first switch enables dynamic control of electron transfer, allowing the system to optimize between reading mode and conversion mode, thereby reducing read noise while maintaining detection accuracy through temporal separation of these functions.
Solution Approach 2:
The patent implements preliminary action by introducing an electron reservoir that can pre-store electrons generated by the infrared photodiode before conversion. This reservoir acts as a buffer, allowing the system to accumulate charge over time and perform conversion at optimized moments, reducing the impact of read noise on the final measurement.
2Measurement precision
If an electron reservoir and switchable configuration are added to separate reading and conversion processes, then read noise is reduced, but the device complexity increases
Solution Approach 1:
The patent applies universality by designing the first switch to serve multiple functions: it controls electron transfer from the infrared photodiode to the reservoir, manages the timing between reading and conversion operations, and can be integrated with existing CMOS pixel structures. This multi-functionality reduces the need for additional dedicated components, thereby limiting the increase in device complexity.
Solution Approach 2:
The patent merges the new electron reservoir and switchable configuration with the existing three-transistor conversion circuit rather than implementing them as completely separate systems. The first switch is integrated into the existing circuit architecture, sharing control mechanisms and physical space with the conversion transistors, thereby reducing overall device complexity compared to a fully separate design.
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 design reduces read noise and dark current, offering improved performance with lower manufacturing costs compared to CCD technology, while maintaining the ability to detect a wide spectral range including visible and infrared radiation.
Implementation Method 1
an infrared photodiode adapted to generate an electric current when it is exposed to optical radiation having a wavelength greater than 950 nanometers (nm)
Data Source
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AI summary
The invention relates to a pixel (14) of a CMOS imager, the pixel (14) comprising: - an infrared photodiode (20) suitable for generating an electrical current when it is exposed to optical radiation with a wavelength greater than 950 nanometres (nm), - a conversion circuit (28) suitable for receiving electrons and for delivering a voltage having a value that varies as a function of the number of electrons received, characterised in that the pixel (14) further comprises: - a first switch (26) connected between the infrared photodiode (20) and the conversion circuit (28).