Basic Principles of CMOS Image Sensors

August 18, 2023

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CMOS is called Complementary Metal-Oxide-Semiconductor (abbreviated as CMOS), which is an integrated circuit design process that can produce NMOS (n-type MOSFET) and PMOS on a silicon wafer template. The basic element of (p-type MOSFET) is called CMOS because NMOS and PMOS are complementary in physical characteristics.
CMOS can be used to make static random access memory of computer appliances, microcontrollers, microprocessors and other digital logic circuit systems, as well as optical instruments, such as CMOS image sensors.
The role of the image sensor is to convert incident light (photons) into electrical signals that can be viewed, analyzed or stored, and image sensors can be classified according to structure types: front-illuminated, back-illuminated, and stacked

Basic composition of CMOS camera
A CMOS camera consists of a lens (Lens), a CMOS image sensing ISP processing chip, and an IO interface chip.

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The role of CMOS image sensor:

Photoelectric conversion: converting optical signals into electrical signals;
Charge accumulation: collect the generated charges as signal charges;
Signal transfer: move the signal charge to the detection node;
Signal detection: converting the signal charge into an electrical signal (voltage);
Analog-to-digital conversion: convert the voltage signal into a digital signal;
The function of the ISP module: convert the original image (usually in Bayer format) collected by the CMOS image sensor into a high-quality and displayable YUV format image through the ISP pipeline processing flow.
Basic structure of CMOS module
CMOS image sensors are generally composed of cover glass, sensor chip, wire bonds, package and contact pads. As shown in the figure, different CMOS image sensors use different Package format, and use Contact Pads to connect with the PCB board.

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CMOS sensor chip structure
A CMOS image sensor chip usually consists of an image sensitive cell array, a row driver, a column driver, a timing control logic, an AD converter, a data bus output interface, and a control interface, as shown in the figure. These parts are usually integrated on the same silicon chip, and its working process can generally be divided into reset, photoelectric conversion, integration, and readout.

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Firstly, when the external light illuminates the pixel array, a photoelectric effect occurs, and corresponding charges are generated in the pixel unit.
The row selection logic unit gates the corresponding row of pixel units as required. The image signals in the row pixel units are transmitted to the corresponding analog signal processing units and A/D converters through the signal buses of the respective columns, and converted into digital image signals for output. The row selection logic unit can scan the pixel array progressively or interlacedly. The row selection logic unit cooperates with the column selection logic unit to realize the window extraction function of the image.
The main function of the analog signal processing unit is to amplify the signal and improve the signal-to-noise ratio. In addition, in order to obtain a practical camera with acceptable quality, various control circuits, such as exposure time control and automatic gain control, must be included in the chip.
In order to make the chip
To operate according to the prescribed beat, multiple timing control signals must be used. In order to facilitate the application of the camera, the chip is also required to output some timing signals, such as synchronization signals, line start signals, field start signals, etc.