On the difference between CMOS and CCD in internal structure and principle

<p> Regardless of any product, the quality is mainly determined by the quality of performance, and the key to performance is related to the product structure and working principle, as is CCD and CMOS.

basic component

CCD is developed on the basis of MOS transistor, its basic structure is MOS (metal-oxide-semiconductor) capacitor structure. Is it to produce a layer with a thickness of about 1000 on the surface of semiconductor P-type silicon (Si) as a substrate by oxidation? ~ 1500? SiO2, a layer of metal (such as aluminum) is evaporated on the surface of SiO2, and a bias voltage (called gate voltage) is added between the substrate and the metal electrode to form a MOS capacitor. Therefore, the CCD is composed of an array of MOS capacitors arranged closely on a silicon substrate.

The most basic CMOS image sensor is a P-type silicon wafer with a low impurity concentration as a substrate, and two highly doped N + -type regions are made on the surface as electrodes by the diffusion method, that is, the source of the field effect tube And the drain, and then a high-temperature oxidation method is applied on the surface of the silicon to cover a layer of silicon dioxide (SiO2) insulating layer, and a layer of aluminum metal is deposited on the insulating layer between the source and the drain as a field The grid of the effect tube. Finally, place the photodiode above the metal aluminum, which constitutes the most basic CMOS image sensor.

In addition, in the CMOS imaging device, the electrical signal is directly read from the CMOS transistor switch array, unlike the CCD need to read line by line.

Internal structure

The CCD imager needs to be supplemented with more peripheral drive circuits to work. It can only output analog electrical signals. Such signals are processed by subsequent address decoders, analog-to-digital converters, and image signal processors. CCD The integration is relatively low. If a CCD digital camera is often composed of six chips, some have as many as eight, and at least three, so that the size of the camera cannot be reduced and the production cost is also high.

The CMOS imager does not require additional peripheral processing circuits. It integrates photodiodes, image signal amplifiers, signal reading circuits, analog-to-digital converters, image signal processors and controllers on a single chip, and is manufactured and processed. Just like the process of semiconductor manufacturers producing integrated circuits. If it constitutes a digital camera, all the components of the digital camera can be integrated into a single chip, that is, a "single chip camera". Therefore, the optoelectronic image system using CMOS chips can not only reduce the overall cost of the system and the time required for assembly, but also greatly reduce the size and complexity of the system.

working principle

In addition, the CCD is a semiconductor imaging device. The charge information stored by the CCD charge coupler needs to be read bit by bit under the control of a synchronization signal. The charge information transfer and reading output require a clock control circuit and three The combination of different power supplies makes the whole circuit more complicated. Therefore, in addition to the relatively slow processing speed of the CCD, the power consumption is also relatively large; and the CMOS photo sensor can take out the electrical signal after photoelectric conversion, the reading is relatively simple, and it can also process the image information of each pixel unit , CMOS photoelectric sensor only needs a single set of power supply to work, and the power consumption is small, which can achieve energy saving.

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