Image Sensor: How do CCD and CMOS Sensors work?

Catalog

Ⅰ Introduction

An image sensor, or photosensitive element, is a device that converts an optical image into an electronic signal. It is widely used in digital cameras and other electronic optical devices. The image sensor uses the photoelectric conversion function of the photoelectric device to convert the light image on the photosensitive surface into an electrical signal in a proportional relationship with the light image. Compared with photosensitive elements such as photodiodes, phototransistors, and other "point" light sources, an image sensor is a functional device that divides the light image on its light-receiving surface into many small units and converts it into usable electrical signals. The image sensor is divided into a light guide camera tube and a solid-state image sensor. Compared with the light guide camera tube, the solid-state image sensor has the characteristics of small size, lightweight, high integration, high resolution, low power consumption, long life, and low price.

The photosensitive device is the core component of the industrial camera. There are two kinds of image sensors which are CMOS (Complementary Metal Oxide Semiconductor) and CCD  (Charged Coupled Device). CCD has the advantages of a good low illumination effect, high signal-to-noise ratio, a strong sense of transparency, and good color reproduction ability. CCD is widely used in high-end fields such as transportation and medical treatment. Because of its advantages in imaging, it will continue to be adopted for a long time, but at the same time, because of its high cost and large power consumption, it also restricts its room for market development. The CMOS sensor uses the most commonly used CMOS process of general semiconductor circuits. It has the characteristics of high integration, low power consumption, fast speed, low cost, etc. In recent years, it has developed rapidly in a wide dynamic range and low illumination. CCD and CMOS have their advantages in different application scenarios.

 Figure1. working process of the image sensor

An image sensor is a semiconductor device that can convert optical images into digital signals. The tiny photosensitive material implanted on the sensor is called a pixel. The more pixels there are on a sensor, the higher the resolution of the picture it provides. It functions as a film, but it converts image pixels into digital signals.

Ⅱ CCD 

1 Definition of CCD image sensor

CCD is a high-end technical component used in photography. CMOS is used in products with lower image quality. The advantages of CMOS are lower manufacturing costs and much lower power consumption than CCD. Although there are big differences in technology, the performance difference between CCD and CMOS is not very big. The requirements of the CMOS camera on the light source are higher, but this problem has been basically solved. The size of the CCD element is mostly 1/3 inch or 1/4 inch. At the same resolution, it is better to choose the larger element size.

CCD was successfully developed in Bell Labs in 1969, and then mass production started by Japanese companies and other companies. Its development history has been nearly 30 years. CCD can be divided into two types: Linear and Area. The linear type is used in image scanners and fax machines, while the area type is mainly used in digital cameras (DSC), camcorders, and surveillance.

 Figure2. Composition of CCD sensor

CCD is a medium that replaces traditional film in the digital age. Its working principle has also evolved with the aid of the principle that the chemical substances on the film sensed light. It is made of high-sensitivity semiconductor materials, which can convert light into electric charge, and convert it into a digital signal through an analog-to-digital converter chip. The digital signal is compressed and stored in the camera's internal flash memory or built-in hard disk card, so you can easily transfer the data to the computer, and use the processing means of the computer to modify the image as needed.

2 Working principle of CCD image sensor

CCD is composed of many photosensitive units, usually in units of megapixels. When the surface of the CCD is exposed to light, each photosensitive unit reflects the charge on the component, and the signals generated by all the photosensitive units are added together to form a complete picture.

The basic unit of the CCD is a MOS capacitor, which can store electric charge. Its structure is shown in Figure 1. Taking P-type silicon as an example, a SiO2 layer is formed on the surface by oxidation on a P-type silicon substrate, and then a layer of metal is deposited on the SiO2 as a gate. Most carriers in P-type silicon are positively charged empty holes, minority carriers are negatively charged electrons. When a positive voltage is applied to the metal electrode, its electric field can repel or attract these carriers through the SiO2 insulating layer. Therefore, the positively charged holes are repelled away from the electrode, leaving the negatively charged minority carriers immobile near the SiO2 layer to form a negative charge layer (depletion layer). This phenomenon forms a trap for electrons. Once the electrons enter the trap, they cannot come back, so it is also called electron potential well.

When the device is exposed to light (light can be injected through the SiO2 layer between the gaps of the electrodes, or through the thin P-type silicon of the substrate), the energy of the photon is absorbed by the semiconductor, and electron-holes are generated. The electrons are attracted to be stored in the potential well. These electrons can be conducted. The stronger the light, the more electrons collected in the potential well. This will turn the strength of light into the number of charges. The conversion of light and electricity, and the electrons collected in the potential well are stored, even if the light is stopped for a certain period, it will not be lost, which realizes the memory of the light.

 Figure3. CCD structure and working principle diagram

In short, the above structure is essentially a tiny MOS capacitor, which is used to form a pixel, which can be "photosensitive" and leave a "latent image". The photosensitive function is to accumulate charge by electrons generated by light intensity, and the latent image is formed by the unequal charge of each pixel, which left in each capacitor. If you can manage to transfer the charge in each capacitor to another place in turn, then form lines and frames, the image transfer can be achieved.

3 Features of CCD image sensor

High resolution: The size of the image point is the μm level, which can sense and identify fine objects and improve the image quality. From 1 inch, 1/2 inch, 2/3 inch, 1/4 inch to the launched 1/9 inch, the number of pixels has increased from more than 100,000 to 4-5 million pixels.

Low noise&High sensitivity: CCD has very low read noise and dark current noise, so it improves the signal-to-noise ratio (SNR), and at the same time has high sensitivity. It can detect incident light with very low light intensity. By the way, its signal will not be covered, making the application of CCD less constrained by the weather.

Wide dynamic range: CCD can simultaneously detect and distinguish strong light and weak light to improve the using range of the system environment. Therefore, signal contrast caused by a large difference in brightness will not occur.

Good linear characteristic curve(Linearity): The intensity of the incident light source and the size of the output signal have a good proportional relationship. The object information will not be lost, and the signal compensation processing cost will be reduced.

High Quantum Efficiency: The very weak incident light irradiation can be recorded. If it is combined with an image intensifier tube and light projector, in the night, distant scenes can still be detected.

Large Field of View: Large-area CCD chips can be manufactured using semiconductor technology. A 35mm CCD that is equivalent to the size of a traditional film has begun to be used in digital cameras, becoming a key component to replace professional optical cameras.

Broad Spectral Response: It can detect light in a wide wavelength range, increase the flexibility of the system, and expand the system application area.

Low Image Distortion: Using the CCD sensor, the image processing will not be distorted, and the original object information will be faithfully reflected.

Small size and light weight: CCD has the characteristics of small size and light weight, so it can be easily installed on artificial satellites and various navigation systems.

Good charge transfer efficiency: This efficiency factor affects the signal-to-noise ratio and resolution. If the charge transfer efficiency is not good, the image will become blurred;

Ⅲ CMOS 

1 Definition of CMOS image sensor

The manufacturing technology of CMOS is no different from that of general computer chips. It is mainly a semiconductor made of silicon and germanium so that it has a negatively charged N pole and a positively charged P pole semiconductor coexisting on the CMOS. The currents generated by the two complementary effects can be recorded and converted into images by the processing chip. Later, it was discovered that CMOS can also be used as an image sensor in digital photography after processing.

CMOS is a complementary metal-oxide-semiconductor, which is mainly a semiconductor made of two elements of silicon and germanium and realizes basic functions through negatively and positively charged transistors on CMOS. The current generated by these two complementary effects can be recorded and interpreted as an image by the processing chip.

A CMOS image sensor is a typical solid-state imaging sensor and has a common historical origin with CCD. CMOS image sensors are usually composed of image sensor cell array, row driver, column driver, timing control logic, AD converter, data bus output interface, control interface, etc. These parts are usually integrated on the same silicon chip. The working process can be divided into reset, photoelectric conversion, integration, and readout.

 Figure4. Composition of CMOS sensor

The photoelectric information conversion function of CMOS is basically similar to that of CCD. The difference is that the information transmission method of the two sensors after photoelectric conversion is different.

2 Working principle of CMOS image sensor

(1) The pixel structure of the MOS tube

Before understanding the CMOS image sensor, we need to know the structure of the bottom layer-the the pixel structure of the MOS tube.

The MOS transistor and photodiode constitute a structural section equivalent to one pixel. During the light integration period, the MOS transistor is cut off, and the photodiode generates corresponding carriers with the intensity of the incident light and stores it in the PN junction of the source (position ① in the figure below).

Figure5. MOS tube pixel structure

When the integration period ends, the scan pulse is applied to the gate of the MOS transistor to turn it on. The photodiode resets the reference potential and the video current flow on the load. MOS transistor source PN junction functions as photoelectric conversion and carrier storage. When a pulse signal is applied to the gate, the video signal is read out.

(2) CMOS image sensor array structure

The pixel structure of multiple MOS tubes constitutes a CMOS image sensitive element array structure. Here is the beginning of the CMOS image sensor sensing light. The CMOS image sensor element array structure is composed of a horizontal shift register, a vertical shift register, and a CMOS image element sensitive element array.

Figure6. CMOS image sensitive element array structure

(1-vertical shift register: 2-horizontal shift register; 3-horizontal scan switch; 4-vertical scan switch; 5-image sensor array; 6-signal line; 7-image sensor)

As mentioned above, each MOS transistor functions as a switch under the pulse drive of the horizontal and vertical scanning circuits. The horizontal shift register sequentially turns on the MOS transistors that perform the horizontal scanning function from left to right, that is, the function of addressing columns, and the vertical shift register sequentially address each row of the array.

Each pixel is composed of a photodiode and a MOS transistor that acts as a vertical switch. The horizontal switch is sequentially turned on under the pulse generated by the horizontal shift register. And the vertical switch is turned on under the pulse generated by the vertical shift register. So we can apply the reference voltage (bias) to the photodiode of the pixel in sequence.

 Figure7. CMOS image sensor array working diagram

The illuminated diode generates carriers to discharge the junction capacitance, which is the accumulation process of the signal during the integration. The above-mentioned process of turning on the bias voltage is also a signal reading process. The size of the video signal formed on the load is proportional to the intensity of the light on the pixel.

(3) Three steps to understanding the workflow of a CMOS image sensor

According to the functional block diagram of the CMOS image sensor, we can find that the workflow of the CMOS image sensor is mainly divided into the following three steps.

Figure8. Functional block diagram of CMOS image sensor

Step 1: The external light illuminates the pixel array and a photoelectric effect occurs. A corresponding charge is generated in the pixel unit.

The scene is focused on the image sensor array through the imaging lens, and the image sensor array is a two-dimensional pixel array. Each pixel includes a photodiode. The photodiode in each pixel converts the light intensity of the array surface into an electrical signal.

Step 2: Select the pixel you want to operate through the row selection circuit and column selection circuit, and read out the electrical signal on the pixel.

In the selection process, the row selection logic unit can scan the pixel array row by row or interlaced, the same applies to the columns. The row selection logic unit and the column selection logic unit are used together to realize the window extraction function of the image.

Step 3: Output the corresponding pixel unit after signal processing.

The image signal in the row pixel unit is transmitted to the corresponding analog signal processing unit and A/D converter through the signal bus of the respective column and converted into a digital image signal for output. Among them, the main function of the analog signal processing unit is to amplify the signal and improve the signal-to-noise ratio.

The pixel electrical signal is amplified and sent to the correlated double sampling CDS circuit for processing. Correlated double sampling is an important method used by high-quality devices to eliminate some interference. The basic principle is that the image sensor leads to two outputs, one is a real-time signal, and the other is a reference signal. The difference between the two signals is used to remove the same or related interference signals.

This method can reduce KTC noise, reset noise, and fixed pattern noise FPN (Fixed Pattern Noise), but also can reduce 1/f noise and improve the signal-to-noise ratio. Also, it can complete signal integration, amplification, sampling, hold, and other functions. The signal is then output to an analog/digital converter and converted into a digital signal output.

Besides, to obtain a qualified practical camera, the chip must contain various control circuits, such as exposure time control, automatic gain control, etc. To make each part of the circuit in the chip operate following the prescribed rhythm, multiple timing control signals must be used. To facilitate the application of the camera, the chip is also required to output some timing signals, such as synchronization signals, line start signals, and field start signals.

Ⅳ Conclusion

Among analog cameras and standard-definition network cameras, CCDs are the most widely used and have dominated the market for a long time. CCD is characterized by high sensitivity, but low response speed, which is not suitable for the high-resolution progressive scanning method used by high-definition surveillance cameras. Therefore, after entering the era of high-definition surveillance, CMOS is gradually recognized by people, and high-definition surveillance cameras generally use CMOS photosensitive devices.

The main advantage of CMOS for CCD is that it is power efficient. Unlike a CCD composed of diodes, CMOS circuits have almost no static power consumption. This makes the power consumption of CMOS only about 1/3 of the ordinary CCD. The problem of CMOS is that when processing fast-converting images because the current conversion is too frequent and overheated. If the dark current suppression effect is not good, noise is likely to appear.

CCD provides good image quality, anti-noise ability, and flexibility in camera design. Although the size of the system and the complexity are increased due to the increase of external circuits, it can be more flexible in circuit design and can improve some of the performance of the CCD camera. CCD is more suitable for applications that have very high requirements on-camera performance and low requirements for cost control, such as astronomy, high-definition medical X-ray images, and other scientific applications that require long exposure and strict image noise requirements.

CMOS is an image sensor that can be produced using contemporary large-scale semiconductor integrated circuit production processes. It has the characteristics of high yield, high integration, low power consumption, and low price. CMOS technology is a technology used by many image sensor semiconductor R&D companies in the world to try to replace CCD. After years of effort, as an image sensor, CMOS has overcome many of the shortcomings of the early days and has developed to a level comparable to CCD technology in terms of image quality. The level of CMOS makes them more suitable for applications requiring small space, small size, low power consumption, and for applications not particularly requiring low image noise and high quality. For example, Most industrial inspection applications with auxiliary light illumination, security applications, and most consumer commercial digital cameras use CMOS.

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