There have been many articles about the advantages of CMOS sensor technology compared to CCD sensor technology, and in everyday life, people almost only see CMOS sensors, but it seems that the debate continues to revolve around which sensor is better. However, in reality, the answer is not easy because the answer depends on many factors. The aspects considered depend on both technology and market, affecting technical feasibility and commercial viability. Photography applications are also very diverse, leading to a variety of accompanying requirements, some applications are suitable for CMOS sensors while others are more suitable for CCD sensors. In this article, we want to clarify this issue in some aspects by examining different situations, as well as explaining the trade-offs and considering the cost factor.

1. A brief history of development

First, CCD (charge coupled device) and CMOS (complementary metal oxide semiconductor) sensors are two different sensor technologies used to convert image signals into digital images, each sensor technology has different strengths and weaknesses that give us advantages in different specific applications.
Both sensor technologies convert light into electrostatic signals that are then processed into digital signals. In CCD technology, the charge at each pixel is transferred through a very small number of output points (nodes) and then converted into a voltage signal, stored and then output from the sensor as an analog signal, all pixels are directly exposed to light, and the output uniformity is higher (a factor that determines image quality). In CMOS technology, each pixel has its own charge-to-voltage converter, and the sensor also includes amplifiers, noise filters, and digital circuits that output the digital signal. These additional functions increase the complexity of the design and reduce the area of ​​the pixel exposed to light. When each pixel is converted separately, the uniformity of the digital signal is reduced, but the signal is processed in parallel, which gives us a faster speed.

Both CCD and CMOS were developed in the late 1960s and 1970s. CCD technology was initially more popular because it provided better image quality than the sensor technology at that time. CMOS technology required higher pixel uniformity, and smaller sizes than semiconductor manufacturers could provide at that time. It was not until the 1990s that scientists were able to create a suitable CMOS sensor design. This has brought back interest in CMOS sensors due to their desirable properties: low power consumption, suitability for on-chip camera integration, low manufacturing costs due to the reuse of logic gates and memories currently in commercial production. Although balancing these benefits while still producing good image quality requires a lot of investment in time, money, and more steps than before, CMOS sensor technology has returned to the race with CCD technology.

2. Popularity in consumer products

With the characteristics of low power consumption and the ability to integrate into small devices, CMOS sensor designers have focused on the field of image sensors for phones (the field that consumes the most image sensors in the world). A large amount of investment has been poured into the development and improvement of CMOS sensors and the field of cMOS sensor manufacturing. And the result of that investment is that we have witnessed a leap in image quality, as well as a rapid decrease in pixel size. It can be seen that in the field of consumer goods and line-scanning cameras, based on most of the criteria that can be mentioned, CMOS technology has outperformed CCD technology.

3. In the field of machine vision

In the field of machine vision, area-weighted CMOS sensors and line sensors have also inherited the achievements of phone camera sensor technology, thereby gradually replacing CCD sensors.

It can be said simply that in the field of machine vision, the two most important factors to evaluate are speed and noise. CMOS and CCD technologies differ in the way they convert signals from light to digital form. For CMOS sensors, the output of pixels is designed in parallel to the maximum, limiting the bottleneck between the sensor and the circuit. Although improved, high-speed CCD sensors can have more outputs but still cannot compare with CMOS sensors. In addition, CCD sensors with voltage signal output also have higher noise than the digital signal of CMOS sensors.

4. However, there are also some areas where CMOS sensors cannot outperform CCD sensors

a. Infrared photography: most CMOS sensors are manufactured for applications that capture light in the visible range, so CMOS sensors are not as sensitive as CCD sensors. If we increase the thickness of the CMOS sensor substrate, we will increase the sensitivity of the sensor but reduce the image resolution. On the contrary, CCD sensors can easily increase the thickness of the epi layer without affecting the image resolution. Therefore, CCD sensors are more suitable than CMOS sensors.

b. Ultraviolet (UV) photography: both sensors have disadvantages and require many measures to improve image quality.

5. In terms of cost

a. For sensors that need to be manufactured according to special requirements: CCD sensors are cheaper to develop than CMOS sensors because CMOS sensors use more expensive silicon masks, and the circuit devices are also more expensive, so in the case of custom CMOS sensors, they may have better capabilities, but sometimes CCD sensors are still the optimal choice.

b. In terms of output: although the development cost is higher, CMOS sensors have higher commercial potential because of the large output, when the output is large, the low cost of producing a product is more important than the low development cost, so CMOS sensor products have lower prices and are therefore more popular.

c. Supply capacity and supply assurance: it will be very difficult when users of the product are left with sensors that have been discontinued, so users should consider choosing a supplier with long-term supply capacity.

6. Conclusion

Choosing a CCD or CMOS sensor is never a simple task, it depends on the requirements, and each application has different requirements. We need to choose appropriately not only in terms of technology but also in terms of price. Therefore, it is difficult to affirm with certainty whether CMOS sensors are really better than CCD sensors.