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I read that CMOS image sensor technology was discovered first but then they had more noise and thus once CCD was invented, CCD got popular because of better image quality.

After decades, now we use CMOS due to its lower power consumption, speed, etc.

What exactly is the reason for CCD having better image quality over CMOS? (At least back then, now lot of research is being done in CMOS to make it better so I dont know if this statement is still valid.)

JRE
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Hari Krishna
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  • CCD sensors are essentially obsolete because CMOS are faster, cheaper and lower noise. In what sense do you think they have "better image quality"? – user1850479 Aug 23 '21 at 16:39
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    Why did we use CCD for some decades even after inventing CMOS sensors ? @user1850479 – Hari Krishna Aug 23 '21 at 16:56
  • You have your assumptions backwards. – Tony Stewart EE75 Aug 23 '21 at 16:58
  • Oops I didn't get you @TonyStewartEE75 – Hari Krishna Aug 23 '21 at 16:59
  • CCD came out long before CMOS – Tony Stewart EE75 Aug 23 '21 at 16:59
  • https://www.teledynedalsa.com/en/learn/knowledge-center/ccd-vs-cmos/#:~:text=CCDs%20and%20CMOS%20imagers%20were,with%20the%20fabrication%20technology%20available. – Hari Krishna Aug 23 '21 at 17:03
  • I read this in multiple papers and sites. Bu then maybe they said MOS technology existed before invention of CCD. It was said the image quality was superior in CCD. @TonyStewartEE75 – Hari Krishna Aug 23 '21 at 17:04
  • You'd have to dig up some old datasheets to compare with modern ones, but the architecture of CCD was a bit "easier" to make: all the pixels essentially dumped into one amplifier-A/D conversion circuit. CMOS has a amplifier-A/D conversion circuit for each pixel so not only do they have to be smaller and/or harder to make, but they have to be well-matched. – vir Aug 23 '21 at 17:11
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    Electric cars came out before 1900, but we don’t count those as being practical technology for Energy density storage until someone learnt how to make Lithium cells reliable and cost effective. Same for CMOS, it was not until the 1990’s that lithography technology could make the pixels small enough to get a cost-effective practical solution. – Tony Stewart EE75 Aug 23 '21 at 17:34
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    @HariKrishna Your link explains why CCDs were used (read the few sentences after what you highlighted). By the way, I remember reading a version of that article back in the 2000s when DALSA was still an independent company. I would try to limit your research to sources from the last 5 or 10 years since technology improves over time. – user1850479 Aug 23 '21 at 17:46
  • Yea these are old sources, I was just curious to know why back in 1980s CCD gave clearer images. since 1990s things have changed. @user1850479 – Hari Krishna Aug 23 '21 at 17:55

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Two big things I can think of: the sensitivity of the device and the speed of the readout circuitry.

For sensitivity, an important figure of merit is QE/RN (quantum efficiency/read noise), kind of like the SNR of the sensor. CMOS used to be half that of CCD but now is about twice as good.

CCD also had an advantage in global shutter since all the accumulated charge was moved simultaneously into a shift register that could store the entire image at once. CMOS had to read out each line separately which took time and if the image moved between the first and last readout, you ended up with a distorted image. Global shutter CMOS has been out for a few years now; ONSemi released the AR0134 back in 2017. I'm not 100% on the technology changes, but my guess is smaller feature size/technology node and back-illumination - where the readout circuitry isn't between the incoming light and the light-gathering area - are contributing factors.

vir
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CCD are still used today by astronomers, x-ray, physics, ...etc, everywhere you need a good signal VS noise ratio. The CMOS is a semiconductor production with all the buffers, shift registers, signal drivers on the same die/chip.

The CCD is bare sensor that got tiny cells charged if the photons expels charge upon collision. All the rest electronics is separated, so it is more expensive, it also needs a mechanical shutter like old camera. To improve SN ratio the CCD can be cooled to a very low temperature, while the CMOS in-built electronics generates spots of noise due to generated heat, even if you cool them.

The CMOS is better for recording fast action due to higher sensitivity, meanwhile the CCD is better for imaging at standstill due to better SN ratio.

Marko Buršič
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  • You can get CMOS sensors now with sub-electron read noise and near-unity QE, so I don't think it is accurate to say CCDs have better SNR, at least in the last 5 or 10 years. Noise in modern CMOS being due to hot spots is also incorrect. – user1850479 Aug 23 '21 at 23:27