29 Dec 2020

CCD vs CMOS – The Scientific Way

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CCD vs CMOS – The scientific way

This article will focus on the comparison between CCD and CMOS detector and explain which is better and more importantly, why?

To begin with the article, first lets briefly talk about CCD and CMOS a bit.

CCD, also known as charge-coupled device (CCD) is an integrated circuit containing an array of linked, or coupled, capacitors. Under the control of an external circuit, each capacitor can transfer its electric charge to a neighboring capacitor. CCD sensors are a major technology used in digital imaging.
In a CCD image sensor, pixels are represented by p-doped metal–oxide–semiconductor (MOS) capacitors. These MOS capacitors, the basic building blocks of a CCD, are biased above the threshold for inversion when image acquisition begins, allowing the conversion of incoming photons into electron charges at the semiconductor-oxide interface; the CCD is then used to read out these charges.

Although CCDs are not the only technology to allow for light detection, CCD image sensors are widely used in professional, medical, and scientific applications where high-quality image data are required.

Now lets define CMOS

CMOS, also known as Complementary metal-oxide-semiconductor sensors are also image sensor, majorly used in applications with less exacting quality demands, such as consumer and professional digital cameras, active pixel sensors

As we see above CCD and CMOS are both metal-oxide-semiconductor sensor which are used to capture image. Now lets define “image capturing” a bit before a in depth comparison.

Image Capturing

Image capturing is a way to convert an image to a analog or digital representation. By image, we mean capturing the photons in a meaningful way, like a matrix of a x b format.

Thus an image could be 1 x 1024 pixels which simply means a line image, or it could be 800 x 600 which is a two dimensional image.

To further define the merits, it should be made clear that there are wide range of applications in image capturing domain; while 1 application’s need may be different from other. To list the merits / demerits, let us define two major applications in image capturing.

  • Consumer Electronics
  • Scientific Research

Consumer Electronics

This segment consisits of all consumer products like digital cameras, mobile phones, etc. The enduser may use these devices in various situations like broad day light photography , or night photography, very slow moving subject, very fast moving subject, etc. Thus, these devices are not designed for one specific purpose but designed to cater to wide variety of scenarios. Thus, the sensor for these application cannot excel at one particular scenario but should perform average at all scenarios.

Scientific Research

This segment consists of specialised application which may have to deal with fixed parameters such as very low light application with very slow moving subject (like imaging galaxies very far away), or very intense light application with very fast subject (like analysing flash lamp). Thus these devices are made for one purpose only, and excels at that application

How the CCD / CMOS works?

Lets touch upon the actual working of CCD and CMOS sensors. I will try to go into details to make the comparison stand out clearly.

Both CCD and CMOS have a fundamental level called a pixel. A pixel is a two-dimensional photosensitive area which captures photons and store as electrical potential energy. As the area acts like a “capacitor”, it can store a charge, proportional to the photons. The amount of charge that can be stored in this capacitor is also known as “well depth”.

The pixels are often described as “bucket”, which can trap photons and further can be converted to proportional electrical signal. I will use this analogy to describe the working of both.

CMOS and CCD both use arrays of silicon pixels (“buckets”) to detect light. When a photon of light hits a silicon atom, it knocks an electron into a higher energy state. This frees the electron to move through the material. It is now referred to as a photoelectron (“rain drop”).

The CCD sensor has a larger pixel size (in range from 3 to 25 micron) vs CMOS sensor (which are in range from 2 to 9 microns). This increases the sensitivity of CCD over CMOS sensor which is critical in scientific application.

The big difference happens when one reads out the sensor. In a Charge Coupled Device (CCD), special electrodes attract and repel electrons, shuffling them out one-by-one to a corner of the chip. In our analogy, water is poured from one bucket to the next, like an old-fashioned fire brigade, until it reaches a corner of the array where it is measured. In a real sensor a couple of on-board transistors make this measurement by converting the number of electrons from a pixel into a voltage. It then goes to some electronics outside of the sensor, which include an analog-to-digital converter. The result is a number for each pixel, describing how much light was detected. Since all the pixels are measured by the exact same electronics, CCD detectors can be made very consistent and accurate.

It should be noted that for CCD, the electronics responsible for conversion of photon to electrical signal is same for all pixels. Thus the error of this electronic circuit is common for all pixels. However, for CMOS, every pixel signal is converted using individual transistor. This introduces tranisistor noise/error which is not common and unique for each pixel. This is not a major problem for consumer electronics but is a great problem for scientific research where very noise count matters, and with CMOS, there is no way to offset it. Also as electronics circuitry to convert data is built into the silicon itself, the sensor tends to heat more which again adds more noise in CMOS output. This is the reason that the CMOS detector are hard to cool and cannot operate at extreme temperatures for scientific research.

The advantage of CMOS here is that they are easy to implement from designers point of view, as the complex electronic circuitry to covert the pixel data is baked into the silicon wafer itself. Also they become faster, which again suits consumer industry. It should be also noted that CMOS are more noisy then CCD due to the above explained difference.

The CMOS sensors are also easier to manufacture, thus are relatively cheaper than similarly performing CCD sensors.

The other major advantage of CCD over CMOS is the dynamic range of CCD. This can be understood as the “depth of the bucket”. CCD tends to have depth from 40,000 to 200,000 while CMOS have depth from 30,000 to 75,000. This is extremely critical for scientific research as it gives higher ADC conversion bits and also does not saturate a part of detector for highly varying intensity of light.

Also the quantum efficiency of CCD is far superior than CMOS

Some other areas where CCD excels over CMOS are:

  • Binning
  • Amp Glow
  • Infrared Imaging – CMOS generally perform poor over 650nm
  • Calibration

Thus we see that for scientific research application, CCD is the way to go. For consumer segment, CMOS is a winner due to its low cost, easy integration in design, and trade offs which are acceptable in consumer segment.

References:

  • https://en.wikipedia.org/wiki/Charge-coupled_device
  • https://en.wikipedia.org/wiki/Active-pixel_sensor
  • https://diffractionlimited.com/ccd-versus-cmos-better/
  • https://camera.hamamatsu.com/jp/en/technical_guides/dissecting_camera/dissecting_camera01/index.html
  • http://ankaa.unibe.ch/forads/sr-009-23.pdf