Image sensors convert an optical image into an electrical signal, i.e. image sensors sense the light coming into the camera lens and convert them into images. The most widely used image sensors available in the market are CCD and CMOS. CCD stands for Charge-Coupled Device while CMOS stands for Complementary Metal-Oxide Semiconductor. Even though both work on the same principle, their functioning is different. With the evolving technology, both sensors have their own significance. While some devices work well with CMOS sensors, few are best suited for CCD sensors. Today, CMOS Images sensors are used more often than CCD Image sensors, even though CCD can provide better image quality. This is because they are easy to manufacture, support higher speed image processing, consume less power, and are less expensive. Though CCD is more advanced and has better quality, CMOS sensors are continuing to improve in technology. CCD is mainly used in DSLR cameras and CMOS image sensors are widely used in digital cameras, mobile phones, tablets, etc.
The CCD image sensor is a semiconductor device that transforms optical images into digital signals. It is used in digital cameras and other electronic optical devices. It is a functional device that divides the light image falling on its light-receiving surface into many small units and converts them into useful electrical signals. It is a highly sensitive photodiode. It is divided into a large number of light-sensitive small areas which are known as pixels. These pixels are used to build up a particular image. A pixel area may not be completely photosensitive. There are areas where photosensitivity is absent. The ratio of the photosensitive area to the total pixel area is known as the fill factor of an image sensor. An image sensor with a high fill factor is always preferred.
When a light photon falls on the area defined by a pixel, those photons are converted into electrons whose number depends on the intensity of the light falling on it. After each cycle, the number of electrons in each pixel is measured and the image is reconstructed.
CCD image sensors are grouped within a family of charge transfer devices that transfer charge through the semiconductor using potential wells. A CCD potential well is made by supplying one of the Metal Oxide Semiconductor (MOS) electrodes with a voltage that is different from that supplied to the other electrodes. When light falls on a CCD sensor, it is transferred to the CCD semiconductor through the electrodes present on top of it. Then they are converted into electric charges which are collected into a potential well beneath the electrodes. This signal packed in the potential well is sequentially transferred through the semiconductor toward the output section. Due to this, they are also known as analog shift registers.
The charge collection in each pixel is done using the electrodes present in each pixel. These electrodes are arranged in such a way that the charge in each pixel is transferred downwards along the pixel columns. When the charges reach the final row, they are transferred out of the CCD and it is measured. This recreates the image taken. The frequency with which individual images are produced is known as the frame rate of a CCD image sensor. The standard unit of frame rate is frames per second (fps).
The charge transfer efficiency of a Charge-Coupled Device (CCD) image sensor is the efficiency with which charge is transferred during each CCD transfer cycle i.e. the efficiency by which the charges are transferred between the pixels to reach the final pixel for the read-out.
The function of a CCD’s photodiodes is to generate free electrons in response to the incident photon. The number of free electrons produced is proportional to the number of photons that fall on a given pixel location. If the pixel collects electrons that were not created by the incident photon, then the electrical representation of the captured image will have inaccuracies. The creation of such electrons occurs continually in CCDs in response to the internal temperature. The current generated by such charge carriers is referred to as dark current.
The Quantum efficiency of a CCD sensor is the efficiency by which the incident photons get converted to electrons. A sensor with 100 % efficiency, for instance, means that all the photons which fall on the photodetector get converted into electrons.
Quantum efficiency is the ratio between the number of charge carriers collected by the photodetector and the number of photons hitting the photosensitive surface. It is an important parameter to determine the quality of the detector and is also known as the spectral response of an image sensor.
The readout noise of a CCD image sensor is a combination of all the noises that arise during the process of amplifying and converting the incident photons into corresponding voltages. The readout noise is a dimensionless value determined from the number of converted electrons and so it is expressed in terms of ‘e’, which stands for electrons. Its value usually lies between 10e and 100e.
Pixel readout frequency is the rate at which the readout process is carried out by each pixel in a CCD image sensor. It is expressed in pixels per second or in hertz (Hz).
In general, CCDs are designed to receive light from the front side where circuit patterns are formed. This type of CCD is called front-illuminated CCD.
The light input surface of the front-illuminated CCD is formed on the front surface of the CCD sensor where thin glass film, poly-silicon electrodes, and gate oxide film are deposited. The electrodes present are not completely transparent, they scatter and reflect the incident light and so reduce overall sensitivity. Furthermore, electrodes make the detection of certain wavelengths nearly impossible because their thickness exceeds the absorption depth. This greatly reduces the quantum efficiency of front-illuminated CCDs.
Back-thinned CCDs were developed to solve such problems. These types of CCDs receive light from the back side of the silicon substrate that does not have a glass film, poly-silicon electrodes, and gate oxide film. Because of this structure, back-thinned CCDs deliver high quantum efficiency over a wide spectral range. In addition to having high sensitivity and low noise, back-thinned CCDs are also sensitive to electron beams, soft x-rays, ultraviolet, visible and infrared regions.
In order for back-thinned CCD to achieve high sensitivity, it is important to make the silicon substrate thin and activate the photosensitive area. The photosensitive area is activated by forming an internal accumulation layer so that the signal charges generated near the backside light input surface are smoothly carried to the CCD potential wells without recombining. Thus they are called back-thinned CCDs and are also sometimes known as back-illuminated CCDs.
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