about choosing a video camera for a family, we wrote about matrices. There we touched on this issue easily, but today we will try to describe both technologies in more detail.

What is a matrix in a camcorder? This is a microcircuit that converts a light signal into an electrical one. Today there are 2 technologies, that is 2 types of matrixes - CCD (CCD) and CMOS (CMOS)... They are different from each other, each has its own pros and cons. It is impossible to say for sure which one is better and which one is worse. They develop in parallel. We will not go into technical details, because they will be trivially incomprehensible, but in general terms we will define their main pros and cons.

CMOS technology (CMOS)

CMOS matrices primarily boast of low power consumption, which is a plus. A camcorder with this technology will last a little longer (depending on battery capacity). But these are trifles.

The main difference and advantage is the random reading of the cells (in the CCD, reading is carried out simultaneously), due to which blurring of the picture is excluded. Have you ever seen "vertical pillars of light" from bright point objects? So CMOS-matrices exclude the possibility of their appearance. And cameras based on them are also cheaper.

There are also disadvantages. The first is the small size of the photosensitive element (in relation to the pixel size). Here, most of the pixel area is occupied by electronics, therefore the area of ​​the photosensitive element is reduced. Consequently, the sensitivity of the matrix decreases.

Because electronic processing is carried out on a pixel, then the amount of noise in the picture increases. This is also a disadvantage, as is the low scan time. Because of this, the effect of a "rolling shutter" arises: when the operator moves, the object in the frame may be distorted.

CCD technology (CCD)

Camcorders with CCD-matrix provide high quality images. Visually, it is easy to notice less noise in video captured with a CCD camcorder compared to video captured with a CMOS camera. This is the very first and most important advantage. And one more thing: the efficiency of CCD-matrices is simply amazing: the fill factor is approaching 100%, the ratio of registered photons is 95%. Take the normal human eye - here the ratio is approximately 1%.

High price and high power consumption are the disadvantages of these matrices. The thing is, the recording process is incredibly difficult here. Image fixation is carried out thanks to many additional mechanisms that are not available in CMOS matrices, therefore, CCD technology is significantly more expensive.

CCD-matrices are used in devices from which it is required to obtain a color and high-quality image, and which, possibly, will shoot dynamic scenes. These are professional camcorders for the most part, although they are also household ones. These are also surveillance systems, digital cameras, etc.

CMOS matrices are used where there are no particularly high requirements for picture quality: motion sensors, inexpensive smartphones ... However, this was the case before. Modern CMOS matrices have different modifications, which makes them very high quality and worthy in terms of competing with CCD matrices.

Now it is difficult to judge which technology is better, because both demonstrate excellent results. Therefore, it is at least silly to put the type of matrix as the only selection criterion. There are many characteristics to consider.

Please rate the article:

After reading the previous part, our reader might get the impression that the CCD is a kind of "black box" that gives out an "electronic negative" after the light image created by the lens has been projected onto its recording surface, and that the quality of the image is influenced exclusively by sensor size.

Sellers of digital photographic equipment adhere to the same point of view, gently but persistently pushing a potential buyer to purchase a model with the largest possible sensor, even if there are no objective reasons for such a purchase. Even more often, various kinds of “unique developments” used to create the matrix, which, oddly enough, are not used by any other manufacturer, act as “bait” for the client.

It is difficult for a novice amateur photographer to distinguish advertising promises from truly effective engineering findings. This article will attempt to "separate the wheat from the chaff," but first you need to familiarize yourself with the basic definitions of digital photography.

How a photon becomes an electron

In charge-coupled devices, the conversion of a photon into an electron is carried out as a result of an internal photoelectric effect: absorption of a light quantum by the semiconductor crystal lattice with the release of charge carriers. It can be either a pair "electron + hole", or a single charge carrier - the latter occurs when donor or acceptor impurities are used in a semiconductor. It is obvious that the formed charge carriers must be somehow preserved until the moment of reading.

For this, the main material of the CCD matrix, a p-type silicon substrate, is equipped with channels of an n-type semiconductor, over which electrodes transparent to photons are made of polycrystalline silicon. After applying an electric potential to such an electrode, a potential well is created in the depletion zone under the n-type channel, the purpose of which is to store the charge "extracted" by means of the internal photoelectric effect. The more photons fall on the CCD element (pixel) and turn into electrons, the higher the charge accumulated by the well will be.

CCD element

CCD pixel cross-section

To obtain an "electronic negative", it is necessary to count the charge of each potential well of the matrix. This charge is called photocurrent, its value is quite small and after reading it requires mandatory amplification.

The charge is read by a device connected to the outermost row of the matrix, which is called a serial shift register. This register is a string of CCD cells, the charges of which are read one by one. When reading the charge, the ability of the CCD elements to move the charges of the potential wells is used - in fact, this is why these devices are called CCD devices. For this, transfer gate electrodes are used, located in the gap between the PSG elements. Potentials are applied to these electrodes, "enticing" the charge from one potential well and transferring it to another.

With the synchronous supply of potential to the transfer electrodes, the simultaneous transfer of all line charges from right to left (or from left to right) is ensured in one working cycle. The charge that turned out to be "superfluous" goes to the output of the CCD-matrix. Thus, the serial shift register converts the charges entering its input in the form of parallel "chains" into a sequence of electrical impulses of different magnitude at the output. To feed these parallel "chains" to the input of the serial register, again a shift register is used, but this time in parallel.

CCD

CCD pixel cross-section

In fact, the parallel register is the CCD itself, which, by means of a combination of photocurrents, creates an electronic "cast" of the light image. A matrix is ​​a set of sequential registers called columns and synchronized with each other. As a result, during the working cycle, there is a synchronous "sliding" of the photocurrents downward, and the charges of the lower row of the matrix that turned out to be "unnecessary" are fed to the input of the sequential register.

As follows from the above, a sufficiently large number of control microcircuits is required to synchronize the supply of potentials to both parallel and serial shift registers. Obviously, the serial register must be completely free of charges in the interval between the clock cycles of the parallel register, therefore, a microcircuit is required that synchronizes both registers with each other.

What a pixel is made of

The so-called full-frame CCD-matrix operates according to the above scheme, its mode of operation imposes some limitation on the camera design: if exposure does not stop during the photocurrent reading, the "extra" charge generated by the photons hitting the pixels is "Smeared" over the frame. Therefore, a mechanical shutter is needed, blocking the flow of light to the sensor for the time required to read the charges of all pixels. Obviously, such a scheme for reading photocurrents does not allow the formation of a video stream at the output from the matrix, therefore it is used only in photographic equipment.

However, the excess charge can accumulate in the potential well during photography - for example, if the exposure is too “long”. "Extra" electrons tend to "spread" over neighboring pixels, which is displayed in the image as white spots, the size of which is related to the overflow value. This effect is called blooming (from English blooming - "blurring"). The fight against blooming is carried out by means of electronic drainage (drain) - removal of excess charge from a potential pit. There are two main types of drainage: vertical (Vertical Overflow Drain, VOD) and lateral (Lateral Overflow Drain, LOD).

CCD side drain

Side drainage scheme

To implement vertical drainage, a potential is applied to the ICT substrate, which, when the potential well depth is overfilled, ensures the outflow of excess electrons through the substrate. The main disadvantage of such a scheme is a decrease in the depth of the potential pit, as a result of which the dynamic range is narrowed. And in matrices with back illumination (in them photons penetrate into the sensor not through the electrode of the potential well, but from the side of the substrate), vertical drainage is generally inapplicable.

Lateral drainage is carried out using special "drainage grooves" into which excess electrons "drain". To form these grooves, special electrodes are laid, to which a potential is applied, which forms the drainage system. Other electrodes create a barrier that prevents premature escape of electrons from the potential well.

As follows from the description, with lateral drainage, the depth of the potential well does not decrease, however, the area of ​​the photosensitive area of ​​the pixel is reduced. Nevertheless, it is impossible to do without drainage, since blooming distorts the image more than all other types of interference. Therefore, manufacturers are forced to complicate the design of matrices.

Thus, the "strapping" of any pixel consists of at least the charge transfer electrodes and the components of the drainage system. However, most CCDs are characterized by a more complex structure of their elements.

Pixel optics

CCDs used in video cameras and in most amateur digital cameras provide a continuous stream of pulses at their output, while the optical path does not overlap. To prevent blurring of the image, interline CCD-matrix is ​​used.

Column Buffered CCD

Column buffered matrix structure

In such sensors, next to each column (which is a sequential shift register) is a buffer column (also a sequential shift register), consisting of CCD elements covered with opaque stripes (usually metal). The set of buffer columns constitutes a buffer parallel register, and the columns of this register are "intermixed" with the columns registering light.

In one working cycle, the photosensitive parallel shift register gives all its photocurrents to the buffer parallel register by means of "horizontal shift" of the charges, after which the photosensitive part is again ready for exposure. Then comes the line-by-line "vertical shift" of the charges of the buffer parallel register, the bottom line of which is the input of the serial matrix shift register.

It is obvious that the transfer of the matrix charge to the buffer parallel shift register takes a short time interval and there is no need to block the light flux with a mechanical shutter - the pits will not have time to overflow. On the other hand, the required exposure time is usually comparable to the read time of the entire buffer parallel register. Due to this, the interval between exposures can be reduced to a minimum - as a result, the video signal in modern video cameras is formed at a frequency of 30 frames per second and higher.

In turn, column buffered sensors fall into two categories. When reading all lines in one clock cycle, we can speak of a matrix with a progressive scan. When odd lines are read in the first cycle, and even lines in the second (or vice versa), we are talking about an interlace scan matrix. By the way, due to the similarity of the sound of the English terms "interlined matrix" and "interlaced" in the domestic literature, sensors with row buffering are often mistakenly called interlaced.

Oddly enough, charge smears also occur in column-buffered matrices. This is caused by the partial flow of electrons from the potential well of the photosensitive CCD element into the potential well of the nearby buffer element. This happens especially often when the photocurrent levels are close to the maximum, caused by very high illumination of the pixel. As a result, a light strip extends up and down from this bright point in the picture, which spoils the frame.

To counteract this phenomenon, the distance between the photosensitive and buffer CCD elements is increased. As a result, the charge exchange becomes more complicated and the time spent on this increases, but the distortion of the frame caused by "smearing" is still too noticeable to be neglected.

Column buffering also allows the implementation of an electronic shutter, which eliminates the need for mechanical blocking of the luminous flux. With the electronic shutter, you can achieve ultra-fast (up to 1 / 10,000th of a second) shutter speeds that are unattainable with a mechanical shutter. This feature is especially relevant when photographing sports, natural phenomena, etc.

To implement an electronic shutter, anti-blooming drainage is required. At very short exposures, which are shorter in duration than the time of charge transfer from the potential well of the photosensitive CCD element to the potential well of the buffer, the drainage plays the role of "cutoff". This "cut-off" prevents electrons generated in the well of the photosensitive element after the exposure time from entering the well of the CCD buffer element.

Pixel structure - microlens and conventional

The degree of concentration of the light flux passing through the microlens depends on the technological level of the matrix manufacturer. There are quite complex designs that provide maximum efficiency for these miniature devices.

However, the use of microlenses significantly reduces the likelihood that light rays incident at a large angle to the normal will penetrate the light-sensitive area. And with a large aperture aperture, the percentage of such rays is quite large. Thus, the intensity of the effect of the light flux on the matrix decreases, that is, the main effect for which the diaphragm is opened.

However, the harm from such rays is no less than the benefit. The fact is that, penetrating into silicon at a large angle, a photon can enter the matrix on the surface of one pixel, and knock out an electron in the body of another. This leads to image distortion. Therefore, in order to weaken the influence of such "armor-piercing" photons, the surface of the matrix, with the exception of light-sensitive areas, is covered with an opaque mask (usually a metal one), which further complicates the design of the matrices.

In addition, microlenses introduce certain distortions into the recorded image, blurring the edges of lines, the thickness of which is on the verge of sensor resolution. But even this negative effect may be partially beneficial. Such thin lines can lead to aliasing of the image, arising from assigning a specific color to a pixel, regardless of whether it is completely covered by a part of the image or only part of it. Jagging results in jagged lines with jagged edges in the image.

It is because of the stepping that cameras with large full-frame sensors are equipped with anti-aliasing filters, and the price of these devices is quite high. Well, matrices with microlenses do not need this filter.

Due to different image quality requirements, column-buffered matrices are mainly used in the amateur technology, while full-frame sensors have settled in professional and studio cameras.

To be continued

This article gives a description, if I may say so, of the pixel geometry. More details about the processes occurring during registration, storage and reading of the charge will be described in the next article.

A single element is sensitive in the entire visible spectral range, therefore, a light filter is used above the photodiodes of color CCD matrices, which allows only one of three colors to pass through: red (Red), green (Green), blue (Blue) or yellow (Yellow), magenta ( Magenta), turquoise (Cyan). And in turn, there are no such filters in the black-and-white CCD.

DEVICE AND OPERATING PRINCIPLE OF THE PIXEL

A pixel consists of a p-substrate covered with a transparent dielectric, on which a light-transmitting electrode is applied, which forms a potential well.

Above the pixel there may be a light filter (used in color matrices) and a converging lens (used in matrices where the sensitive elements do not completely occupy the surface).

A positive potential is applied to the light-transmitting electrode located on the surface of the crystal. Light falling on a pixel penetrates deep into the semiconductor structure, forming an electron-hole pair. The resulting electron and hole are pulled apart by the electric field: the electron moves to the carrier storage area (potential well), and the holes flow into the substrate.

A pixel has the following characteristics:

• The capacity of a potential well is the number of electrons that the potential well can hold.
• Spectral pixel sensitivity - the dependence of the sensitivity (the ratio of the photocurrent to the luminous flux) on the radiation wavelength.
• Quantum efficiency (measured as a percentage) is a physical quantity equal to the ratio of the number of photons, the absorption of which caused the formation of quasiparticles, to the total number of absorbed photons. In modern CCD matrices, this figure reaches 95%. In comparison, the human eye has a quantum efficiency of the order of 1%.
• Dynamic range is the ratio of the saturation voltage or current to the rms voltage or dark noise current. Measured in dB.

CCD AND CHARGE TRANSFER DEVICE

The CCD is divided into lines, and in turn, each line is divided into pixels. The lines are separated by stop layers (p +), which do not allow the overflow of charges between them. To move a data packet, parallel, vertical (VCCD) and sequential, horizontal (HCCD) shift registers are used.

The simplest cycle of operation of a three-phase shift register begins with the fact that a positive potential is applied to the first gate, as a result of which a well is formed, filled with the formed electrons. Then we apply a potential to the second gate that is higher than the first one, as a result of which a deeper potential well is formed under the second gate, into which electrons will flow from under the first gate. To continue the movement of the charge, you should reduce the value of the potential at the second gate, and apply a greater potential to the third. Electrons flow under the third gate. This cycle continues from the accumulation point to the directly read horizontal resistor. All electrodes of the horizontal and vertical shift registers form phases (phase 1, phase 2 and phase 3).

Color classification of CCDs:

• Black and white
• Colored

Architecture classification of CCDs:

Photosensitive cells are shown in green, opaque areas are in gray.

The CCD has the following characteristics:

• The charge transfer efficiency is the ratio of the number of electrons in the charge at the end of the path along the shift register to the number at the beginning.
• The fill factor is the ratio of the area filled with light-sensitive elements to the total area of ​​the light-sensitive surface of the CCD matrix.
• Dark current is an electric current that flows through a photosensitive element in the absence of incident photons.
• Readout noise is the noise that occurs in the conversion and amplification circuits of the output signal.

Frame transfer matrices. (English frame transfer).

Advantages: The ability to occupy 100% of the surface with photosensitive elements; Readout time is lower than that of a full-frame transfer sensor; Smudge less than full-frame transfer CCD; Has a duty cycle advantage over full frame architecture: the frame transfer CCD collects photons all the time. Disadvantages: When reading data, cover the light source with the shutter to avoid blurring; The path of charge movement has been increased, which negatively affects the efficiency of charge transfer; Manufacturing and production of these matrices is more expensive than devices with full-frame transfer.

Matrices with interline transfer or matrix with column buffering (English Interline-transfer).
	Advantages: There is no need to use a shutter; No lubrication. Disadvantages: The ability to fill the surface with sensitive elements no more than 50%. The read speed is limited by the speed of the shift register; The resolution is lower than that of CCDs with frame and full frame transfer.

Matrices with line-frame transfer or matrix with column buffering (English interline).

Advantages: The processes of accumulation and transfer of charge are spatially separated; The charge from the storage elements is transferred to the transfer registers that are closed from the light of the CCD matrix; The transfer of the charge of the entire image is carried out in 1 clock cycle; No lubrication; The interval between exposures is minimal and is suitable for video recording. Disadvantages: The ability to fill the surface with sensitive elements no more than 50%; Resolution is lower than that of CCDs with frame and full frame transfer; The path of charge movement has been increased, which negatively affects the efficiency of charge transfer.

APPLICATION OF CCD MATRIXES

	SCIENTIFIC APPLICATION for spectroscopy; for microscopy; for crystallography; for fluoroscopy; for natural sciences; for biological sciences.
	SPACE APPLICATION in telescopes; in star sensors; in tracking satellites; when probing planets; onboard and manual crew equipment.
	INDUSTRIAL APPLICATIONS to check the quality of welds; to control the uniformity of painted surfaces; to study the wear resistance of mechanical products; for reading barcodes; to control the quality of product packaging.
	APPLICATION FOR PROTECTION OF OBJECTS in residential apartments; at airports; at construction sites; at workplaces; in "smart" cameras that recognize a person's face.
	APPLICATION IN PHOTOGRAPHY in professional cameras; in amateur cameras; in mobile phones.
	MEDICAL APPLICATION in fluoroscopy; in cardiology; in mammography; in dentistry; in microsurgery; in oncology.
	AUTO ROAD APPLICATION for automatic license plate recognition; for speed control; for traffic management; for admission to the parking lot; in police surveillance systems.

How distortion occurs when shooting moving objects with a linear shutter sensor:

(lang: ‘ru’)

I continue the conversation about the device started in the previous publication.

One of the main elements of a digital camera that distinguishes it from film cameras is a photosensitive element, the so-called image intensifier tube or photosensitive digital camera... We have already spoken about camera matrices, but now we will consider the device and the principle of the matrix operation in more detail, although rather superficially so as not to tire the reader too much.

Most digital cameras nowadays are equipped with CCD matrices.

CCD-matrix. Device. Principle of operation.

Let's take a look at the device CCD sensors.

Semiconductors are known to be divided into n-type and p-type semiconductors. In an n-type semiconductor there is an excess of free electrons, and in a p-type semiconductor there is an excess of positive charges, "holes" (and hence a lack of electrons). All microelectronics is based on the interaction of these two types of semiconductors.

So, the element Digital camera CCD is arranged as follows. See Figure 1:

Fig. 1

Without going into details, a CCD element or a charge-coupled device, in English transcription: charge-coupled-device - CCD, is a MIS (metal-dielectric-semiconductor) capacitor. It consists of a p-type substrate - a silicon layer, an insulator of silicon dioxide and electrode plates. When a positive potential is applied to one of the electrodes, a zone depleted in major carriers - holes, is formed under it, since they are pushed aside by the electric field from the electrode deep into the substrate. Thus, a potential well is formed under this electrode, i.e., the energy zone is favorable for the movement of minority carriers - electrons into it. A negative charge accumulates in this pit. It can be stored in this well for a long time due to the absence of holes in it and, therefore, the reasons for the recombination of electrons.

In photosensitive matrices the electrodes are films of polycrystalline silicon, transparent in the visible region of the spectrum.

The photons of the light incident on the matrix fall into the silicon substrate, forming a hole-electron pair in it. Holes, as mentioned above, are displaced deep into the substrate, and electrons accumulate in the potential well.

The accumulated charge is proportional to the number of photons falling on the element, that is, to the intensity of the light flux. Thus, a charge relief corresponding to the optical image is created on the matrix.

Moving charges in the CCD matrix.

Each CCD element has several electrodes to which different potentials are applied.

When a potential higher than that on a given electrode is applied to a neighboring electrode (see Fig. 3), a deeper potential well is formed under it, into which the charge moves from the first potential well. Thus, the charge can move from one CCD cell to another. The CCD element shown in Fig. 3 is called three-phase, there are also 4-phase elements.

Fig. 4. Scheme of operation of a three-phase device with a charge coupled - a shift register.

To convert charges into pulses of current (photocurrent), sequential shift registers are used (see Fig. 4). Such a shift register is a string of CCD elements. The amplitude of the current pulses is proportional to the amount of charge transferred, and is thus proportional to the incident luminous flux. The sequence of current pulses generated by reading the sequence of charges is then fed to the input of the amplifier.

Rows of closely spaced CCD elements are combined into CCD... The work of such a matrix is ​​based on the creation and transfer of a local charge in potential wells created by an electric field.

Fig. 5.

The charges of all CCD elements of the register are synchronously moved to adjacent CCD elements. The charge that was in the last cell is fed to the output from the register, and then fed to the input of the amplifier.

The serial shift register is charged with perpendicularly spaced shift registers, collectively referred to as a parallel shift register. Parallel and sequential shift registers make up the CCD matrix (see Fig. 4).

The shift registers perpendicular to the serial register are called columns.

The movement of charges in the parallel register is strictly synchronized. All charges of one row are shifted simultaneously to the next. The charges of the last line go into a sequential register. Thus, in one working cycle, the line of charges from the parallel register enters the input of the sequential register, freeing up space for the newly formed charges.

The operation of the serial and parallel registers is synchronized by a clock generator. Part digital camera matrix also includes a microcircuit that supplies potentials to the register transfer electrodes and controls their operation.

This type of image intensifier is called a full-frame CCD-matrix. For its operation, it is necessary to have an opaque cover, which first opens the image intensifier for exposure to light, then, when it receives the number of photons necessary to accumulate a sufficient charge in the matrix elements, it closes it from light. Such a cover is a mechanical shutter, as in film cameras. The absence of such a shutter leads to the fact that when the charges move in the shift register, the cells continue to be irradiated with light, adding extra electrons to the charge of each pixel, which do not correspond to the luminous flux of a given point. This leads to "smearing" of the charge, respectively, to the distortion of the resulting image.

Introduction

In this course work I will consider general information about CCD devices, parameters, history of creation, characteristics of modern CCD cameras in the mid-infrared range.

As a result of the course work, I studied the literature on the creation, principle of operation, technical characteristics and application of CCD cameras in the mid-IR range.

CCD. The physical principle of the CCD. CCD

A charge-coupled device (CCD) is a series of simple MIS structures (metal-dielectric-semiconductor) formed on a common semiconductor substrate in such a way that strips of metal electrodes form a linear or matrix regular system in which the distance between adjacent electrodes is sufficient small (Fig. 1). This circumstance determines the fact that the mutual influence of neighboring MIS structures is decisive in the operation of the device.

Figure 1 - CCD structure

The main functional purposes of photosensitive CCDs are converting optical images into a sequence of electrical pulses (forming a video signal), as well as storing and processing digital and analog information.

CCDs are manufactured on the basis of monocrystalline silicon. For this, a thin (0.1-0.15 μm) dielectric film of silicon dioxide is created on the surface of a silicon wafer by thermal oxidation. This process is carried out in such a way as to ensure the perfection of the semiconductor - insulator interface and to minimize the concentration of recombination of centers at the interface. The electrodes of individual MIS elements are made of aluminum, their length is 3-7 microns, the gap between the electrodes is 0.2-3 microns. Typical number of MIS-elements is 500-2000 in linear and matrix CCD; the area of ​​the plate Under the extreme electrodes of each row, p-n - junctions are made, intended for the input - output of a portion of charges (charge packs) electric. method (injection by p-n-junction). With photoelectric the input of the charge packets, the CCD is illuminated from the front or rear side. Under frontal illumination, in order to avoid the shading effect of the electrodes, aluminum is usually replaced by films of heavily doped polycrystalline silicon (polysilicon), transparent in the visible and near-IR regions of the spectrum.

How the CCD works

The general principle of CCD operation is as follows. If a negative voltage is applied to any metal electrode of the CCD, then under the action of the arising electric field, the electrons, which are the main carriers in the substrate, leave the surface deep into the semiconductor. At the surface, a depletion region is formed, which on the energy diagram represents a potential well for minority carriers - holes. Holes falling into this region in any way are attracted to the insulator - semiconductor interface and are localized in a narrow subsurface layer.

If now a negative voltage of greater amplitude is applied to the neighboring electrode, then a deeper potential well is formed and the holes pass into it. Applying the necessary control voltages to various CCD electrodes, it is possible to provide both storage of charges in certain near-surface regions and directed movement of charges along the surface (from structure to structure). The introduction of a charge packet (recording) can be carried out either by a pn-junction located, for example, near the extreme CCD element, or by light generation. The removal of the charge from the system (reading) is also easiest to carry out using a pn junction. Thus, a CCD is a device in which external information (electrical or light signals) is converted into charge packets of mobile carriers, located in a certain way in the near-surface regions, and information processing is carried out by the controlled movement of these packets along the surface. It is obvious that digital and analog systems can be built on the basis of CCDs. For digital systems, only the fact of the presence or absence of a charge of holes in a particular element of the CCD is important; in analog processing, they deal with the values ​​of moving charges.

If a light flux carrying an image is directed to a multielement or matrix CCD, then the photogeneration of electron-hole pairs will begin in the volume of the semiconductor. When entering the depletion region of the CCD, the carriers are separated and holes accumulate in the potential wells (moreover, the value of the accumulated charge is proportional to the local illumination). After a certain time (on the order of several milliseconds), sufficient for the perception of the image, the picture of the charge packets corresponding to the distribution of illumination will be stored in the CCD matrix. When the clock is turned on, the charge packets will move to the output reader, which converts them into electrical signals. As a result, the output will be a sequence of pulses with different amplitudes, the envelope of which the video signal gives.

The principle of operation of the CCD on the example of a fragment of the line of the CCD controlled by a three-cycle (three-phase) circuit is illustrated in Figure 2. During cycle I (perception, accumulation and storage of video information), the so-called. storage voltage Uxp, pushing back the main carriers - holes in the case of p-type silicon - deep into the semiconductor and forming depleted layers with a depth of 0.5-2 microns - potential wells for electrons. Illumination of the PCCD surface generates excess electron-hole pairs in the silicon volume, while electrons are drawn into potential wells, localized in a thin (0.01 μm) surface layer under electrodes 1, 4,7, forming signal charge packets.

charge link camera infrared

Figure 2 - operation diagram of a three-phase charge-coupled device - a shift register

The amount of charge in each packet is proportional to the exposure of the surface near the given electrode. In well-formed MIS structures, the generated charges near the electrodes can persist for a relatively long time, but gradually, due to the generation of charge carriers by impurity centers, defects in the bulk, or at the interface, these charges will accumulate in potential wells until they exceed the signal charges and even completely fill the wells.

During cycle II (charge transfer), a readout voltage higher than the storage voltage is applied to electrodes 2, 5, 8 and so on. Therefore, deeper potentials arise under electrodes 2, 5 and 8. wells than under electrons 1, 4 and 7, and due to the proximity of electrodes 1 and 2, 4 and 5,7 and 8, the barriers between them disappear and electrons flow into neighboring, deeper potential wells.

During cycle III, the voltage on electrodes 2, 5, 8 decreases to a, is removed from electrodes 1, 4, 7.

That. all charge packets are transferred along the CCD line to the right by one step equal to the distance between adjacent electrodes.

During the entire operation, a small bias voltage (1-3 V) is maintained at the electrodes that are not directly connected to the potentials, which ensures the depletion of charge carriers on the entire semiconductor surface and weakening of the recombination effects on it.

Repeating the process of switching voltages many times, all charge packets, excited, for example, by light in a line, are sequentially output through the extreme r-h-junction. In this case, voltage pulses appear in the output circuit, proportional to the amount of charge of the given packet. The illumination pattern is transformed into a surface charge relief, which, after moving along the entire line, is converted into a sequence of electrical impulses. The greater the number of elements in a row or matrix (the number of 1 - IR receivers; 2 - buffer elements; 3 - CCD, an incomplete transfer of the charge packet from one electrode to the neighboring one occurs and the resulting information distortion is amplified. the time of light transfer, spatially separated areas of perception - accumulation and storage - readout are created on the FPCD crystal, and in the former they provide maximum photosensitivity, and the latter, on the contrary, screen from light. 1 in one cycle, are transferred to register 2 (from even elements) and to register 3 (from odd elements). While information is transferred through these registers through output 4 to signal combining circuit 5, a new video frame is accumulated in line 1. FPSS with frame transfer (Figure 3), the information received by the accumulation matrix 7 is quickly "dumped" into the storage matrix 2, from which the successive but read by CCD register 3; at the same time, matrix 1 accumulates a new frame.

Figure 3 - accumulation and reading of information in a linear (a), matrix (b) charge-coupled photosensitive device and in a device with charge injection.

In addition to CCDs of the simplest structure (Figure 1), other types of them have also become widespread, in particular, devices with overlapping polysilicon electrodes (Figure 4), in which active photoelectric effect is provided on the entire semiconductor surface and a small gap between the electrodes, and devices with asymmetry of near-surface properties (for example ., with a dielectric layer of variable thickness - Figure 4), operating in a two-stroke mode. The structure of a CCD with a volumetric channel (Figure 4) formed by diffusion of impurities is fundamentally different. Accumulation, storage, and charge transfer occur in the bulk of the semiconductor, where the recombination of centers is less than on the surface and the carrier mobility is higher. The consequence of this is an order of magnitude increase and decrease in comparison with all types of CCD with a surface channel.

Figure 4 - Varieties of CCD devices with surface and volume channels.

To perceive color images, one of two methods is used: dividing the optical flow using a prism into red, green, blue, perceiving each of them by a special FPCD - a crystal, mixing pulses from all three crystals into a single video signal; creation of a film line or mosaic coding filter on the surface of the FPZS, which forms a raster of multi-colored triads.