Applications of cathode ray tube
Cathode ray tubes are used in oscilloscopes to measure voltage and phase angles, analyze current or voltage waveforms, etc. These tubes are used in television and radar installations.
Cathode ray tubes There are different types. According to the method of producing an electron beam, they are divided into tubes with a cold and heated cathode. Cold cathode tubes are used relatively rarely, since their operation requires very high voltages (30-70 kV). Tubes with a heated cathode are widely used. These tubes are also divided into two types according to the method of controlling the electron beam: electrostatic and magnetic. In electrostatic tubes, the electron beam is controlled using an electric field, and in magnetic tubes, using a magnetic field.
Electrostatically controlled cathode ray tubes They are used in oscilloscopes and are extremely diverse in design. It is enough for students to familiarize themselves with the principle of construction of such a tube containing the main standard elements. A tube of type 13LOZ7, which is presented in the table with some simplifications, meets these goals.
A cathode ray tube is a well-evacuated glass container containing electrodes. The wide end of the tube - the screen - is coated on the inside with a fluorescent substance. The screen material glows when struck by electrons. The source of electrons is an indirectly heated cathode. The cathode consists of a filament 7 inserted into a thin porcelain tube (insulator), on which is placed a cylinder 6 with an oxide coating on the end (cathode), due to which electron radiation is achieved in only one direction. The electrons emitted from the cathode rush to anodes 4 and 3, which have a fairly high potential relative to the cathode (several hundred volts). To shape a beam of electrons and focus it on a screen, the beam passes through a series of electrodes. However, students should pay attention to only three electrodes: modulator (control cylinder) 5, first anode 4 and second anode 3. The modulator is a tubular electrode to which a negative potential is applied relative to the cathode. Due to this, the electron beam passing through the modulator will be compressed into a narrow beam (beam) and directed by the electric field through the hole in the anode towards the screen. By increasing or decreasing the potential of the control electrode, you can regulate the number of electrons in the beam, i.e., the intensity (brightness) of the screen glow. Using anodes, not only an accelerating field is created (acceleration of electrons is ensured), but by changing the potential of one of them, you can more accurately focus the electron beam on the screen and obtain greater sharpness of the luminous point. Typically, focusing is accomplished by changing the potential of the first anode, which is called focusing.
The electron beam, coming out of the hole in the anode, passes between two pairs of deflection plates 1,2 and hits the screen, causing it to glow.
By applying voltage to the deflection plates, you can cause the beam to deflect and the luminous spot to shift from the center of the screen. The magnitude and direction of the bias depend on the voltage applied to the plates and the polarity of the plates. The table shows the case when voltage is applied only to vertical plates 2. With the indicated polarity of the plates, the electron beam is shifted to the right under the influence of electric field forces. If voltage is applied to horizontal plates 1, then the beam will shift in the vertical direction.
The lower part of the table shows a method for controlling a beam using a magnetic field created by two mutually perpendicular coils (each coil is divided into two sections), the axes of which have vertical and horizontal directions. The table shows the case when there is no current in the horizontal coil and the vertical coil provides beam displacement only in the horizontal direction.
The magnetic field of the horizontal coil causes the beam to shift in the vertical direction. The combined action of the magnetic fields of the two coils ensures that the beam moves across the entire screen.
Magnetic tubes are used in televisions.
Federal Agency for Education
Kuzbass State Pedagogical Academy
Department of Automation of Production Processes
Essay
in radio engineering
Subject:Oscillographic cathode ray tube. Transmitting television tubes
Electron beam indicators
1.1 Basic parameters of CRT
1.2 Oscilloscope electron tubes
II. Transmitting television tubes
2.1 Transmitting television tubes with charge accumulation
2.1.1 Iconoscope
2.1.2 Supericonoscope
2.1.3 Orticon
2.1.4 Superorthikon
2.1.5 Vidicon
Bibliography
I. Electron beam indicators
Electron beam is an electronic vacuum device that uses a stream of electrons concentrated in the form of a beam or beam of beams.
Cathode ray devices that have the shape of a tube extended in the direction of the beam are called cathode ray tubes (CRT). The source of electrons in a CRT is a heated cathode. The electrons emitted by the cathode are collected into a narrow beam by the electric or magnetic field of special electrodes or coils with current. The electron beam is focused on a screen, for the production of which the inside of the glass tube is coated with a phosphor - a substance that can glow when bombarded with electrons. The position of the spot on the screen visible through the glass of the balloon can be controlled by deflecting the flow of electrons by exposing it to an electric or magnetic field of special (deflecting) electrodes or coils with current. If the electron beam is formed and controlled using electrostatic fields, then such a device is called an electrostatically controlled CRT. If not only electrostatic, but also magnetic fields are used for these purposes, then the device is called a magnetically controlled CRT.
Schematic representation of a cathode ray tube
Fig.1
Figure 1 schematically shows the CRT device. The tube elements are placed in a glass container, from which air is evacuated to a residual pressure of 1-10 μPa. In addition to the electron gun, which includes a cathode 1, a grid 2 and an accelerating electrode 3, the electron ray tube has a magnetic deflection and focusing system 5 and deflection electrodes 4, which make it possible to direct the electron beam to various points on the inner surface of the screen 9, which has a metal anode grid 8 with a conductive phosphor layer. The voltage is applied to the anode grid with the phosphor through high-voltage input 7. A beam of electrons falling at high speed onto the phosphor causes it to glow, and a luminous image of the electron beam can be seen on the screen.
Modern focusing systems provide a diameter of the luminous spot on the screen of less than 0.1 mm. The entire system of electrodes that form the electron beam is mounted on holders (traverses) and forms a single device called an electron spotlight. To control the position of the luminous spot on the screen, two pairs of special electrodes are used - deflecting plates, located mutually perpendicular. By changing the potential difference between the plates of each pair, it is possible to change the position of the electron beam in mutually perpendicular planes due to the effect of the electrostatic fields of the deflecting plates on the electrons. Special generators in oscilloscopes and televisions generate a linearly varying voltage, which is applied to the deflection electrodes and creates a vertical and horizontal scan of the image. As a result, a two-dimensional picture of the image is obtained on the screen.
A magnetically controlled CRT contains the same electronic spotlight as an electrostatically controlled CRT, except for a second anode. Instead, a short coil (focusing) with current is used, placed on the neck of the tube near the first anode. The non-uniform magnetic field of the focusing coil, acting on the electrons, acts as a second anode in the electrostatic focusing tube.
The deflection system in a magnetically controlled tube is made in the form of two pairs of deflection coils, also placed on the neck of the tube between the focusing coil and the screen. The magnetic fields of the two pairs of coils are mutually perpendicular, which makes it possible to control the position of the electron beam when the current in the coils changes. Magnetic deflection systems are used in tubes with a high anode potential, which is necessary to obtain high screen brightness, in particular in television receiving tubes - picture tubes. Because the magnetic deflection system is located outside the CRT cylinder, it is convenient to rotate it around the axis of the CRT, changing the position of the axes on the screen, which is important in some applications, such as radar displays. On the other hand, the magnetic deflection system is more inertial than the electrostatic one and does not allow the beam to move with a frequency of more than 10-20 kHz. Therefore, oscilloscopes - instruments designed to observe changes in electrical signals over time on a CRT screen - use electrostatically controlled tubes. Note that there are CRTs with electrostatic focusing and magnetic deflection.
1.1 BasicoptionsCRT
The color of the screen glow may be different depending on the composition of the phosphor. Most often, screens with white, green, blue, and violet colors are used, but there are CRTs with yellow, blue, red, and orange colors.
Afterglow is the time required for the brightness of the glow to decrease from nominal to initial after the cessation of electronic bombardment of the screen. The afterglow is divided into five groups: from very short (less than 10 -5 s) to very long (more than 16 s).
Resolution is the width of the luminous focused line on the screen or the minimum diameter of the luminous spot.
The brightness of the screen is the intensity of light emitted by 1 m 2 of the screen in the direction normal to its surface. Deflection sensitivity is the ratio of the displacement of a spot on the screen to the value of the deflection voltage or magnetic field strength.
There are different types of CRTs: oscillographic CRTs, receiving television tubes, transmitting television tubes, etc. In my work I will consider the design and principle of operation of an oscillographic CRT and transmitting television tubes.
1.2 Oscilloscope cathode ray tubes
Oscilloscope tubes are designed to capture images of electrical signals on a screen. Typically these are electrostatically controlled CRTs, which use a green screen color for observation and cyan or blue for photography. To observe fast periodic processes, CRTs with increased brightness and a short afterglow (no more than 0.01 s) are used. Slow periodic and single fast processes are best observed on CRT screens with a long afterglow (0.1-16 s). Oscillographic CRTs are available with round and rectangular screens ranging in size from 14x14 to 254 mm in diameter. To simultaneously observe two or more processes, multibeam CRTs are produced, in which two (or more) independent electronic spotlights with corresponding deflection systems are mounted. The spotlights are mounted so that the axes intersect in the center of the screen.
II. Transmitting television tubes
Transmitting television tubes and systems convert images of transmission objects into electrical signals. Based on the method of converting images of transmission objects into electrical signals, transmitting television tubes and systems are divided into instantaneous tubes and systems and tubes with charge accumulation.
In the first case, the magnitude of the electrical signal is determined by the light flux that at a given moment of time falls either on the cathode of the photocell, or on the elementary section of the photocathode of the transmitting television tube. In the second case, light energy is converted into electrical charges on the storage element (target) of the transmitting television tube during the frame scanning period. The distribution of electrical charges on the target corresponds to the distribution of light and shadow over the surface of the transmitted object. The totality of electrical charges on the target is called potential relief. The electron beam periodically runs around all the elementary areas of the target and writes off the potential relief. In this case, the useful signal voltage is released at the load resistance. Tubes of the second type, i.e. with accumulated light energy, have greater efficiency than tubes of the first type, so they are widely used in television. That is why I will consider in more detail the structure and types of tubes of the second type.
Transmitting television tubes with charge accumulation
Iconoscope
The most important part of the iconoscope (Fig. 1a) is the mosaic, which consists of a thin sheet of mica 0.025 mm thick. A large number of small silver grains 4, isolated from each other, oxidized and treated in cesium vapor, are deposited on one side of the mica.
A cathode ray tube, invented back in 1897, is an electron-vacuum device that has much in common with a conventional vacuum tube. Externally, the tube is a glass flask with an elongated neck and a flat end part—a screen.
Inside the bulb and neck, as well as inside the cylinder of an electronic lamp, there are electrodes, the leads of which, like those of the lamp, are soldered to the legs of the base.
The main purpose of a cathode ray tube is to produce a visible image using electrical signals. By applying appropriate voltages to the electrodes of the tube, you can draw on its screen graphs of alternating voltages and currents, the characteristics of various radio devices, and also obtain moving images similar to those we see on a movie screen.
Rice. 1. Wonderful pencil.
All this makes the cathode ray tube an indispensable part of televisions, radars, and many measuring and computing instruments.
What kind of “fast pencil” manages to sketch current pulses on the screen of a cathode ray tube that last millionths of a second? How do you manage to select the tones of a complex pattern? How can you instantly “erase” one image from the screen and create another with the same speed? (Fig. 1).
Fluorescent screen to electron beam. The operation of a cathode ray tube is based on the ability of certain substances (willite, zinc sulfide, zinc aluminate:) to glow (luminesce) under the influence of electron bombardment.
If the anode of a conventional electron tube is coated from the inside with such a luminescent substance, it will glow brightly due to bombardment by electrons forming the anode current. By the way, such a luminescent anode is used in one of the special electron tubes - the 6E5C optical tuning indicator. The inside of the thickened end of the flask is coated with a luminescent composition, thus forming a luminescent screen of a cathode ray tube. Using a special device—an “electron gun”—a narrow beam of electrodes—an “electron beam”—is directed from the neck of the tube onto the screen.
Rice. 2. The screen glows under the action of a beam of electrons.
In the place where the electrons hit the luminescent layer, a luminous point is formed on the screen, which is clearly visible (from the end) from the outside of the tube through the glass. The greater the number of electrons forming a beam and the faster these electrons move, the brighter the luminous point on the luminescent screen.
If the electron beam is moved in space, then the luminous point will also move across the screen, and if the beam moves quickly enough, then our eye will see solid luminous lines on the screen instead of a moving point (Fig. 2).
If you quickly trace the entire screen line by line with an electron beam and at the same time change the beam current (i.e., the brightness of the luminous point) accordingly, then you can get a complex and fairly clear picture on the screen.
Thus, the image on the luminescent screen of the tube is obtained using a sharply directed beam of electrons and therefore, just like in an electron tube, the main processes in the tube are associated with the production and ordered movement of free electrons in a vacuum.
Cathode ray tube and triode
A cathode ray tube is in many ways similar to an amplification tube - a triode. Just like a lamp, the tube contains a cathode that emits the electrons needed to produce the electron beam. From the cathode of the tube, electrons move to the screen, which, like the anode of the triode, has a high positive potential relative to the cathode.
Rice. 3. Emergence of secondary electrons
However, applying positive voltage directly to the screen is difficult, since the luminescent substance is a semiconductor. Therefore, positive voltages on the screen have to be created indirectly. The inside of the flask is coated with a layer of graphite, to which a positive voltage is applied. The electrons forming the beam, hitting the luminescent substance with force, “knock out” the so-called “secondary” electrons from it, which move in an orderly manner towards the graphite coating under the influence of a positive voltage on it (Fig. 3).
At the first moment, the number of secondary electrons leaving the screen is much greater than the number of beam electrons entering it. This leads to the formation of a shortage of electrons in the atoms of the luminescent substance, i.e. the screen acquires a positive potential. Equilibrium between the number of electrons hitting the screen and the number of secondary electrons knocked out of it will be established only when the voltage on the tube screen is close to the voltage on the graphite coating. Thus, the current in the cathode ray tube is closed along the path cathode - screen - graphite coating, and therefore, it is the graphite coating that plays the role of the anode, although the electrodes flying out of the cathode do not directly hit it.
Near the cathode of the tube there is a control electrode (modulator), which plays the same role as the control grid of the triode. By changing the voltage on the control electrode, you can change the amount of beam current, which in turn will lead to a change in the brightness of the point glowing on the screen.
However, along with the similarities between an amplifying electron tube and a cathode ray tube, there are features in the operation of the latter that fundamentally distinguish it from a triode.
First, electrons move from the cathode to the tube screen in a narrow beam, while they move in a “broad front” to the anode of the lamp.
Secondly, in order to create an image on it by moving a luminous point across the screen, it is necessary to change the direction of movement of the electrons flying towards the screen and, thus, move the electron beam in space.
From all this it follows that the most important processes that distinguish a tube from a triode are the formation of a thin electron beam and the deflection of this beam in different directions.
Formation and focusing of the electron beam
The formation of an electron beam begins already near the cathode of the cathode ray tube, which consists of a small nickel cylinder with a cap coated with an emitting material (well emitting electrons when heated) material. An insulated wire—a heater—is placed inside the cylinder. Thanks to this cathode design, electrons are emitted from a much smaller surface area than in a conventional vacuum tube. This immediately creates some directionality of the beam of electrons flying from the cathode.
The cathode of the cathode ray tube is placed in a heat shield - a metal cylinder, the end part of which, directed towards the bulb, is open. Due to this, electrons do not move from the cathode in all directions, as is the case in a lamp, but only in the direction of the luminescent screen. However, despite the special design of the cathode and the heat shield, the flow of moving electrons remains excessively wide.
A sharp narrowing of the electron flow is carried out by the control electrode, which, although it plays the role of a control grid, structurally has nothing in common with the grid. The control electrode is made in the form of a cylinder covering the cathode, in the end part of which a round hole with a diameter of several tenths of a millimeter is made.
A significant (several tens of volts) negative bias is applied to the control electrode, due to which it repels electrons, which, as is known, have a negative charge. Under the influence of a negative voltage, the trajectories (paths of movement) of electrons passing through a narrow hole in the control electrode are “compressed” towards the center of this hole and thus a rather thin electron beam is formed.
However, for the tube to operate normally, it is necessary not only to create an electron beam, but also to focus it, i.e., to ensure that the trajectories of all the electrons of the beam converge on the screen at one point. If the beam is not focused, then a rather large luminous spot will appear on the screen instead of a luminous point and, as a result, the image will be blurry or, as amateur photographers say, “unsharp.”
Rice. 4. Electron gun and its optical analogy.
The beam is focused by an electronic optical system, which acts on moving electrons in the same way as conventional optics on light rays. The electronic optical system is formed by electrostatic lenses (static focusing) or electromagnetic lenses (magnetic focusing), the end result of which is the same.
An electrostatic lens is nothing more than (Fig. 4a) an electric field formed with the help of special electrodes, under the influence of which the trajectories of the beam electrons are bent. In a tube with static focusing (Fig. 4, b) there are usually two lenses, for the formation of which they use a control electrode already known to us, as well as two special electrodes: the first and second anodes. Both of these electrodes are metal cylinders, sometimes of different diameters, to which a large positive (relative to the cathode) voltage is applied: the first anode is usually 200-500 V, the second is 800-15,000 V.
A first lens is formed between the control electrode and the first anode. Its optical analogue is a short-focus collecting lens, consisting of two elements: a biconvex and a biconcave lens. This lens produces an image of the cathode inside the first anode, which in turn is projected onto the tube screen using the second lens.
The second lens is formed by the field between the first and second anodes and is similar to the first lens, except that its focal length is much longer. Thus, the first lens plays the role of a condenser, and the second lens acts as the main projection lens.
Inside the anodes there are thin metal plates with holes in the center - diaphragms, which improve the focusing properties of the lenses.
By changing the voltage on any of the three electrodes that form electrostatic lenses, you can change the properties of the lenses, achieving good focusing of the beam. This is usually done by changing the voltage at the first anode.
A few words about the names of the electrodes “first anode” and “second anode”. Previously, we established that the role of the anode in a cathode ray tube is played by the graphite coating near the screen. However, the first and second anodes, mainly intended for focusing the beam, due to the presence of a large positive voltage on them, accelerate electrons, i.e., they do the same as the anode of an intensifying lamp. Therefore, the names of these electrodes can be considered justified, especially since some of the electrons escaping from the cathode fall on them.
Rice. 5. Magnetic focusing tube. 1—control electrode; 2—first anode; 3—focusing coil; 4—graphite coating; 5—luminescent screen; 6—flask.
In cathode ray tubes with magnetic focusing (Fig. 5), there is no second anode. The role of a collecting lens in this tube is played by a magnetic field. This field is formed by a coil covering the neck of the tube through which a direct current is passed. The magnetic field of the coil creates a rotational movement of electrons. At the same time, electrons move at high speed parallel to the axis of the tube towards the luminescent screen under the influence of a positive voltage on it. As a result, electron trajectories form a curve “resembling a helix.
As they approach the screen, the speed of the electrons' translational motion increases, and the effect of the magnetic field weakens. Therefore, the radius of the curve gradually decreases and near the screen the electron beam is stretched into a thin straight beam. Good focusing is usually achieved by changing the current in the focusing coil, that is, by changing the magnetic field strength.
The entire system for producing an electron beam in tubes is often called an “electron gun” or “electron spotlight.”
Electron beam deflection
The deflection of the electron beam, as well as its focusing, is carried out using electric fields (electrostatic deflection) or using magnetic fields (magnetic deflection).
In tubes with electrostatic (Fig. 6a) deflection, the electron beam, before hitting the screen, passes between four flat metal electrode plates, which are called deflection plates.
Rice. 6. Beam control using. a—electrostatic and b—magnetic fields.
A cathode ray tube (CRT) uses a beam of electrons from a heated cathode to produce an image on a fluorescent screen. The cathode is made of oxide, indirectly heated, in the form of a cylinder with a heater. The oxide layer is deposited on the bottom of the cathode. Around the cathode there is a control electrode, called a modulator, cylindrical in shape with a hole in the bottom. This electrode serves to control the density of the electron flow and to pre-focus it. A negative voltage of several tens of volts is applied to the modulator. The higher this voltage, the more electrons return to the cathode. Other electrodes, also cylindrical in shape, are anodes. There are at least two of them in a CRT. At the second anode the voltage ranges from 500 V to several kilovolts (about 20 kV), and at the first anode the voltage is several times less. Inside the anodes there are partitions with holes (diaphragms). Under the influence of the accelerating field of the anodes, electrons acquire significant speed. The final focusing of the electron flow is carried out using a non-uniform electric field in the space between the anodes, as well as thanks to diaphragms. A system consisting of a cathode, modulator and anodes is called an electron projector (electron gun) and is used to create an electron beam, that is, a thin stream of electrons flying at high speed from the second anode to the luminescent screen. An electronic spotlight is placed in the narrow neck of the CRT bulb. This beam is deflected by an electric or magnetic field, and the intensity of the beam can be changed by means of a control electrode, thereby changing the brightness of the spot. The luminescent screen is formed by applying a thin layer of phosphor to the inner surface of the end wall of the conical part of the CRT. The kinetic energy of electrons bombarding the screen is converted into visible light.
CRT With electrostatic control.
Electric fields are commonly used in small screen CRTs. In electric field deflection systems, the field vector is oriented perpendicular to the initial beam trajectory. Deflection is accomplished by applying a potential difference to a pair of deflection plates (see figure below). Typically, deflection plates make the deflection in the horizontal direction proportional to time. This is achieved by applying a voltage to the deflection plates, which increases uniformly as the beam moves across the screen. Then this voltage quickly drops to its original level and begins to increase evenly again. The signal that requires research is supplied to plates that deflect in the vertical direction. If the duration of a single horizontal scan is equal to the period or corresponds to the repetition rate of the signal, one period of the wave process will be continuously reproduced on the screen.
1 - CRT screen, 2 - cathode, 3 - modulator, 4 - first anode, 5 - second anode, P - deflection plates.
Electromagnetic controlled CRT
In cases where large deflection is required, using an electric field to deflect the beam becomes ineffective.
Electromagnetic tubes have an electron gun, the same as electrostatic ones. The difference is that the voltage at the first anode does not change, and the anodes are only designed to speed up the electron flow. Magnetic fields are required to deflect the beam in large screen CRT televisions.
The electron beam is focused using a focusing coil. The focusing coil is wound in a row and fits directly onto the tube bulb. The focusing coil creates a magnetic field. If electrons move along the axis, then the angle between the velocity vector and the magnetic field lines will be equal to 0, therefore, the Lorentz force is zero. If an electron flies into a magnetic field at an angle, then due to the Lorentz force the electron trajectory will deviate towards the center of the coil. As a result, all electron trajectories will intersect at one point. By changing the current through the focusing coil, you can change the location of this point. Make sure that this point is in the plane of the screen. The beam is deflected using magnetic fields generated by two pairs of deflection coils. One pair is vertical deflection coils, and the other is coils in such a way that their magnetic field lines on the center line will be mutually perpendicular. The coils have a complex shape and are located at the neck of the tube.
By using magnetic fields to deflect the beam at large angles, the CRT is short and also allows for larger screen sizes.
Picture tubes.
CRTs are classified as combined CRTs, that is, they have electrostatic focusing and electromagnetic beam deflection to increase sensitivity. The main difference between picture tubes and CRTs is the following: the electron gun of picture tubes has an additional electrode, which is called an accelerating electrode. It is located between the modulator and the first anode, a positive voltage of several hundred volts is applied to it relative to the cathode, and it serves to further accelerate the electron flow.
Schematic structure of a kinescope for black-and-white television: 1- filament of the cathode heater; 2- cathode; 3- control electrode; 4- accelerating electrode; 5- first anode; 6- second anode; 7- conductive coating (aquadag); 8 and 9 - coils for vertical and horizontal beam deflection; 10- electron beam; 11- screen; 12 - terminal of the second anode.
The second difference is that the kinescope screen, unlike a CRT, is three-layered:
1st layer - outer layer - glass. The glass of the kinescope screen is subject to increased requirements for parallelism of the walls and the absence of foreign inclusions.
Layer 2 is a phosphor.
Layer 3 is a thin aluminum film. This film performs two functions:
Increases the brightness of the screen, acting like a mirror.
The main function is to protect the phosphor from heavy ions that fly out of the cathode along with electrons.
Color picture tubes.
The principle of operation is based on the fact that any color and shade can be obtained by mixing three colors - red, blue and green. Therefore, color picture tubes have three electron guns and one common deflection system. The screen of a color picture tube consists of separate sections, each of which contains three phosphor cells that glow in red, blue and green. Moreover, the sizes of these cells are so small and they are located so close to each other that their glow is perceived by the eye as a total. This is the general principle for constructing color picture tubes.
Mosaic (triads) of a color picture tube screen with a shadow mask: R-red, G-green, B-blue phosphor “dots”.
Electrical conductivity of semiconductors
Intrinsic conductivity of semiconductors.
An intrinsic semiconductor is an ideally chemically pure semiconductor with a homogeneous crystal lattice whose valence orbit contains four electrons. Silicon is most commonly used in semiconductor devices. Si and germanium Ge.
The electron shell of a silicon atom is shown below. Only four outer shell electrons, called valence electrons, can participate in the formation of chemical bonds and the process of conduction. Ten internal electrons do not participate in such processes.
The crystal structure of a semiconductor on a plane can be represented as follows.
If an electron receives an energy greater than the band gap, it breaks the covalent bond and becomes free. In its place, a vacancy is formed, which has a positive charge equal in magnitude to the charge of the electron and is called hole. In a chemically pure semiconductor, the electron concentration n equal to the hole concentration p.
The process of formation of a pair of charges, an electron and a hole, is called charge generation.
A free electron can take the place of a hole, restoring the covalent bond and emitting excess energy. This process is called charge recombination. During the process of recombination and charge generation, the hole seems to move in the opposite direction from the direction of electron motion, therefore the hole is considered to be a mobile positive charge carrier. Holes and free electrons resulting from the generation of charge carriers are called intrinsic charge carriers, and the conductivity of a semiconductor due to intrinsic charge carriers is called intrinsic conductivity of the conductor.
Impurity conductivity of conductors.
Since the conductivity of chemically pure semiconductors depends significantly on external conditions, impurity semiconductors are used in semiconductor devices.
If a pentavalent impurity is introduced into a semiconductor, then 4 valence electrons restore covalent bonds with the semiconductor atoms, and the fifth electron remains free. Due to this, the concentration of free electrons will exceed the concentration of holes. The impurity due to which n> p, called donor impurity. A semiconductor with n> p, is called a semiconductor with electronic type of conductivity, or semiconductor n-type.
In a semiconductor n-type electrons are called majority charge carriers and holes are called minority charge carriers.
When a trivalent impurity is introduced, three of its valence electrons restore a covalent bond with the atoms of the semiconductor, and the fourth covalent bond is not restored, i.e., a hole occurs. As a result, the hole concentration will be greater than the electron concentration.
An impurity at which p> n, called acceptor impurity.
A semiconductor with p> n, is called a semiconductor with hole type conductivity, or semiconductor p-type. In a semiconductor p-type holes are called majority charge carriers and electrons are called minority charge carriers.
Formation of electron-hole transition.
Due to the uneven concentration at the interface R And n semiconductor, a diffusion current arises, due to which electrons from n-regions go to p-region, and in their place remain uncompensated charges of positive ions of the donor impurity. Electrons arriving in the p-region recombine with holes, and uncompensated charges of negative ions of the acceptor impurity arise. Width R-n transition - tenths of a micron. At the interface, an internal electric field of the p-n junction arises, which will be inhibitory for the main charge carriers and will reject them from the interface.
For minority charge carriers, the field will be accelerating and will transfer them to the region where they will be the majority ones. The maximum electric field strength is at the interface.
The potential distribution across the width of a semiconductor is called a potential diagram. Potential difference at R-n transition is called contact difference potentials or potential barrier. In order for the main charge carrier to overcome R-n transition, its energy must be sufficient to overcome the potential barrier.
Direct and reverse connection p-ntransition.
Let us apply an external voltage plus to R-regions The external electric field is directed towards the internal field R-n transition, which leads to a decrease in the potential barrier. The majority charge carriers can easily overcome the potential barrier, and therefore through R-n transition, a relatively large current will flow, caused by the majority charge carriers.
Such inclusion R-n transition is called direct, and the current through R-n The transition caused by majority charge carriers is also called forward current. It is believed that when connected directly R-n the passage is open. If you connect external voltage to minus p-region, and a plus on n-region, then an external electric field arises, the lines of intensity of which coincide with the internal field R-n transition. As a result, this will lead to an increase in the potential barrier and width R-n transition. The main charge carriers will not be able to overcome R-n transition, and it is believed that R-n the crossing is closed. Both fields - internal and external - are accelerating for minority charge carriers, therefore minority charge carriers will pass through R-n transition, producing a very small current, which is called reverse current. Such inclusion R-n transition is also called inverse.
Properties p-ntransition.Current-voltage characteristic p-ntransition
To the main properties R-n transitions include:
- property of one-way conductivity;
Temperature properties R-n transition;
Frequency properties R-n transition;
Breakdown R-n transition.
One-way conductivity property R-n Let's look at the transition using the current-voltage characteristic.
Current-voltage characteristic (CVC) is a graphically expressed dependence of the amount of flow through R-n transition of current from the magnitude of the applied voltage I= f(U) – Fig. 29.
Since the magnitude of the reverse current is many times less than the forward current, the reverse current can be neglected and it can be assumed that R-n The junction conducts current in one direction only. Temperature property R-n transition shows how work changes R-n transition when temperature changes. On R-n The transition is largely affected by heating, and to a very small extent by cooling. As the temperature increases, the thermal generation of charge carriers increases, which leads to an increase in both forward and reverse current. Frequency properties R-n transitions show how it works R-n transition when high-frequency alternating voltage is applied to it. Frequency properties R-n transitions are determined by two types of transition capacitance.
The first type of capacitance is the capacitance caused by the immobile charges of donor and acceptor impurity ions. It is called charging or barrier capacitance. The second type of capacitance is diffusion capacitance, caused by the diffusion of mobile charge carriers through R-n transition when switched on directly.
If on R-n transition to supply alternating voltage, then capacitance R-n transition will decrease with increasing frequency, and at some higher frequencies the capacitance may become equal to the internal resistance R-n transition during direct switching. In this case, when turned back on, a sufficiently large reverse current will flow through this capacitance, and R-n the transition will lose the property of one-way conductivity.
Conclusion: the smaller the capacity R-n transition, the higher frequencies it can operate at.
The frequency properties are mainly influenced by the barrier capacitance, since diffusion capacitance occurs during direct connection, when the internal resistance R-n little transition.
Breakdown p-ntransition.
As the reverse voltage increases, the electric field energy becomes sufficient to generate charge carriers. This leads to a strong increase in reverse current. The phenomenon of a strong increase in reverse current at a certain reverse voltage is called electrical breakdown R-n transition.
Electrical breakdown is a reversible breakdown, i.e. when the reverse voltage decreases R-n the transition restores the property of one-way conductivity. If the reverse voltage is not reduced, the semiconductor will become very hot due to the thermal effect of the current and R-n the transition burns out. This phenomenon is called thermal breakdown R-n transition. Thermal breakdown is irreversible.
Semiconductor diodes
A semiconductor diode is a device consisting of a semiconductor crystal, usually containing one p-n junction and having two terminals. There are many different types of diodes - rectifier, pulse, tunnel, reverse, microwave diodes, as well as zener diodes, varicaps, photodiodes, LEDs, etc.
Diode marking consists of 4 designations:
K S -156 A
Phosphors are applied to the screen of a cathode ray tube in the form of tiny dots, and these dots are collected in groups of three; in each three, or triad, there is one red, one blue and one green dot. In the figure I showed you several such triads. In total, there are about 500 thousand triads on the tube screen. The picture you see on TV consists entirely of luminous dots. Where image details are lighter, more electrons hit the dots and they glow brighter. Accordingly, fewer electrons fall into the dark areas of the image. If there is a white detail in a color image, then everywhere within that detail all three points in each triad glow with the same brightness. On the contrary, if there is a red detail in a color image, then everywhere within this detail only the red dots of each triad glow, and the green and blue dots do not glow at all.
Do you understand what it means to create a color image on a TV screen? This is, firstly, to force electrons to fall into the right places, that is, to those phosphor points that should glow, and not to fall into other places, that is, to those points that should not glow. Second, the electrons must get to the right places at the right time. After all, the image on the screen is constantly changing, and where at some point, for example, there was a bright orange spot, a moment later, say, dark purple should appear. Finally, thirdly, the right number of electrons must fall into the right place and at the right time. More - where the glow should be brighter, and less - where the glow is darker.
Since there are almost one and a half million phosphor dots on the screen, the task at first glance seems extremely difficult. In fact - nothing complicated. First of all, a cathode ray tube has not one, but three separate heated cathodes. Exactly the same as in a regular vacuum tube. Each cathode emits electrons and creates an electron cloud around it. Near each cathode there is a grid and an anode. The number of electrons passing through the grid to the anode depends on the voltage across the grid. So far everything is happening as in a regular three-electrode lamp - triode.
What's the difference? The anode here is not solid, but with a hole in the very center. Therefore, most of the electrons moving from the cathode to the anode are not retained at the anode - they fly out through the hole in the form of a round beam. The structure, consisting of a cathode, grid and anode, is called an electron gun. The gun shoots a beam of electrons, and the number of electrons in the beam depends on the voltage on the grid.
Electron guns aimed so that the beam emitted from the first cannon always hits only the red dots of the triads, the beam from the second cannon only hits the green dots, and the beam from the third cannon only hits the blue dots. In this way, one of the three problems of creating a color image is solved. By applying the required voltages to the grids of each of the three guns, the required intensities of red, green and blue light are set, and therefore provide the desired coloring for each detail of the image.
