Emission of electrons from metal. Electron emission Physics emission
The electrons of the conductor move freely within its boundaries, and when sufficient energy is absorbed, they can also go outside, breaking the wall of the potential well near the surface of the body (Fig. 10.6). This phenomenon is called electron emission (in a single atom, a similar phenomenon is called ionization).
At T = 0 the energy required for emission is determined by the difference between the levels W= 0 and the Fermi level E R(Fig. 10.6) and is called the work function. The energy source can be photons (see paragraph 9.3), causing photoemission (photoelectric effect).
Rice. 10.6
The cause of thermionic emission is the heating of the metal. When the electron distribution function is distorted (see Fig. 10.5, b) this “tail” can go beyond the cutoff of the potential well, i.e. some electrons have enough energy to leave the metal. This is usually used to supply electrons to a vacuum.
The simplest device that uses thermal emission is an electrovacuum diode (Fig. 10.7, a). Its cathode K is heated from the EMF source ? and and emits electrons, which create a current iodine by the action of an electric field between the anode and cathode. An electrovacuum diode differs from a photodiode mainly in the source of energy that caused the emission of electrons, so their current-voltage characteristics are similar. The more tension U a between the anode and the cathode, the greater part of the electrons from their cloud at the cathode is drawn by the electric field per unit time. Therefore, as the voltage increases U a current I is growing. At some voltages, the zero pulls already all electrons leaving the cathode, and a further increase in voltage does not lead to an increase in current - saturation occurs.
Rice. 10.7
QUESTION. Why is the saturation current at T, more than with G, (Fig. 10.7, b)? ANSWER. At T 2 > D, more electrons leave the cathode per unit time.
With the reverse polarity of the applied voltage (“minus” is connected to the anode, and “plus” to the cathode), the electrons are not accelerated, but slowed down, therefore, the electrovacuum diode is able to pass current only in one direction, i.e. he has one-way conduction. This allows it to be used for rectifier current(Fig. 10.7, in): during the action of a positive half-wave of voltage, the diode passes current, but during a negative half-wave, it does not.
In 1907, the American Lee de Forest added a third grid electrode to the diode, which made it possible to amplify electrical signals. Such a triode was then supplemented with other electrodes, which made it possible to create various kinds amplifiers, generators and converters. This led to the rapid development of electrical engineering, radio engineering and electronics. Then the baton was picked up by semiconductor devices, which replaced vacuum tubes, but in CRT, X-ray tubes, electron microscopes and some vacuum tubes, thermal emission is still relevant.
Another source of electron emission can be the bombardment of the material surface by various particles. Secondary electron-electron emission arises as a result of impacts of external electrons, which transfer part of their energy to the electrons of the substance. Such emission is used, for example, in a photomultiplier tube (PMT) (Fig. 10.8, a). His photocathode 1 emits electrons when exposed to light. They are accelerated towards the electrode (dynode) 2, from which they knock out secondary electrons, they are accelerated towards the dynode 3 etc. As a result, the primary photocurrent is multiplied to such an extent that the PMT is able to register even individual photons.
Rice. 10.8
The same principle was applied in the image intensifier tube (see paragraph 9.3) of the new generation. It contains hundreds of thousands of photomultipliers (according to the number of pixels that form images of objects), each of which is a metallized microchannel ~ 10 μm wide. Along this channel, electrons move in the same zigzag manner, like light in an optical fiber and like electrons in a PMT, multiplying at each collision with the channel walls due to secondary emission. Since the electron trajectory differs negligibly from a rectilinear one (only within the channel width), a package of such channels located between the photocathode and the screen (Fig. 10.8, b) eliminates the need to focus photoelectrons (compare with Fig. 9.4). Each channel carries out not only the reproduction of electrons, but also their transfer to the required point, which ensures the clarity of the image.
In secondary ion-electron emission, the primary particles - energy carriers are ions. AT gas-discharge devices they ensure the reproduction of electrons from the cathode, which then multiply by ionization of gas molecules (see paragraph 5.9).
There is also a very exotic type of emission, the origin of which is explained by the Heisenberg uncertainty principle. If the metal surface has an electric field that accelerates electrons, then a straight line is superimposed on the potential ledge 1 ex(2 in Fig. 10.6), and the ledge turns into a barrier 3. If the total energy of the electron is equal to W, those. on A W less than the height of the barrier, then, according to classical ideas, “take” it, i.e. go outside, he can't. However, according to quantum concepts, an electron is also wave, which not only reflected from an optically denser medium, but also refracted. At the same time, the presence of a function inside the barrier means the finite probability of finding an electron there. In the "classical" view, this is impossible, since complete electron energy W, and its component potential energy - is equal in this area W+ AVK, i.e. the part is greater than the whole! At the same time, there is some uncertainty AVK energy that depends on time At stay of an electron inside the barrier: AWAt>h. Decreasing At: uncertainty A.W. can reach the required value, and the solution of the Schrödinger equation gives finite values | p | 2 s outside barrier, i.e. there is a chance that the electron will get out without jumping over the barrier! It is higher the lower AW n At.
These conclusions are confirmed in practice by the presence of a tunnel, or sub-barrier, effect. It even finds application, providing the emission of electrons from metal in fields of ~10 6 -10 7 V/cm. Since such emission occurs without heating, irradiation, or particle bombardment, it is called field emission. Usually it occurs from all kinds of points, protrusions, etc., where the field strength increases sharply. It can also lead to electrical breakdown of the vacuum gap.
In 1986, the Nobel Prize in Physics was awarded for the invention of the scanning electron microscope based on the tunneling effect. Its laureates are the German physicists E. Ruska and G. Binnig and the Swiss physicist G. Rohrer. In this device, a thin needle scans along the surface at a small distance from it. The tunneling current that arises in this case carries information about the energy states of the electrons. Thus, it is possible to obtain an image of the surface with atomic precision, which is especially important in microelectronics.
The tunnel effect is responsible for recombination during ion-electron emission (see above), for electrification by friction, in which electrons from atoms of one material tunnel to atoms of another. It also determines the socialization of electrons in a covalent bond, leading to a splitting of energy levels (see Fig. 10.5, a).
Electronic emission the emission of electrons from the surface of a solid or liquid. E. e. arises in cases when, under the influence of external influences, a part of the electrons of the body acquires energy sufficient to overcome the potential barrier (See Potential barrier) at the boundary of the body, or if, under the influence of an electric field, the surface potential barrier becomes transparent for a part of the electrons that have the highest energies inside the body . E. e. can occur when bodies are heated (Thermionic Emission) ,
bombarded by electrons (Secondary Electron Emission), ions (Ion Electron Emission), or photons (Photo Electron Emission) .
Under certain conditions (for example, when passing current through a semiconductor with high electron mobility or when a strong electric field pulse is applied to it), conduction electrons can “heat up” much more than the crystal lattice, and some of them can leave the body (emission of hot electrons) . For observation E. e. it is necessary to create an externally accelerating electric field near the surface of the body (emitter), which "sucks" the electrons from the surface of the emitter. If this field is large enough (≥ 10 2 in/cm),
then it reduces the height of the potential barrier at the boundary of the body and, accordingly, the work function (Schottky effect) ,
as a result of which E. e. increases. In strong electric fields (Electronic emission10 7 in/cm) the surface potential barrier becomes very thin and there is a tunnel "leakage" of electrons through it (Tunneling emission) ,
sometimes also called field emission. As a result of the simultaneous action of 2 or more factors, thermoauto- or photoautoelectronic emission may occur. In very strong pulsed electric fields (Electronic emission 5․10 7 in/cm) tunneling emission leads to the rapid destruction (explosion) of micropoints on the surface of the emitter and to the formation of a dense plasma near the surface (see Plasma). The interaction of this plasma with the surface of the emitter causes a sharp increase in the current E. e. up to 10 6 a with a duration of current pulses of several tens ns(explosive emission). With each current pulse, microquantities are transferred (Electronic emission 10 -11 G) of the emitter substance to the anode. Lit.: Dobretsov L. N., Gomoyunova M. V., Emission electronics, Moscow, 1966; Bugaev S. P., Vorontsov-Velyaminov P. N., Iskoldsky A. M., Mesyats S. A., Proskurovskiy D. I., Fursey G. N., The phenomenon of explosive electron emission, in the collection: Discoveries in the USSR 1976 year, M., 1977. T. M. Lifshitz.
Great Soviet Encyclopedia. - M.: Soviet Encyclopedia. 1969-1978 .
See what "Electronic emission" is in other dictionaries:
Electron emission is the emission of electrons from the surface of a solid or liquid. Types of Emission Thermionic Emission The electron emission resulting from heating is called thermionic emission (TE). The phenomenon of TE ... ... Wikipedia
Emission of electrons by the surface of a condensed medium. E. e. occurs in cases where part of the body's electrons acquires as a result of external. impact energy sufficient to overcome the potential. barrier on its border, or if external ... ... Physical Encyclopedia
Emission of electrons by the surface of a condensed medium. E. e. arises in cases where a part of the body's electrons acquires as a result of external. impacts, energy sufficient to overcome the potential barrier at its border, or if external ... ... Physical Encyclopedia
ELECTRONIC emission, the emission of electrons by a solid or liquid under the influence of an electric field (field emission), heating (thermionic emission), electromagnetic radiation (photoelectronic emission), electron flow ... ... Modern Encyclopedia
Big Encyclopedic Dictionary
Electronic emission- ELECTRONIC EMISSION, the emission of electrons by a solid or liquid under the influence of an electric field (field emission), heating (thermionic emission), electromagnetic radiation (photoelectronic emission), electron flow ... ... Illustrated Encyclopedic Dictionary
electronic emission- Emission of electrons from the surface of the material into the surrounding space. [GOST 13820 77] Topics electrovacuum devices ... Technical Translator's Handbook
electronic emission- the emission of electrons by the surface of a solid or liquid. Electronic emission occurs when, under the influence of external influences, a part of the body's electrons acquires energy sufficient to overcome ... ... Encyclopedic Dictionary of Metallurgy
Emission of electrons by a solid or liquid under the influence of an electric field (field emission), heating (thermionic emission), electromagnetic radiation (photoelectronic emission), electron flow (secondary electron ... ... encyclopedic Dictionary
Emission of electrons in vom. Depending on the method of excitation, a trace is distinguished. main types of E. e .: thermionic emission, photoelectron emission (see External photoelectric effect), secondary electron emission, field emission ... Big encyclopedic polytechnic dictionary
Books
- Explosive electron emission, G. A. Mesyats, ... Category: Electricity and Magnetism
- Secondary electron emission , I. M. Bronshtein , B. S. Fraiman , The book is devoted to one of the issues of modern physical electronics - secondary electron emission. Measurement methods are considered: secondary emission coefficient (SE), inelastic and elastic ... Category: Solid state physics. Crystallography Series: Engineer's Physical and Mathematical Library Publisher:
The electron emission resulting from heating is called thermionic emission (TE). The TE phenomenon is widely used in vacuum and gas-filled devices.
- Electrostatic or Autoelectronic emission
Electrostatic (field emission) is called the emission of electrons due to the presence of a strong electric field near the surface of the body. In this case, additional energy is not imparted to the electrons of the solid body, but due to a change in the shape of the potential barrier, they acquire the ability to escape into vacuum.
Photoelectronic emission (PE) or external photoelectric effect - the emission of electrons from a substance under the action of radiation incident on its surface. FE is explained on the basis of the quantum theory of a solid body and the zone theory of a solid body.
Emission of electrons by the surface of a solid when it is bombarded by electrons.
Emission of electrons by a metal when it is bombarded with ions.
Emission of electrons as a result of local explosions of microscopic areas of the emitter.
Emission of electrons by ultracold surfaces cooled to cryogenic temperatures. little studied phenomenon.
see also
Write a review on the article "Electronic emission"
An excerpt characterizing the electronic emission
"Requesting reinforcements?" Napoleon spoke with an angry gesture. The adjutant bowed his head affirmatively and began to report; but the emperor turned away from him, took two steps, stopped, turned back and called Berthier. “We need to give reserves,” he said, spreading his arms slightly. - Whom to send there, what do you think? - he turned to Berthier, to this oison que j "ai fait aigle [the caterpillar that I made an eagle], as he later called him.- Sovereign, send Claparede's division? - said Berthier, who remembered by heart all the divisions, regiments and battalions.
Napoleon nodded his head in the affirmative.
The adjutant galloped to Claparede's division. And after a few minutes the young guards, standing behind the mound, moved from their place. Napoleon silently looked in that direction.
“No,” he suddenly turned to Berthier, “I cannot send Claparède. Send Friant's division, he said.
Although there was no advantage in sending Friant's division instead of Claparède, and there was even an obvious inconvenience and delay in stopping Claparede now and sending Friant, the order was carried out with precision. Napoleon did not see that in relation to his troops he played the role of a doctor who interferes with his medicines - a role that he so correctly understood and condemned.
Friant's division, like the others, disappeared into the smoke of the battlefield. Adjutants continued to jump up from different directions, and all, as if by agreement, said the same thing. Everyone asked for reinforcements, everyone said that the Russians were holding their positions and were producing un feu d "enfer [hell fire], from which the French army was melting.
An important role in ensuring the conductivity of the arc gap is played by electrons supplied by the cathode under the influence of various reasons. This process of release of electrons from the surface of the cathode electrode or the process of release of electrons from the bond with the surface is called electron emission. For the process of emission, it is necessary to expend energy.
The energy that is sufficient to release electrons from the surface of the cathode is called the work function ( U out )
It is measured in electron volts and is usually 2-3 times less than the work of ionization.
There are 4 types of electron emission:
1. Thermionic emission
2. Field emission
3. Photoelectronic emission
4. Emission under the impact of heavy particles.
Thermionic emission proceeds under the influence of strong heating of the surface of the electrode - cathode. Under the action of heating, the electrons located on the cathode surface acquire such a state when their kinetic energy becomes equal to or greater than the forces of their attraction to the atoms of the electrode surface, they lose contact with the surface and fly out into the arc gap. Strong heating of the end of the electrode (cathode) occurs because at the moment of its contact with the part, this contact occurs only at certain points on the surface due to the presence of irregularities. This position, in the presence of current, leads to a strong heating of the contact point, as a result of which an arc is initiated. The surface temperature greatly affects the simulation of electrons. The emission is usually estimated by the current density. The relationship between thermionic emission and cathode temperature was established by Richardson and Deshman.
where j0 is the current density, A/cm2;
φ is the electron work function, e-V;
BUT- a constant, the theoretical value of which is A \u003d 120 a / cm 2 deg 2 (experimental value for metals A \u003e 62.2).
In autoelectronic emission, the energy necessary for the release of electrons is imparted by an external electric field, which, as it were, “sucks” the electrons beyond the limits of the influence of the electrostatic field of the metal. In this case, the current density can be calculated from the formula
, (1.9)
where E is the electric field strength, V/cm;
With an increase in temperature, the value of autoelectronic emission decreases, but at low temperatures its influence can be decisive, especially at a high electric field strength (10 6 - 10 7 V / cm), which, according to Brown M.Ya. and G.I. Pogodin-Alekseev can be obtained in the near-electrode regions.
When radiation energy is absorbed, electrons of such high energy can appear that some of them leave the surface. The photoemission current density is determined by the formula
where α - reflection coefficient, the value of which for welding arcs is unknown.
The wavelengths that cause photoemission as well as for ionization are determined by the formula
Unlike ionization, the emission of electrons from the surface of alkali and alkaline earth metals is caused by visible light.
The surface of the cathode can be subjected to impacts of heavy particles (positive ions). Positive ions in case of impact on the cathode surface can:
First of all, give away the kinetic energy they possess.
Secondly, can be neutralized on the cathode surface; while they give the electrode ionization energy.
Thus, the cathode acquires additional energy, which is used for heating, melting and evaporation, and some part is spent again on the escape of electrons from the surface. As a result of a sufficiently intense emission of electrons from the cathode and the corresponding ionization of the arc gap, a stable discharge is established - an electric arc with a certain amount of current flowing in the circuit at a certain voltage.
Depending on the degree of development of a particular type of emission, three types of welding arcs are distinguished:
Hot cathode arcs;
Cold cathode arcs;
Vacuum is understood as a gas or air in a state of the highest rarefaction (pressure of the order of ). Vacuum is a non-conductive medium, since it contains an insignificant amount of electrically neutral particles of matter.
To obtain an electric current in a vacuum, a source of charged particles - electrons is needed, and the movement of electrons in vacuum occurs practically without collisions with gas particles.
The source of electrons is usually a metal electrode - the cathode. In this case, the phenomenon of the release of electrons from the surface of the cathode into the environment, called electron emission, is used.
Free electrons in a metal in the absence of an external electric field randomly move between the ions of the crystal lattice.
Rice. 13-6. Double electrical layer on the metal surface.
At room temperature, no electrons escape from the metal due to the insufficient value of their kinetic energy. Part of the electrons with the highest kinetic energy, during their movement, goes beyond the surface of the metal, forming an electron layer, which, together with the layer of positive ions of the crystal lattice located under it in the metal, forms a double electric layer (Fig. 13-6). The electric field of this double layer counteracts the electrons tending to leave the conductor, i.e., it is inhibitory for them.
For an electron to go beyond the metal surface, it is necessary for the electron to impart energy equal to the work that it must do to overcome the retarding effect of the double layer field. This work is called the work function. The ratio of the output energy to the electron charge is called the output potential, i.e. .
The work (potential) of the output depends on the chemical nature of the metal.
The values of the output potential for some metals are given in Table. 13-1.
Table 13-1
Depending on the way in which the additional energy necessary to exit the metal is imparted to the electrons, the types of emission are distinguished: thermionic, electrostatic, photoelectronic, secondary, and under the impact of heavy particles.
Thermionic emission is the phenomenon of the release of electrons from the cathode, due solely to the heating of the cathode. When a metal is heated, the speeds of electrons and their kinetic energy increase and the number of electrons leaving the metal increases. All electrons emerging from the cathode per unit time, if they are removed from the cathode by an external field, form an electric emission current. As the cathode temperature rises, the emission current increases slowly at first, and then faster and faster. On fig. 13-7 curves of the emission current density, i.e., the emission current per unit cathode surface, expressed in A/cm2, are given as a function of temperature T for various cathodes.
Rice. 13-7. Curves of emission current density depending on temperature for various cathodes: a - oxide; b - tungsten, covered with thorium; c - uncoated tungsten.
The dependence of the emission current density on temperature and work function is expressed by the Richardson-Dashman equation:
where A is the emission constant; for metals it is equal to; T is the absolute temperature of the cathode, K; - base of natural logarithms; - work function, eV; is the Boltzmann constant.
Thus, the emission current density increases proportionally and so that a cathode made of a material with a low work function and a high operating temperature is needed to obtain a large emission current.
If the electrons that have flown out of the cathode (the emitted electrons) are not removed from it by an external accelerating field, then they accumulate around the cathode, forming a volume negative charge (electron cloud), which creates a decelerating electric field near the cathode, which prevents the further escape of electrons from the cathode.
Electrostatic electron emission is the phenomenon of the release of electrons from the cathode surface, due solely to the presence of a strong electric field near the cathode surface.
The force acting on an electron in an electric field is proportional to the charge of the electron and the field strength F - ee. At a sufficiently high strength of the accelerating field, the forces acting on an electron located near the cathode surface become large enough to overcome the potential barrier and eject electrons from the cold cathode.
Electrostatic emission finds use in mercury valves and some other appliances.
Photoelectron emission is the phenomenon of the release of electrons, due solely to the action of radiation absorbed by the cathode, and not associated with its heating. In this case, the cathode electrons receive additional energy from light particles - photons.
Radiant energy is emitted and absorbed by certain portions - quanta. If the quantum energy, determined by the product of the Planck constant of the radiation frequency v, i.e., is greater than the work function for the material of the given cathode, then the electron can leave the cathode, i.e., photoelectron emission will take place.
Photoelectronic emission is used in solar cells.
Secondary electron emission is the phenomenon of the exit of secondary electrons, due solely to the impact of primary electrons on the surface of a body (conductor, semiconductor). Flying electrons, called primary, meeting a conductor on their way, hit it, penetrate into its surface layer and give part of their energy to the electrons of the conductor. If the additional energy received by the electrons upon impact is greater than the work function, then these electrons can go beyond the conductor.
Secondary electron emission is used, for example, in photomultipliers to amplify the current.
Secondary emission can be observed in vacuum tubes in which the anode is exposed to electrons flying from the cathode. In this case, secondary electrons can create a flow that is opposite to the “working” one, which worsens the operation of the lamp.
Electron emission under the impact of heavy particles is the phenomenon of the release of electrons, due solely to the impact of ions or excited atoms (molecules) on the surface of the body - the electrode. This type of emission is similar to the secondary electron emission considered above.