Jul 16, 2019 in this case, thermionicphotothermionic emission occurs only for. Electronic transition imaging of carbon based materials. Of course, inside the metal the electric field is zero so an electron. The cathode is heated so electron evaporation is possible, and a potential di. A useful description of the process of th ermionic emission of electrons is provided by the model of an essentially free electron gas in a potential well the metal boundaries. Field emission and thermionic field tf emission are considered as the phenomena responsible for the excess currents observed both in the forward and reverse directions of schottky barriers formed on highly doped semiconductors.
An electric force can pull these electrons away from the wire the larger the electric force, the larger the resulting current of electrons. This occurs because the thermal energy given to the carrier overcomes the forces restraining it. Although the physical mechanism has been known for over a century, it has yet to be consistently realized in a manner practical for largescale deployment. Statistical analysis and spread of the data suggest that the conduction variability in same cnt devices results from differences in local surface potential of the. The two laws were derived on the basis of sommerfeld free electron model 1, 2,where the fermidirac statistics is applied to an atmosphere of threedimensional 3d free electrons in metals. Application of the thermionic field emission model in the. Field emission and thermionic fi eld emission tunneling through a schottky barrier on an ntype semiconductor. Sep 19, 2009 we suggest a general approach to considering the thermionic, field, and thermionic field emissions of electrons from metals. Field assisted thermionic emission and field emission have quite a number of applications in radio and television broadcasting, radars for space and aerospace applications, microwave communications, xray tubes and portable xray generators, microwave amplifiers, field emission microscopy, parallel electron beam microscopy, nanolithography. Field emission article about field emission by the free. Tf guns are proposed as an attractive alternate to cold cathode guns as high average and high peak current density excitation sources. Field emission fe is a process in which a metal surface will emit current when that surface is exposed to a sufficiently high electric field typically on the order of 1 vnm and under high vacuum to avoid an electric arc. However, many groups reports ignored the thermionic emission process in the low electric field.
With rising temperature and decreasing e, temperature and field, or tf, emission gives way to field enhanced thermionic emission the schottky effect. Unlike thermionic devices, which suffer poor emission rates at low temperature, the thermalfield nanowire converter can produce. Although the theories of thermionic and field emission of electrons from metals are very well understood, the two types of emission have usually been studied separately by first specifying the range of. Pdf theories of field and thermionic electron emissions. The cathode is heated so electron evaporation is possible, and. The thermionic emission is that the thermal energy given to charge carriers overcomes the binding potential, also known as work function of metals such that electrons escape out from the surface of solid. Thermionic and field electron emissions governed by richardsondushman and fowlernordheim equations, respectively, normally coexist under typical experimental conditions.
Strictly speaking, the term thermal emittance should only be applied to thermionic emission, but the concept of thermal emittance or the intrinsic emittance of the cathode can be applied to the three forms of electron emission. In this regime, the combined effects of fieldenhanced thermionic and field emission can be modeled by the murphygood equation for thermofield tf emission. Mar 15, 2020 thermionic emission forms the basic principle on which many of the devices used in the field of electronics and communication operates. Term paper field assisted thermionic emission, field. The emission is a thermionic emission if the energy responsible for it is in the form of heat energy. Calculation of thermionic emission te current predicted current levels much lower than experimentally seen, so that other mechanisms such as thermionicfield emission tfe6 or impurityassisted tunneling were considered. Thermionic emission is the liberation of electrons from an electrode by virtue of its temperature releasing of energy supplied by heat. Icse solutions for class 10 physics thermionic emission. Depending on how you deliver the energy to the metal surface, the emission is of different types. Pdf taking into account the effect of the lowenergy band structure of carbon nanotubes cn, we develop the theories of cn field and thermionic. In this paper the emission is treated from a unified point of view in order to establish the ranges of.
Other effects at thermionic emission are mentioned. Calculation of thermionic emission te current predicted current levels much lower than experimentally seen, so that other mechanisms such as thermionic field emission tfe6 or impurityassisted tunneling were considered. Field, thermionic and secondary electron emission spectroscopy it seems that youre in usa. Modinos, field, thermionic and secondary electron spectroscopy, plenum, ny 1984. Thermionic and field electron emission devices from diamond. After emission, a charge will initially be left behind in the. The induced deep level defect band through high mg doping led to a reduction of the depletion layer width in the pga n near the interface and an increase in the probability of thermionic field emission. The phenomenon was first observed 1883 by thomas a. Field emission is a quantummechanical phenomenon in which electrons tunnel through a potential barrier at the surface of a solid as a result of the application of a large electric field. Pdf thermionic field emission of electrons from metals and. Thermionic emission is the heatinduced flow of charge carriers from a surface or over a potentialenergy barrier.
Although the theories of thermionic and field emission of electrons from metals are very well understood, the two types of emission have usually been studied separately by first specifying the range of temperature and field and then constructing the appropriate expression for the current. Emitter evaluation in a diode having concentric cylinders. Breakdown of richardsons law in electron emission from. Field and thermionicfield emission in schottky barriers. Thermionic field emission of electrons from metals and. In the absence of a strong electric field, an electron must acquire a certain minimum energy, called the work function, to escape through the surface of a given material, which acts as a barrier to electron passage. The charge carriers can be electrons or ions, and in older literature are sometimes referred to as thermions. After emission, a charge will initially be left behind in the emitting. With recent research, the author intends to outline the framework of the field emission of zno nanostructures.
Starting in 1901, owen richardson studied this phenomenon and in 1929 he received the nobel prize in physics for his work. Voltagecurrent characteristics are derived for field and thermionicfield emission in the forward and reverse regime. Glenn edwards, committee member harald ade, committee member robert nemanich, committee chair keith weninger, committee member ron scattergood, committee member. Here we present the study of the thermionic emission of the hybrid structure phosphorus, p doped diamond nano crystals grown on conical carbon nanotubes ccnts. Superior photothermionic electron emission from illuminated. Childs law that describes relation between anode current at given cathodeanode potential is also derived. Quantum tunneling and field electron emission theories. A pure field emission is called cold field emission cfe, but at higher temperatures, a combination of thermionic and field emission called schottky emission can occur. It occurs because the wave function of an electron does not vanish at the classical turning point, but decays exponentially into the barrier where. Field emission, also called cold emission, discharge of electrons from the surface of a material subjected to a strong electric field.
In field emission, electrons tunnel through a potential barrier, rather than escaping over it as in thermionic or photoemission. State two characteristics of the substance used as thermionic emitter. Field, thermionic and secondary electron emission spectroscopy. Materials with low work function values thermionic electron emission, which is a key phenomenon for waste heat recovery applications. As can be seen in eqn lo, the thermionic field emission process is taken into account through a parameter 6 and, when the tunneling mechanism is neglected 6 0, this equation becomes the thermionic emission boundary condition similar to that used in ref. If the cathode incorporates both thermionic and field emissions at a poorer vacuum, the thermal energy assists the electron emission. Thermionic emission of electrons from metal at high temperature is discussed. State three factors on which rate of emission of electrons from a metal surface depend. Implementing the general thermalfield emission equation to the high electric field nanoprotrusion model.
Paper analysis of thermionic and thermofield emission in. Thermionic field emission transport in carbon nanotube. Thermionic emission, discharge of electrons from heated materials, widely used as a source of electrons in conventional electron tubes e. The emission current density is a function of the cathode material work function and the local electric field magnitude over the surface and can. We derive richardsons law that describes emission of electrons from a metal at given temperature. Field emission spectroscopy of singly adsorbed atoms. Voltagecurrent characteristics are derived for field and thermionic field emission in the forward and reverse regime. This is a general method with potentially wide applicability, requiring only the effective mass m and relative dielectric constant.
Apr 05, 2016 field assisted thermionic emission and field emission have quite a number of applications in radio and television broadcasting, radars for space and aerospace applications, microwave communications, xray tubes and portable xray generators, microwave amplifiers, field emission microscopy, parallel electron beam microscopy, nanolithography. Thermionic field emission of electrons from metals and explosive electron emission from micropoints article pdf available in journal of experimental and theoretical physics 1092. Analysis by thermionic field emission is difficult due to its multivariate nature. Thermionic and field emission are experiencing a transition toward 2d materials, especially carbonbased materials. This occurs because the thermal energy given to the carrier overcomes the work function of the material. The photothreshold of melanin and thermionic field emission from diamond. Tunneling through a schottky barrier has been analyzed theoretically by padovani and stratton, 60 and by crowell and rideout. The observed exponentially increasing relationship of i sat and. A hot wire will be surrounded by evaporated electrons. It also resulted in an increase in current flow under forward bias condition, which was not analyzed using the thermionic emission model. The net thermionicphotothermionic flux thus may be expressed as 30. As can be seen in eqn lo, the thermionicfield emission process is taken into account through a parameter 6 and, when the tunneling mechanism is neglected 6 0, this equation becomes the thermionic emission boundary condition similar to that used in ref.
The thermionic emission is a phenomena related to field emission. The emission of electrons from the surface of a conductor into a vacuum under the influence of a high electric field is an elementary effect comparable with thermionic emission, photoelectric emission, or secondary emission. Mar 18, 2005 the induced deep level defect band through high mg doping led to a reduction of the depletion layer width in the pga n near the interface and an increase in the probability of thermionic field emission. Analysis of thermionic and thermofield emission in.
Electronic emission types, thermionic, photoelectric. The energy spectrum of the electrons leaving the metal in field emission is narrow figure 2, page 56. Levine, physics laboratory manual, prenticehall, inc. Unlike field emission, the electrode is heated by a heat source which further increases the probability of electron emission. Thermionic emission, for example, is known to be a key process in arc formation, and models for thermionic emission in arcs have been recently developed 6. The effect is purely quantummechanical, with no classical analog. In recent work, we developed a mathematical stability approach that greatly simplified the evaluation of the multivariate thermionic field emission parameters.
Thermionic energy conversion tec is the direct conversion of heat into electricity by the mechanism of thermionic emission, the spontaneous ejection of hot electrons from a surface. Results are presented for a program to validate the technology for thermionic field emission tf guns. For this purpose, based on the standard model of free electrons in a metal, we suggest a numerical method for determining the transmission coefficient through the potential barrier at the metalvacuum interface suitable for an arbitrary barrier. Frontiers thermionic energy conversion in the twenty. Combined field and thermionic emission process in zno. Thermionicfield emission in aigaasgaas 1wodimensional. Field emission and thermionicfield tf emission are considered as the phenomena responsible for the excess currents observed both in the forward and reverse directions of schottky barriers formed on highly doped semiconductors. The laws governing the behaviors of electron emission include richardsons law for thermionic emission and fowlernordheim law for field emission. Research in field emission from carbon structures was first studied as a. Pdf thermionic field emission of electrons from metals. Pdf we suggest a general approach to considering the thermionic, field, and thermionic field emissions of electrons from metals.
Field emission simion 2019 supplemental documentation. In contrast to thermionic emission or photoemission, in which electrons acquire sufficient energy via heating or energy exchange with photons to overcome the potential barrier, electrons need. Electron thermionic emission from graphene and a thermionic. Example applications of thermionic emission include vacuum tubes, diode valves, cathode ray tube, electron tubes, electron microscopes, xray tubes, thermionic converters, and electrodynamic tethers. Thermionic emission properties of novel carbon nanostructures. Cathode behaves like a thermionic emitter the cathode is modified by adding zro to lower the value of the work function a schottky gun is a fieldassisted thermionic emitter resolution suffers at low voltages due to larger energy spread life is much shorter than the cold field emitter. We suggest a general approach to considering the thermionic, field, and thermionic field emissions of electrons from metals. In order to resolve the mechanisms, a numerical calculation was performed to calculate the tfe current. Introduction diamond and carbon nanostructures exhibit unique properties which have led to numerous studies of electron emission, and more recently thermionic emission. However, field emission can take place from solid or liquid surfaces, into vacuum, air, a fluid, or any nonconducting or. The most common context is field emission from a solid surface into vacuum. A recently published field emission cathode parameter extraction method x. Electronic emission types, thermionic, photoelectric, secondary, field if we provide energy to an electrode then the electrons in the outer most shell of the atom of that metal gain enough energy to erect form the surface of that metal or electrode.
813 527 618 913 1304 1387 1180 1029 1252 37 664 9 844 546 69 633 482 1420 699 611 1097 898 473 1113 347 704 332 687 848 1133 285 69 393