Emission thermionic

The traditional current-voltage expression representing thermionic emission is given by 

Equation 8.5 is a representation of the carrier flux from the semiconductor to the metal, with the barrier being voltage (V) dependent, ( )b — 12 If from the metal to the semiconductor, with the barrier fixed at ( )b, there exists a parasitic resistance in the circuit such as semiconductor resistance (Rg), the thermionic-emission current expression is modified as 

In the reverse direction, the barrier lowering becomes more important. In such a case (using Js instead of Jteo for saturation current). 

The contribution by the generation-recombination current to the overall current is negligibly small for ZnO even for very small effective carrier lifetimes, because the intrinsic carrier concentration is nearly zero at room temperature (RT). Even at 

For practical comparison of the suitability of different kinds of refractories as cathode materials in generating high electronic emission currents, a figure of merit was defined by the 

Heated cathodes in television picture tubes, x-ray tubes, and the vacuum tubes of yesterday are used to produce large quantities of electrons which are then accelerated to the anode by a high voltage. The process by which these electrons are produced is called thermionic emission. The number of electrons produced by heating the cathode material can be obtained by integrating over the high-energy portion of the F-D distribution 

Accounting for the momenta of the electrons traveling in the direction of the surface, the thermionic current is given by the Richardson equation. 

The equation has the Arrhenius form so plotting log / against 1/kT offers an alternative method for obtaining the work function. Also, it is clear that efficient cathode materials must have a small work function. 

The Fermi sphere for atoms with one valence electron per atom can easily fit in a bcc reciprocal lattice (fee direct lattice). If a divalent element is added, the additional electrons will increase the diameter of the Fermi sphere. When the Fermi sphere begins to touch the Brillouin zone, the additional electrons must either go to the unfilled states in the comers of the first Brillouin zone, which are higher energy states, go across the Brillion zone, which costs energy because of the energy gap, or imdergo a solid-phase transition to a new reciprocal lattice with a Brillouin that can accommodate more electrons. This model explains the solid-phase transformations of many mixed-valency binary alloys in which the phases progress from fee to bcc to hep as more divalent component is added. 

As the electrons move through the lattice imder the influence of an applied field, they interact with the lattice and transfer momentum to the lattice (or vice versa). This momentum transfer is accounted for by assigning the electron an effective mass that is defined as m = h - d E/dk ). Since the effective mass is inversely proportional to the curvature of the energy band, bands that curve upward have positive effective mass and those that curve downward have negative effective mass. Electrons occupying sharply curved bands have small effective mass and tend to be very mobile, while those occupying flat bands are heavy and sluggish. 

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Reimann A L 1934 Thermionic Emission (London Chapman and Flail)... 

Shelton FI 1957 Thermionic emission from a planar tantalum crystal Phys. Rev. 107 1553-7... 

One important sem source that is not based on thermionic emission is the field emission (fe) source. Fe-sem systems typically give images of much higher resolution than conventional sems due to the much narrower energy distribution (on the order of 0.25 eV) of the primary electron beam. A fe source is a pointed W tip from which electrons tunnel under the influence of a large electric field. This different mechanism of electron generation also results in a brightness comparable to a conventional thermionic source with much less current. 

Tungsten with the addition of as much as 5% thoria is used for thermionic emission cathode wires and as filaments for vibration-resistant incandescent lamps. Tungsten—rhenium alloys are employed as heating elements and thermocouples. Tantalum and niobium form continuous soHd solutions with tungsten. Iron and nickel are used as ahoy agents for specialized appHcations. 

The results of several studies were interpreted by the Poole-Erenkel mechanism of field-assisted release of electrons from traps in the bulk of the oxide. In other studies, the Schottky mechanism of electron flow controlled by a thermionic emission over a field-lowered barrier at the counter electrode oxide interface was used to explain the conduction process. Some results suggested a space charge-limited conduction mechanism operates. The general lack of agreement between the results of various studies has been summari2ed (57). 

Thermionic Emission - Because of. the nonzero temperature of the cathode, free electrons are continuously bouncing inside. Some of these have sufficient energy to overcome the work function of the material and can be found in the vicinity of the surface. The cathode may be heated to increase this emission. Also to enhance this effect, cathodes are usually made of, or coated with, a low work-function material such as thorium. 

For example, the //V characteristics of devices based on the aluminum chelate complex Alq3, where Ag-Mg or ln-Mg are used as the cathode, can be described by thermionic emission of electrons over the barrier height at the electron injection contact/Alq3 [78]. 

The boundary conditions are given by specifying the panicle currents at the boundaries. Holes can be injected into the polymer by thermionic emission and tunneling [32]. Holes in the polymer at the contact interface can also fall bach into the metal, a process usually called interlace recombination. Interface recombination is the time-reversed process of thermionic emission. At thermodynamic equilibrium the rates for these two time-reversed processes are the same by detailed balance. Thus, there are three current components to the hole current at a contact thermionic emission, a backflowing interface recombination current that is the time-reversed process of thermionic emission, and tunneling. Specifically, lake the contact at Jt=0 as the hole injecting contact and consider the hole current density at this contact. 

For the same the single layer devices based on Alq3, Peyghambarian et al. [83] found that the 1/V characteristics can also be described by an SCL current flow in the low cu ire lit regime. However, in the low current regime the 1/V characteristics can be qualitatively modeled by the Fowler-Nordheim equation (even if, quantitatively, the real device current differs from the calculated by seven orders of magnitude) [83] and thermionic emission ]78]. 

For typical polymer LED device parameters, currenl is space charge limited if the energy barrier to injection is less than about 0.3-0.4 eV and contact limited if it is laiger than that. Injection currents have a component due to thermionic emission and a component due to tunneling. Both thermionic emission and tunneling... 

A polymer layer al a contact can enhance current How by serving as a transport layer. The transport layer could have an increased carrier mobility or a reduced Schottky barrier. For example, consider an electron-only device made from the two-polymer-layer structure in the top panel of Figure 11-13 but using an electron contact on the left with a 0.5 eV injection barrier and a hole contact on the right with a 1.2 cV injection barrier. For this case the electron current is contact limited and thermionic emission is the dominant injection mechanism for a bias less than about 20 V. The electron density near the electron injecting contact is therefore given by... 

The analytic theory outlined above provides valuable insight into the factors that determine the efficiency of OI.EDs. However, there is no completely analytical solution that includes diffusive transport of carriers, field-dependent mobilities, and specific injection mechanisms. Therefore, numerical simulations have been undertaken in order to provide quantitative solutions to the general case of the bipolar current problem for typical parameters of OLED materials [144—1481. Emphasis was given to the influence of charge injection and transport on OLED performance. 1. Campbell et at. [I47 found that, for Richardson-Dushman thermionic emission from a barrier height lower than 0.4 eV, the contact is able to supply... 

The forward current at a semiconductor-metal junction is mainly determined by a majority carrier transfer i.e. electrons for n-type, as illustrated in Fig. 1 d. In this majority carrier device the socalled thermionic emission model is applied according to which all electrons reaching the surface are transferred to the metal. In this case we have ... 

The free-electron gas was first applied to a metal by A. Sommerfeld (1928) and this application is also known as the Sommerfeld model. Although the model does not give results that are in quantitative agreement with experiments, it does predict the qualitative behavior of the electronic contribution to the heat capacity, electrical and thermal conductivity, and thermionic emission. The reason for the success of this model is that the quantum effects due to the antisymmetric character of the electronic wave function are very large and dominate the effects of the Coulombic interactions. 

In an electromagnetic tube, electrons are produced by a hot filament. Electrons are emitted from the surface of the filament in a process known as thermionic emission. 

There is further emphasis on adsorption isotherms, the nature of the adsorption process, with measurements of heats of adsorption providing evidence for different adsorption processes - physical adsorption and activated adsorption -and surface mobility. We see the emergence of physics-based experimental methods for the study of adsorption, with Becker at Bell Telephone Laboratories applying thermionic emission methods and work function changes for alkali metal adsorption on tungsten. 

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