Emission current from metals electron

Table 4. Electron Emission Current from Metals at Elevated Temperatures...

To further substantiate the proposed model, they have carried out some investigations connected with modification of semiconductor electron subsystem [174, 175]. Temperature is one of the important factors. Having no effect on the electron emission from the metal under the action of RGMAs, temperature strongly affects the current-transfer processes at the metal - semiconductor contacts. The impact of temperature on the interaction of RGMAs with Au/ZnO structures can be evaluated as follows. 

Fig. 4.8. Field-emission spectrum of Mo(lOO). The quantity displayed, Jf, is the ratio between the observed field-emission current and the prediction based on a free-electron model, Eq. (4.20). As shown, the field-emission spectrum of Mo(lOO) near the Fermi level is substantially different from a free-electron-metal behavior. (After Weng, 1977.)...

At present, very high ultimate values of tunnel emission flow density can be obtained ( 10 ° A cm-2). The tunnel emission of electrons from metals is widely used in modern technology in developing various devices where high currents or intensive electron beams are required. 

Bilayered polysilane LEDs have been obtained by inserting a SiOx thin layer between the cathode and a Wurtz synthesized PMPS emitter film.94 The SiOx layers were prepared by 02 plasma treatment of the PMPS film surfaces. It was found that the external quantum efficiency was significantly enhanced by this treatment. This enhancement has been attributed to an increased electron injection via tunneling, resulting in a reduced hole current caused by the blocking effect of the thin SiOx layer. The weak visible emission observed from single-layer polysilane LEDs is almost completely eliminated. It was concluded that the visible emission is caused by the erosion of the PMPS surfaces due to the collision with hot metal particles during the vacuum deposition of the cathode, and this erosion process is avoided by the SiOx layer. 

The multilayered structure and electroluminescent mechanism of OLEDs is illustrated in Figure 4.45. Depending on whether small organic molecules or long repeating-unit polymers are used (Figure 4.46), the diodes are referred to as OLEDs or PLEDs, respectively. Under positive current, electrons and holes are injected into the emissive layer from opposite directions - from the cathode and anode, respectively. The metal... 

Thermionic emission. The number of electrons which escape from the metal surface increases rapidly with temperature (thermionic emission). In general, the higher the temperature and the lower the work function, the higher is the electron emissivity. The current density can be calculated by the Richardson-Dushman equation (in the absence of an external electrical field), according to i — AT exp(—rp/kT), where A is the Richardson constant (A cm K ), T is the temperature (K), and

work function (eV). For pure tungsten A — 60.2 (A cm K ) [1.91]. The thermionic current (A cm ) can then be calculated as i — 60.2r exp(—52230/T) [1.37]. 

Fig. 30. Electron emission from metals with differently shaped surface barriers. j—current density in amp/em1, F—field in volts/A. Calculations for cf> = tf>2 = 4.5 ev, tj> j = 5.5 ev width of conduction band = 0.4 ev d = 2.5A. Slopes of F-N curves identical to better than 1%.

Arcs with hot cathode spots. If the cathode is made from lower-melting-point metals like copper, iron, silver, or mercury, the high temperatme required for emission caimot be sustained permanently. Electric current flows in this case through hot spots that appear, move fast, and disappear on the cathode surface. Current density in the spots is extremely high (10" -10 A/cm ), which leads to intensive but local and short heating and evaporation of the cathode material while the rest of the cathode actually stays cold. The mechanism of electron emission from the hot spots is thermionic field emission. Cathode spots appear not only on the low-melting-point cathodes but also on refractory metals at low currents and low pressures. 

The cathodes spots are the localized current centers, which appear on the cathode when significant current should be provided but the cathode carmot be heated enough as a whole. The most typical cause of cathode spots is the application of metals with relatively low melting points. The cathode spots can also be caused by low arc currents, which are only able to provide the necessary electron emission when concentrated to a small area. The cathode spots also appear at low gas pressures (<1 Torr), when metal vapor from the cathode provides atoms to generate positive ions bringing their energy to the cathode to sustain the electron emission. To provide the required evaporation, cmrent is concentrated in spots at pressures <1 Torr and currents 1-10 A, such spots occur even on refractory metals. 

However, experimental ]V curves often deviate from the ideal /scl- In these cases, the measured current /inj is injection limited caused by a nonohmic contact or poor surface morphology. When the MO interface is nonohmic, carrier injection can be described by the Richardson-Schottky model of thermionic emission the carriers are injected into organic solid only when they acquire sufficient thermal energy to overcome the Schottky barrier ((()), which is related to the organic ionization potential (/p), the electron affinity (AJ, the metal work function (O, ), and the vacuum level shift (A) [34,35]. Thus, the carrier injection efficiency (rj) can be calculated by the following equation ... 

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