Thermionic emission from nanocrystalline cathodes

The thermionic emission current from a metal cathode possessing a nanocrystalline (fractal) structure is calculated within the framework of a model assuming that thermoelectrons form a nondegenerate gas with an arbitrary dimension parameter, which characterizes the density and fractal properties of the gas. A method is proposed for determining the fractal dimension from the temperature dependence of the thermionic emission current.     

Thermionic emission from electron traps

Electron emission from traps filled by previous irradiation (exoelectron emission) is described in terms of thermionic emission from a surface layer of definite depth (isolated box model). From the kinetic equations a “thermionic range” is derived which is a common measure for the increasing temperature promoting the emission and the rejecting surface potential. Relative to the emission depth the emission efficiency is formulated which separates regions of strong and weak emission. The energy levels of the traps are readily computed in the case of strong emission. A check on F-centres in LiF confirms the theoretical model.     

Thermionic emission model for contact resistance in organic field-effect transistor

Injection mechanism of top-contact pentacene field-effect transistor (OFET) was investigated in respect to the internal field. The contact resistance was evaluated by the transmission line method for various applied external voltages as well as various pentacene film thicknesses. The behaviour of contact resistance was described in terms of the thermionic emission model (Schottky injection) and internal electric field generated by excess charges accumulated on pentacene–gate insulator interface. It was shown that pentacene film thickness changes the internal electric field affecting the carrier injection barrier. It was concluded that the space-charge field effect made a significant contribution for smaller pentacene film thicknesses and therefore in accordance to the thermionic model was able to decrease contact resistance representing the potential drop.     

Thermionic electrodes technology

In the thermionic two component system consisting of a refractory metal surface and Cs the only parameter that allows for an enhancing of efficiency is the nature of the metal surface. Surfaces obtained by the follwing techniques are studied: 1. radio-frequency inductive plasma sputtering, 2. van der Schueren evaporation, a sublimation technique, 3. vapor plating. The surfaces obtained by the mentioned techniques are studied as to their adhesion and grain size. A modified Shelton retarding current method allows for the determination of the distribution of work functions. The improvements in diode-efficiency to be obtained by improving the metal surfaces are probably limited. It has been shown that significant improvements can be hoped for by introducing a third component in the system. Thus techniques for the handling of barium are studied.     

Thermionic valve effect and cathode crater rhythm in a low-voltage cold-cathode vacuum arc

A vacuum arc with a cold cathode burns rhythmically. The period of the rhythm is determined by a thermionic valve effect which occurs between the hot crater and the cold cathode as a result of the brief retention of the heat in the crater by the oppositely propagating electron flux. Pis’ma Zh. Tekh. Fiz. 24, 67–72 (July 26, 1998)     

Study of galvanomagnetic effects in Bi-Sr-Ca-Cu-O films

Galvanomagnetic effect has been studied at 77 K on Bi-Sr-Ca-Cu-O films as a function of d.c. bias current. These films were prepared by spray pyrolysis and screen-printing techniques, withT _c (R=0) 80 K and 100 K respectively. Magnetic field dependence ofI _c of sprayed-film showed very small hysteresis whereas screen-printed film showed greater hysteresis. Application of small magnetic field on these films destroyed the zero-resistance state and a finite resistance was developed. The slope of the resistance vs magnetic field curveΔR/ΔH depended on the current flowing through the sample. To increaseΔR/ΔH, meander-shape pattern was prepared on the sprayed film. In generalΔR/ΔH increased by a factor of 10³ after sample patterning. The possibility of using these films as a sensitive magnetic field sensor is discussed.     

Thermionic emission from the 2DEG assisted by image-charge-induced barrier lowering in AlInN/AlN/GaN heterostructures

The effect of image charges on current transport mechanisms investigated at the nanoscale in Al(1-x)In(x)N/GaN heterostructures was studied. Current-voltage (I-V) measurements were performed locally using a conductive AFM-tip as a nanoprobe and the conduction mechanism was modeled to explain the observed behavior. This model suggests that current transport is controlled by thermionic emission (TE) of the two-dimensional electron gas (2DEG) across the potential barrier at the heterointerface, where the image charges generated by the 2DEG induce a barrier lowering at the Al(1-x)In(x)N/GaN interface, enhancing electron transport. This barrier lowering depends on the 2DEG characteristics, such as 2DEG density n(2D), first subband energy E? and the average distance x? of the 2DEG from the interface. By fitting the experimental I-V curves with the present model the 2DEG density was evaluated. The obtained results were in very good agreement with the Hall measurements.     

Determination of galvanomagnetic and thermomagnetic effects in semiconductors

The factors with an appreciable influence on the accuracy of determination of the basic galvanomagnetic and thermomagnetic effects in semiconductors are examined. Formulas are given for the Hall emf, magneto resistance, and transverse Nernst-Ettingshausen effect with allowance for the nonisothermicity of the specimens and heat transfer from their surface to the surrounding medium.     

Structural and galvanomagnetic properties of thin antimony films

The structural and electrical properties of thin antimony films evaporated onto glass substrates were studied. The influence of the deposition conditions (in particular the substrate temperature and the evaporation rate) on the condensation mechanism and the shape, size and orientation of the crystallites was determined. The structure of the films was studied by transmission electron microscopy. The degree of texturization of the films was also determined. Measurements of the electrical resistivity were carried out over the temperature range 4.2–300 K; the Hall effect and the magnetoresistance were measured at 77 and 300 K. From these data the electrical conductivity σ, the temperature coefficient of resistivity β, the Hall coefficient R_H, the mobilities μ_e and μ_h of electrons and holes and their respective concentrations (n = p) were determined for films of various structures and thicknesses. The influence of the structure on the electrical properties of thin antimony films was observed.The results obtained were analysed on the basis of the Fuchs-Sondheimer and Mayadas-Schatzkes theories. The mean free path of the carriers as well as the scattering parameters p at the film surface and R at the grain boundaries were determined.     

CdxHg1−xTe thin film galvanomagnetic devices

The present paper concerns evaporated thin film Hall generators of Cd_xHg_1?xTe. We first give some information about the technology of Cd_xHg_1?xTe thin films and then we consider the construction and characteristics of these galvanomagnetic devices.     

Thermionic current transport in a dielectric channel

The results are given of an experimental investigation of the transport of thermionic current in a dielectric channel. The results of an analysis of the current-voltage characteristics are used to demonstrate that the Child-Langmuir law does not extend to the case when the expansion of an electron flow due to the space charge effect is restricted by the channel walls. Furthermore, it is possible to obtain a value of the current density at the wall that is several orders of magnitude higher than its limiting value associated with the restriction with respect to the space charge.     

Automatic measurement of galvanomagnetic parameters of narrow-gap semiconductor devices

Translated from Izmeritel'naya Tekhnika, No. 8, pp. 42–43, August, 1991.     

Thermoelectric and galvanomagnetic properties of systems with mutually penetrating components

The effective coefficients of thermal conductivity, electrical conductivity, thermal emf, the effective Hall mobility, and the effective Hall coefficient are determined. The analytical dependences obtained are compared with experimental results for a Bi-Cd alloy.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 33, No. 3, pp. 439–448, September, 1977.     

Emissivity of Lanthanum Hexaboride Thermionic Electron Gun Cathode

The emissivity of the thermionic electron gun cathode material lanthanum hexaboride (LaB \(_6\) ) at an operating temperature of 1622 K has been measured for wavelengths from 550 nm to 2400 nm. The emissivity is calculated from scanning monochromator spectral measurements calibrated with a tungsten lamp and the published emissivity of 0.82 at 1600 K for LaB \(_6\) at 650 nm. The results show a higher emissivity at shorter wavelengths (up to 0.86 at 729 nm) and a lower emissivity in the near-IR to mid-IR (down to 0.41 at 2146 nm). These measurements were motivated by the need to make fast, accurate optical measurements of the temperature of the LaB \(_6\) cathode in our thermionic microwave gun, and to select the wavelength most readily absorbed by the cathode for manipulation of its surface temperature as a means of counteracting back-bombardment heating.     

Thermodynamic Calculation of the Characteristics of Metal Thermionic Emission

The thermodynamic equilibria of the “Fermi–Dirac gas–Maxwell gas–Boltzmann gas” and “metal–saturated vapor” systems are considered (with allowance for charged particles). It is shown that the work function of a metal at 0 K is equal to the Mulliken electronegativity of its atoms. For the latter case, it is proposed to take into account the relationship between the work-function thermal coefficient and the thermodynamic functions of positive and negative metal ions.     

A two-band model for the galvanomagnetic properties of thin semimetal films

A two-band model is proposed to describe the galvanomagnetic properties in thin samples of transition metals or of sernimetals such as bismuth which are characterized by two overlapping partially filled bands. The formalism proposed by Sondheimer and Wilson for transition metals and the Cottey model of carrier scattering at the external surfaces are used to solve size effect problems in a thin film subjected to a transverse magnetic field. The general case where the number of carriers per unit volume differs for the two bands is studied. The influence of the temperature is accounted for introducing a parameter x which corresponds to the fraction of the current carried by the conduction electrons. The results of these calculations are presented within the limit of low magnetic fields. Compared with the classical F-S theory, the variation of the Hall effect with film thickness is found to be determined by three essential parameters: the specularity parameterp, the ratio,y, of the number of carriers in the two bands, and the temperature-dependent parameterx. A large scatter of theoretical results is reported; in particular for typical sets of values forp, x andy the size effects can be exactly opposite to those predicted by one-carrier models. Tentative attempts to fit previously published data on the basis of the present model are undertaken. Qualitative agreement is observed, the theory is found to be able to explain the change in the sign of the Hall coefficient with increasing thickness sometimes observed in bismuth films. Difficulties in controlling morphology and geometrical surface properties of films with various thicknesses are outlined. As a consequence, one may find here a possible explanation for quantitative differences between theory and experiments.     

Studies on photoelectrical,optical and galvanomagnetic properties of CdSe films

CdSe films were deposited onto glass substrates by the hot wall technique at a system pressure of 10^–6torr. The optimum deposition conditions for obtaining device grade films were determined. Photoelectrical and optical properties of the films deposited at the optimum conditions were studied. Information on the refractive index and absorption coefficient were derived from the above measurements and the data were analysed in the light of the existing theories. Direct transitions corresponding to band gaps of 1.7 and 1.92 eV were obtained. Measurement of mobility of CdSe films as a function of temperature indicated ionized impurity scattering to be predominant in our temperature range of measurements. The scattering parameter and the effective massm _e ^* of carriers was determined from thermoelectric power measurements.