Space-charge effects and current self-quenching in a metal/CdS/LaS cold cathode

Abstract

We analyze the importance of space-charge effects in the cathode to anode gap region of a recently proposed metal/CdS(cadmium sulfide)/LaS(lanthanum sulfide) cold cathode. Our approach is based on an ensemble Monte Carlo description of electron transport assuming ballistic injection across the CdS and LaS layers. Under this approximation, the energy spectrum of the injected beam entering the air gap can be determined exactly as a function of the applied bias across the CdS layer. The effects of shot noise in the injected current are taken into account. For some of the biasing conditions considered here, space-charge effects are quite drastic and lead to dynamical effects which are responsible for the onset of current self-quenching similar to the Child–Langmuir regime of operation of thermionic cathodes. The limiting anode current density is found to be much larger than the Child–Langmuir limit. In the presence of strong space-charge effects, large oscillations in the minimum of the electrostatic potential in front of the cathode lead to oscillations in the measured anode current within the tens of GHz frequency range.

Onset of current self-quenching in a metal/CdS/LaS cold cathode in the presence of inelastic scattering in the CdS layer

“…However, the limiting value of the measured average anode current density is much larger than the Child-Langmuir limit. 1 In this current self-quenching regime, we have shown in the past that the chopping action of the beam is sometimes accompanied by high-frequency ͑several tens of GHz͒ fluctuation in the minimum of the electrostatic potential in the air gap ͑see Fig. 1͒.…”

“…These oscillations lead to large oscillations in the anode current. 1 In this article, we investigate in further detail the chopping action of the electrostatic potential in front of the cathode and its effect on the energy spread of the carriers collected at the anode. As it turns out, the chopping action of the beam is quite substantial, leading to an energy spread at the anode with a full width at half maximum of only a few k B T. This is important for practical applications such as flat panel displays or traveling wave tubes which require electron beams that are as monoenergetic as possible.…”

“…In the case of Fowler-Nordheim injection, assumed here, an expression for h(E) was derived in Ref. 1.…”

Interplay of current crowding and current self-quenching effects in planar cold cathodes

“…This phenomenon is called dc current crowding. [6][7][8][9][10] The part of the beam reflected as the result of space-charge will increase the amount of charge collected by the semimetallic thin film, hence influence the amount of current crowding described earlier. For a cold cathode with a rectangular emission window dc current crowding leads to emitted current which decreases symmetrically and monotonically from the edges of the emission window towards the center where it reaches a minimum, as illustrated in Fig.…”

Transition from sub-Poissonian to super-Poissonian shot noise in planar cold cathodes

“…22 Preliminary ensemble Monte Carlo ͑EMC͒ simulations of the space-charge effects in the vacuum gap between anode and cathode of the structure shown in Fig. 23 These oscillations lead to large oscillations in the anode current density 23 which can strongly affect the correlations among carriers, hence the shot noise power spectrum of the anode current fluctuations. 23 and 24.…”