| Popis: |
A quantum-mechanical wave impedance approach is used to calculate the shot noise power spectrum of the emission current from planar metallic cathodes starting with the Landauer-Buttiker formalism. This formalism takes into account the effects of the image force potential in front of the cathode. For metals with low work function, the Fano factor which characterizes the reduction of the shot noise power below the Schottky result 2 q I (where q is the magnitude of the charge of the electron and I is the time average of the anode current) is calculated as a function of the externally applied electric field. Simple analytical expressions for the Fano factor are derived for the cathode operated in the thermionic (Richardson) and tunneling (Fowler-Nordheim) regimes. The value of the Fano factor is found to be reduced substantially below the value calculated when the effects of the image charge potential are neglected. As a practical example, we consider cathodes consisting of rare-earth sulfide materials which, in their rocksalt structure, could present a more stable alternative to alkali metals for reaching low or negative electron affinity at the surface of III-V semiconductor compounds. |
