  Under certain conditions, space-charge limited emission in vacuum microdiodes
manifests as clearly defined bunches of charge with a regular size and
interval. The frequency corresponding to this interval is in the Terahertz
range. In this computational study it is demonstrated that, for a range of
parameters, conducive to generating THz frequency oscillations, the frequency
is dependant only on the cold cathode electric field and on the emitter area.
For a planar micro-diode of given dimension, the modulation frequency can be
easily tuned simply by varying the applied potential. Simulations of the
microdiode are done for 84 different combinations of emitter area, applied
voltage and gap spacing, using a molecular dynamics based code with exact
Coulomb interaction between all electrons in the vacuum gap, which is of the
order 100. It is found, for a fixed emitter area, that the frequency of the
pulse train is solely dependent on the vacuum electric field in the diode,
described by a simple power law. It is also found that, for a fixed value of
the electric field, the frequency increases with diminishing size of the
emitting spot on the cathode. Some observations are made on the spectral
quality, and how it is affected by the gap spacing in the diode and the initial
velocity of the electrons.
