  We study a near-field radiative thermal transistor analogous to an electronic
one made of a VO 2 base placed between two silica semi-infinite plates playing
the roles of the transistor collector and emitter. The fact that VO 2 exhibits
an insulator to metal transition is exploited to modulate and/or amplify heat
fluxes between the emitter and the collector, by applying a thermal current on
the VO 2 base. We study the transistor behavior in 4 typical regimes where the
emitter-base and base-collector separation distances can be larger or smaller
than the thermal wavelength, and in which the VO 2 layer can be opaque or
transparent. Thermal currents variations with the base temperatures are
calculated and analyzed. An optimum configuration for base thickness and
separation distance maximizing the thermal transistor modulation factor is
found.
