Nonuniform thermal conductance in avalanche microwave oscillators

The local dc current density
j(x)
in avalanche microwave oscillators is a function of the local breakdown voltage
V_{b}(x)
, which in turn depends on local temperature
T(x)
and field
E(x)
. Recent progress in junction fabrication has led to very uniform field distributions such that local temperature variations of the order of a few degrees are no longer negligible, calculations of
T(x)
for various diode configurations show that the spherical heat flow near the diode edge causes considerable variations of
T(x)
. The resulting variations in
j(x)
are so significant even in the case of a reasonably ideal mounting (flip-chip mesa on a copper substrate) that they will affect the oscillator performance with respect to efficiency and oscillator noise. The dynamic thermal properties are discussed in terms of a simplified oscillator model consisting of two parallel subdiodes with different but uniform current densities. Nonuniform breakdown combined with an internal current redistribution resulting from nonuniform internal heating causes AM and FM modulation and is also responsible for the often observed delayed onset of microwave oscillations under pulsed conditions.