Thermal effects of the operation of high average power Gunn devices

A lumped thermal resistance model is described for GaAs Gunn and LSA diodes. Thermal resistances are defined for the active layer, contact layer, bond interface, package and heat sink. This permits the calculation of tha maximum device temperature T
M
= T
0
+ P Σ
i
R
i
and the critical temperature difference across the active layer ΔT
A
= PR
A
. A transient analog incorporates thermal time constants T
i
= R
i
C
i
to consider high duty cycle pulsed operation. The carrier mobility is modeled as varying as
1/T
in the range from 300 to 500°K. This permits thermal measurements based upon changes in resistance. The thermal calculations were also in agreement with blackbody infrared data. The mobility decline with temperature is shown to act as a link between the thermal profile and device performance. A thermally induced avalanching point, as well as device efficiency, are influenced by the peak-to-valley current ratio. This ratio is reduced from its theoretical value of over 2:1 by the active layer resistivity ratio
\bar{partial}/\partial_{\max}
. Thus thermal gradients in the active layer act to create mobility gradients which alter the observed peak-to-valley ratio. The steps necessary for maintaining high average power efficient operation require low thermal resistances to minimize the active layer temperature gradients and a high current drop back ratio. Total experimental thermal resistances of 6.5°C/watt for an X-band CW diode and 17°C/watt for a thick LSA diode have been observed and fit the model presented.