Thermal management of power HBT in pulsed operating mode

We focus this paper on the improvement of the thermal management of power transistor based on the InGaP/GaAs HBT technology and specially for pulsed mode application applied to very HBT high power transistor, from 10 W to 3 W, respectively for L to Ku bands radar applications, a specific study has been done to suggest new opportunities if we take into account the transient or dynamic behavior of the transistor in pulse operating mode. From very short pulse (1 μs) to very long pulse (≈ 1 ms), a analysis has been performed with as a consequence a strong improvement of thermal impedance (Zth) through specific designs of the thermal shunt (material-shape) present at the front side. We chose to develop the concept of "thermal sponge" on power HBT transistor acting as a very efficient thermal capacitance to suppress thermal variation inside the pulse and improving as a consequence the CW thermal resistance. Two approaches have been compared: the first one with the support of very thick metal growth directly on the thin gold thermal shunt, the second one with a very small part of diamond substrate directly on top the same thermal shunt. As a conclusion, for long pulse application greater than 200 μs up to 1 ms, the diamond approach gives superior result with 30% of improvement on the temperature rising.