On the determination of the thermal impedance of microwave bipolar transistors

This paper has two main axis: firstly, we address the experimental characterization of the frequency-dependent thermal impedance of microwave bipolar transistors from continuous wave (CW)-only measurements (both DC and AC). From the experimental perspective, we will review some of the already available methods and propose a new method based on a recent observation. It will be shown that under proper measurement control, a reasonable precision of the computed value can be achieved. The method is applied to characterize the global (external) behavior of a multi-finger Heterojunction Bipolar Transistor (HBT), whose physical structure is known. A distributed thermal circuit, entirely derived from 3D thermal simulations, is incorporated into a complete distributed electrothermal model of the device, whose global behavior is validated by measurements. Then from a distributed electrothermal simulation perspective, we will address the power and temperature distribution between fingers as a function of the power dissipated by the device, and will show that the global behavior in measurements is close to the worst case in terms of highest temperature among individual fingers.