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Determination of self-heating in bipolar and heterojunction bipolar transistors by measurement and simulation Reid, Adam Robert


Self-heating in bipolar transistors, the effect which causes a rise in the device junction temperature due to the electrical power dissipation, is of interest for several reasons, including device reliability, model parameter extraction and electrothermal modeling. Self-heating is most commonly quantified by a thermal resistance, which relates the device steady-state temperature and power dissipation. In this work we present an improved setup for measuring thermal resistance, based on an isothermal characterization of the device by means of a pulsed-bias experiment. A 30 times increase in bias-pulse speed, compared to previous setups, allows for the first time, accurate measurements to be made on devices with submicron emitter dimensions. Simultaneous to increased performance, we have achieved a significant reduction in the complexity of the setup through the elimination of specialized sample preparation, enabling measurements with conventional microwave on-wafer probes. To facilitate comparison with experimental measurements, an analysis tool based on the finite element method has been developed to solve detailed 3-D thermal models of bipolar transistors, including emitter metalization and trench-isolation effects. Thermal resistance measurements have been made on a collection of 9 single emitter silicon bipolar and 6 trench-isolated SiGe heterojunction bipolar transistors and compared to results from theoretical models. In addition, a collection of 15 GaAs heterojunction bipolar transistors have been studied theoretically and compared to measurements from a previous study. Results for silicon bipolar devices show that significant reductions in thermal resistance are achieved by using high aspect ratio emitter geometries. Comparison of results for silicon bipolar devices with trench-isolated SiGe devices of a similar emitter geometry show a threefold increase in thermal resistance due to the presence of trench isolation. This increase in thermal resistance is found to be attributable in roughly equal parts to the two isolation components, namely the deep and shallow trenches. The thermal resistance is found to scale weakly with emitter area for the small devices studied here, an effect which is most apparent in the GaAs devices. The origin of this effect is found to be the 3-D nature of the heat flow in the smallest devices. Emitter metalization is found to play a role in reducing the thermal resistance for high thermal resistance devices. Additionally, the emitter metalization is found to significantly increase the uniformity of the temperature across the emitter.

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