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Self-heating and isothermal characterization of heterojunction bipolar transistors Kleckner, Todd Christopher

Abstract

Self-heating, the process by which power dissipation causes a rise in the operating temperature of the device, causes significant changes in the operation of bipolar transistors and can compromise the accuracy of device models if the operating environment of the device is not identical to the one in which it was characterized. As conventional methods utilized to construct device models are based in part upon measurements of device performance in which the static power dissipation results in an elevated device temperature, the extracted model contains information pertaining to the thermal environment in which it was characterized. This hidden temperature dependence is often ignored, and results in erroneous parameter extraction. Extending a technique developed by workers at Hewlett-Packard, a pulsed bias measurement system has been developed which addresses the need for temperature-independent device characterization. This system can be used to measure the isothermal collector characteristics at constant base-emitter voltage, and the isothermal Gummel plot of collector current. The resulting data can be used to extend the region of model validity by de-embedding the thermal effects from the inherent isothermal electrical response of the device. The system can also be used to determine the values of the thermal resistance and capacitance, macroscopic thermal parameters that can be used with device model thermal subcircuits. Using this measurement technique, the first experimental studies of thermal parameter scaling in deep-trench Si/SiGe and non-trench-isolated AlGaAs/GaAs heterojunction bipolar transistors has been performed. Scaling coefficients were determined for the dependence of thermal resistance and capacitance on emitter area. Comparison of measured thermal resistances to model predictions show substantial deviations for small devices and for large devices. Results for small devices suggest the interconnect metallization plays a significant role, and those for large devices show that electrothermal feedback is important and must be accounted for. For each of the two technologies studied, measured thermal capacitances vary linearly with the square root of the emitter area and depend only weakly on the emitter geometry. The result is a simple thermal capacitance scaling law which allows estimates for unknown structures in a given technology from a single measurement.

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