Studies of Safe Operating Area of InGaP/GaAs Heterojunction Bipolar Transistors

The safe operating area (SOA) of InGaP/GaAs heterojunction bipolar transistors has been studied in detail both experimentally and theoretically. Devices without ballasting resistors were measured in dc to reveal the intrinsic SOA characteristics, which are influenced by both self-heating and the breakdown effect. Two distinct regions in the SOA boundary were observed indicating two different dominating failure mechanisms at different bias conditions. The theoretical analysis, which took into consideration all the relevant effects, was able to explain all the features in the measured results. Secondary unstable points beyond the SOA boundary were found theoretically. These secondary failure points and the gap between the two branches of the SOA boundary explain why the device failure points when measured in constant Ib mode were different from those measured in constant Vb mode.