For the first time, we present a comprehensive study of the dependence of the RF breakdown voltage of a Silicon Germanium (SiGe) Heterojunction Bipolar Transistor (HBT) on the DC breakdown metrics, device size, geometry, bias conditions and temperature. We explore the RF breakdown voltage of a SiGe HBT based on the 4 ^th generation GLOBALFOUNDRIES 350nm PA technology and show that the collector-base breakdown voltage with emitter open (BV _CBO ) is more important in determining the RF breakdown of the HBT than the collector-emitter breakdown voltage with base open (BV _CEO ). We also show that the RF breakdown voltage and the Safe Operating Area (SOA) of the device are strongly dependent on the device size, and that the thermal effects significantly impact the RF breakdown voltage of the device.