The maximum operating region of a SiGe HBT has been experimentally investigated by a direct microwave waveform measurement. Dynamic RF load lines are used as a probe to detect the limit of the RF operation. For the first time, it is found that SiGe HBTs operate beyond the conventional BVceo, while GaAs HBTs cannot survive at that voltage. The conventional BVceo limits the average Vc of the maximum load lines, but has no influence on the peak voltage. Another BVceo measured with a voltage generator is proposed to represent the irreversible avalanche breakdown instead of the conventional one. A pulsed breakdown measurement is also performed to reveal the time constant of the phenomena.

A microwave waveform measurement system below 18 GHz was developed and verified with a conventional RF measurement. The current and voltage RF waveforms of AlGaAs HBTs at the fundamental frequency of 1 GHz were directly measured with the system. A new direct method of sweeping and measuring dynamic RF load lines is proposed to measure the operating limits of the device. The maximum operating region was experimentally investigated with this method. The limits with a small input power are found to come from thermal runaway and the avalanche breakdown of the device. With a large input power, the HBT was found to operate beyond the DC limit of thermal runaway. The base ballasting resistance was also found to enhance large signal operating limits beyond those expected from the conventional DC theory.