High-frequency characteristics of inverted-mode heterojunction bipolar transistors

It is shown that in emitter-down heterojunction bipolar transistors (HBT´s), parasitics can be reduced sufficiently that intrinsic transit-time delays become the dominant limitations to high-frequency performance. In this situation it is found that the dependence of the unilateral gain on frequency can be significantly different from the simple 6-dB/octave decrease usually assumed. It is found that the device exhibits negative output conductance over certain bands of frequencies, and that when this occurs a series of resonances are observed in the gain versus frequency characteristics. Explanations of this behavior are given in terms of the phase delay of the common-base current gain. The generality and relevance of these observations to other types of transistors, and the utilization of the negative output conductance to enhance high-frequency operation are also discussed.