Physical origin of the negative output resistance of heterojunction bipolar transistors

An analytical expression for the small-signal output impedance of a heterojunction bipolar transistor (HBT) is obtained based on device physics. It is shown that, when external parasitics are sufficiently reduced so that intrinsic time delays become the dominant limitations to high-frequency performance, the output resistance can be negative over certain bands of frequencies due to transit time delays. Under such conditions amplifiers with high gains can be designed at frequencies where the power gain of the device due to conventional transistor action is very low, and power gain vs. frequency characteristics of the device can be drastically different from the traditional 6-dB/octave rolloff. Analytical expressions that can be used in the design of these devices are derived to obtain the full benefit of negative output resistance.