Frequency dependence of the unilateral gain in bipolar transistors

The frequency dependence of the current gain and the unilateral gain in advanced bipolar transistors is analyzed. A GaAlAs/GaAs heterostructure bipolar transistor (HBT) is used as an example to show that the extrinsic parasitics still dominate the gain rolloff. Transit-time effects, even though they are a significant component of the time constants that determine the f/sub T/ and f/sub max/ of transistors, do not cause a change in the rolloff of unilateral gain with a frequency from 6 to 12 dB/octave in these advanced devices. Devices with very large transit times, and hence low f/sub T/ and f/sub max/ may, however, exhibit the rolloff change if designed to have a very low fringing capacitance and other parasitics.<>