Complex current gain and cutoff frequency determination of HBTs

The behaviour of the modulus and phase Phi(h(21)) of the complex current ga in h(21) of high frequency heterojunction bipolar transistors (HBTs) has be en investigated using an InGaAs/ InAlAs/InP HBT, and compared with the beha viour predicted by a one pole/one zero analytical expression for h(21). It is demonstrated that the phase value Phi(h(21)) = -90 degrees reliably loca tes the frequency range in which \h(21)\ frequency fall-off reaches -20dB/d ec, the criterion for determining f(T) An analytical fit of the measured ga in frequency product GF centred within this frequency rang provides a repro ducible and fully automated determination of f(T).