Complex current gain and cutoff frequency determination of HBTs

The behaviour of the modulus and phase Φ(h_2l) of the complex current gain h_2l of high frequency heterojunction bipolar transistors (HBTs) has been investigated using an InGaAs/InAlAs/InP HBT, and compared with the behaviour predicted by a one pole/one zero analytical expression for h₂₁ It is demonstrated that the phase value Φ(h_2l)=-90° reliably locates the frequency range in which (h_2l) frequency fall-off reaches -20 dB/dec, the criterion for determining f_T. An analytical fit of the measured gain frequency product GF centred within this frequency range provides a reproducible and fully automated determination of f_T