Current Dependence of Small Signal Base - Collector Capacitance in Microwave AlGaAs/GaAs HBTs

The current dependence of small signal base-collector capacitance, C
bc
and base series resistance, R
b
, extracted directly from S-parameters, has been investigated in microwave n-p-n AlGaAs/GaAs HBTs. It is found that the magnitude of C
bc
at V
ce
=3V in our devices is relatively constant at low current densities but reduces at current density of 2×10
³
A/cm
²
and then increases rapidly at a critical value of 3×10
⁴
A/cm
²
. The results obtained are compared with a physics based analytical model and shown to exhibit good agreement.