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

The current dependence of small signal base-collector capacitance, Cbc and base series resistance, Rb, extracted directly from S-parameters, has been investigated in microwave n-p-n AlGaAs/GaAs HBTs. It is found that the magnitude of Cbc at Vce=3V in our devices is relatively constant at low current densities but reduces at current density of 2×10³A/cm² and then increases rapidly at a critical value of 3×10⁴A/cm². The results obtained are compared with a physics based analytical model and shown to exhibit good agreement.