Abstract
The anomalous dip in scattering parameter S11 of InGaP/GaAs heterojunction bipolar transistors (HBTs) is explained quantitatively. Our results show that for InGaP/GaAs HBTs, the input impedance can be represented by a “shifted” series RC circuit at low frequencies and a “shifted” parallel RC circuit at high frequencies. The appearance of the anomalous dip of S11 in a Smith chart is caused by this inherent ambivalent characteristic of the input impedance. It is found that under constant collector-emitter voltage (VCE), an increase of base current (which corresponds to a decrease of base-emitter resistance and an increase of transconductance) enhances the anomalous dip. In addition, the anomalous dip in S11 of InGaP/GaAs HBTs can also be interpreted in terms of poles and zeros.
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