An Analysis of Base Bias Current Effect on SiGe HBTs
Journal
IEEE Transactions on Electron Devices
Journal Volume
52
Journal Issue
1
Pages
132-136
Date Issued
2005
Author(s)
Lin, Yo-Sheng
Abstract
The anomalous dip in scattering parameter S/sub 11/ of SiGe heterojunction bipolar transistors (HBTs) is explained quantitatively for the first time. Our results show that for SiGe 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 very accurately. The appearance of the anomalous dip of S/sub 11/ in a Smith chart is caused by this inherent ambivalent characteristic of the input impedance. In addition, it is found that under constant collector-emitter voltage (V/sub CE/), an increase of base current (which corresponds to a decrease of base-emitter resistance (r/sub /spl pi//) and an increase of transconductance (g/sub m/)) enhances the anomalous dip, which can be explained by our proposed theory.
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