An analysis of the bias dependence of scattering parameters S/sub 11/ and S/sub 22/ of SiGe heterojunction bipolar transistors (HBTs)

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. 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. It is found that under constant collector-emitter voltage (V/sub CE/), an increase of base current (which corresponds to a decrease of r/sub /spl pi// and an increase of g/sub m/) enhances the anomalous dip. In addition, the anomalous dip in S/sub 11/ of SiGe HBTs can also be interpreted in terms of poles and zeros.

I. Introduction

The anomalous dip in scattering parameter 8₂₂of MOSFETs/MESFETs and BJTs/HBTs, which 'has been explained quantitatively [1]–[3], can be seen frequently in the literature [4]–[6]. However, the anomalous dip in scattering parameter S₁₁of BJTs/HBTs has never been reported. In this paper, the anomalous dip in S₁₁of SiGe HBTs is reported and explained quantitatively for the first time. It is found that under constant V_CE, an increase of base current (I_B) enhances the anomalous dip. That is to say, for devices with lower and higher g_m, the anomalous dip is more prominent