Modeling of Third-order Distortion in Common-Emitter SiGe HBTs Using Mextram 505.00

Abstract

Mextram is an advanced compact model for bipolar transistors. Mextram 504.12 does not provide a good OIP3 simulation near peak OIP3. Mextram 505.00 improves peak OIP3 by modifying intrinsic base-collector depletion capacitance. As for the intrinsic CB depletion capacitance, Mextram 505.00 modifies control voltage Vjunc and smooth transition width Vch to help OIP3 simulation. At the same time, Mextram 505.00 provides several options for Vjunc and Vch. In Mextram 504.12, Vjunc is the main reason for distorted peak OIP3. Vjunc goes up quickly once high injection occurs, as the zero injection epilayer resistance is used to calculate Vjunc as VB2C1 +Vxio, with Vxio being xi = 0 resistance calculated epilayer voltage drop. This Vjunc needs a very large Vch to smooth the control voltage so that the intrinsic CB depletion capacitance increases gradually, but this Vch also distorts both the intrinsic CB depletion capacitance and peak OIP3. Mextram 505.00 fixes Vjunc and Vch to provide a much better OIP3 simulation. To investigate how the intrinsic CB depletion capacitance affects OIP3. Contributions to the third order output current are individually examined. The third order main current is found to be dominant, and sensitive to Vjunc and Vch. The intrinsic CB depletion capacitance affects the third order main current through circuit action. The extrinsic CB depletion capacitance also has a big effect on OIP3 through the third order main current. Decreasing the extrinsic CB depletion capacitance improves the peak OIP3.