Effect of Ge profile design on the performance of an n–p–n SiGe HBT-based analog circuit

Two new germanium profiles, graded double box (GDB) and graded step box (GSB), are considered in the base region for an n–p–n silicon–germanium heterojunctions bipolar transistors (SiGe HBTs). Their effects on the performance of the HBT-based common emitter amplifier circuit are investigated so as to predict the behavior of analog circuits in the high-frequency region. An analytical model of an n–p–n SiGe HBT with uniform impurity doping in the base is described, and used to extract SPICE parameters for the devices based on GDB and GSB as well as other two accepted profiles i.e. box and triangular profiles. The frequency response, current gain roll-off and gain-bandwidth product of HBT-based common emitter amplifier circuit for all the four profiles have been compared and contrasted.