Abstract: The influence of a heterojunction in the vicinity of a graded BC junction on the performance of npn SiGe HBTs is studied.SiGe HBTs differing only in heterojunction position in the vicinity of a graded BC junction are simulated by means of 2D Medici software for DC current gain and frequency characteristics.In addition,the simulated DC current gains and cut-off frequencies are compared at different collector-emitter bias voltages.Through the simulation results,both DC and HF device performance are found to be strongly impacted by degree of confinement of the neutral base in the SiGe layer, even in the absence of a conduction band barrier.This conclusion is of significance for designing and analyzing SiGe HBTs.

Key words: SiGe HBT, BC junction, HBE, relative position, device performance