SIMULATION STUDY OF PARASITIC BARRIER FORMATION IN Si/SiGe HETEROSTRUCTURES

by BREED, ANIKET AJITKUMAR

In this study, the effects of displacement of a p-n junction from coincidence with a SiGe/Si heterojunction have been investigated using a simple analytical model. The phenomenon is of interest for understanding the degradation in performance of NPN SiGe/Si heterojunction bipolar transistors where Ge is employed in the base when there is boron outdiffusion that produces p-n junction displacement at one or both of the emitter and the collector-base junctions. This analysis describes the formation of parasitic barriers in the conduction band associated with this p-n junction displacement. The barrier is significantly larger for the heterojunction located on the p-side, but the barrier height is also a function of the Germanium content, doping level and the magnitude of the displacement. Junction bias and high current density flow also modulate the barrier’s height. These barriers degrade the performance of the SiGe HBT in terms of its current gain, cutoff frequency, power gain and maximum frequency of oscillation. The results of the analytical model are compared with those obtained using a commercial numerical device simulator ATLAS from SILVACO International based on a simple, three-region model of a heterojunction with a nearby p-n junction. The implications of p-n junction displacement on the formation of parasitic barriers at the emitter and the collector junctions in SiGe transistors and their dependence on device operation are discussed.