4.2.1 Influence of Collector Doping on HBT Performance

Briefly, fT can be improved by epitaxy and processing and while the layout mainly improves fmax. By decreasing the base and collector thickness, the transit frequency fT can be improved but at the same time fmax may be affected adversely. In this work, we have investigated the effect of collector doping on the improvement of fT. At a high collector current density ( sat dC C v N q J > ), the electron density entering the base collector depletion region exceeds the doping level and this changes the electric field profile at the junction. This will cause na increase of hole injection from base to collector and so incerasing base width. The extended base width results in increased base transit time and aslo degraded current gain. This high current density effect is the well-known Kirk Effect [2,3]. Doping of the collector (NC) may oush the Kirk current density to higher levels.