Current crowding effects in SOI-SiGe HBTs with low doped emitters

Anomalous limitation of collector and base current in SOI SiGe HBTs at moderate bias levels is shown to be the result of current crowding in the low doped emitter rather than in the base as is the case for homojunction transistors. Experimental results from devices with decreasing emitter window dimension, re-enforced by 2D simulation, show clearly a trend for crowding at the centre as the emitter resistance increases. The results have significance for achieving full optimisation of HBTs and in particular for medium power devices.