PARASITIC ENERGY BARRIERS IN SIGE HBTS

The improvement in electrical performance of SiGe heterojunction bipolar transistors compared to conventional homojunction transistors depends strongly on the position of the SiGe/Si transition relative to the emitter and collector junctions. Parasitic energy barriers can easily be introduced during processing. Measurements and calculations of experimental n-p-n HBT's will be presented, showing that a parasitic conduction-band barrier at the base-collector junction reduces the collector current and the cutoff frequency. A simple analytical model explains the f(T) degradation, caused by the reduction of the collector current and a pileup of minority carriers in the base. With the model the effective height and width of the barrier can also be derived from the measured collector current enhancement factor I(c)(SiGe)/I(c)(Si).