VELOCITY SATURATION IN THE COLLECTOR OF SI/GEXSI1-X/SI HBTS

The effects of velocity saturation on the unity gain-bandwidth product ft, and transconductance gm of n-p-n and p-n-p hetero-junction bipolar transistors (HBT’s) with GexSi1-x bases are described and simulated. For the n-p-n device, velocity saturation combined with a valence-band offset at the base-collector junction causes accelerated gm and ft, rolloff for current densities greater than the knee current for the Kirk effect. For the p-n-p device, the gm and ft, are degraded for all current densities. These limitations combine with the limits imposed by dislocation formation due to strain in the pseudomorphic layer to impose constraints on the design of Si/GexSi1-x/Si HBT's. © 1990 IEEE