Velocity saturation in the collector of Si/Ge/sub x/Si/sub 1-x//Si HBT's

The effects of velocity saturation on the unity gain-bandwidth product f/sub t/ and transconductance g/sub m/ of n-p-n and p-n-p heterojunction bipolar transistors (HBTs) with Ge/sub x/Si/sub 1-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 g/sub m/ and f/sub t/ rolloff for current densities greater than the knee current for the Kirk effect. For the p-n-p device, the g/sub m/ and f/sub t/ 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/Ge/sub x/Si/sub 1-x//Si HBTs.<>