Base thickness and high-frequency performance of SiGe HBT's

The influence of the base thickness on the high-frequency performance of about 20 fabricated HBTs (heterojunction bipolar transistors) has been analyzed by plotting the highest obtained f/sub T/ value against base thickness. Base thickness is defined as the distance between the two Si/SiGe interfaces which usually coincide with the metallurgical pn-junctions. A steady increase of f/sub T/ with decreasing base thickness can be clearly seen: starting from the first published HBTs with 50 nm base thickness which did not exceed 20 GHz, transistors with 40 nm base had 37-52 GHz and devices with 28-30 nm bases exhibited 58-91 GHz. The latest two samples had only 25 and 22 nm base thicknesses, leading to an f/sub T/ of 95 GHz (f/sub max/=50 GHz), the highest value reported for Si transistors. The base doping in the latter sample was 8*10/sup 19/ cm/sup -3/ to maintain a base sheet resistance of about 1.2 k Omega .