Standard SiGe technologies operating at 4 K for front-end readout of SQUID arrays

We present some experimental results showing that two different standard BiCMOS SiGe technologies can operate at 4 K. DC and low frequency measurements were carried out on two SiGe heterojunction bipolar transistors (HBT) of 0.8 mum and 0.35 mum AMS BiCMOS SiGe technologies. We report that for both SiGe HBTs, the transconductance gm increases at cryogenic temperature making possible the use of such devices down to 4 K for low noise voltage amplification. However, the current gain beta remains highly dependant on the fabrication process. The discussion is centred on the effects which could explain the behaviors of beta and gm measurements at temperature down to 4 K