Cryogenic SiGe Hetero-Junction Bipolar Transistors From Standard Technologies For Low Noise FLL

Dynamic range of superconducting quantum interference devices (SQUID) can be considerably increased by using a flux-locked-loop (FLL). The FLL noise is mainly determined by the amplifier noise used in the feedback loop. The use of a very low noise amplifier working at cryogenic temperature can lead to reach the limit of SQUID intrinsic noise (typically few 10^-6 Phi₀/radicHz). To realise the amplifier of the FLL, the use of standard technologies allows to design an ASIC including several amplifiers which are necessary for the read-out of a SQUID array. We present here 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 hetero-junction 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 temperatures 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. Thus, we found that the current gain of the 0.8 mum technology HBTs decreases at low temperature whereas it increases for the 0.35 mum technology. The discussion is centred on the effects which could explain the behaviours of beta and gm at temperature down to 4 K. Noise measurements are also presented confirming the existence of an effective temperature which increases the noise expected at 4 K. The presented results can lead to the realisation of cryogenic ASICs for low noise FLL readout of bolometers arrays dedicated to cosmic microwave background (CMB) observations.