Use of SiGe bipolar transistors for cryogenic readout of SQUIDs

We have measured characteristics and noise of four commercially available SiGe heterojunction bipolar transistors at 4.2 K. One device exhibits 75 pV Hz^−1/2 white noise and a 1/f noise corner of 20 kHz at 9 mW power dissipation, proving suitable for readout of SQUIDs immersed in liquid He. Another device exhibits 370 pV Hz^−1/2 white noise and a 1/f noise corner of 5 kHz at 50 µW dissipation, and still gives useable gain at 7 µW dissipation. This device is useful for SQUID multiplexers intended for closed-cycle refrigerators, in which the cooling power is limited. The achievable dissipation is even compatible for operation in the low-temperature stages of dilution refrigerators or ADRs. Furthermore, SiGe-based dc SQUID readout with 0.35 µΦ0 Hz^−1/2 flux noise was demonstrated. Applying negative flux feedback yielded a slew rate exceeding 50Φ0 µs⁻¹.