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.
- bipolar transistors
- superconducting transistors