Operation of SiGe HBTs Down to 70 mK

We present the first measurement results of a highly scaled, 90-nm silicon–germanium heterojunction bipolar transistor (SiGe HBT) operating at cryogenic temperatures as low as 70 mK. The SiGe HBT exhibits a transistor-like behavior down to 70 mK, but below 40 K, the transconductance suggests the presence of non-equilibrium transport mechanisms. Despite the non-ideal base current at cryogenic temperatures, a $dc$ current gain $(\beta) > 1$ is achieved for $I_{C} > 1$ nA, suggesting that ultra-low-power low-noise amplifiers should be viable. Exposure of the SiGe HBT to strong magnetic fields (±14 T) is also presented to help understand the nature of the non-ideal current.