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This paper provides insight into the transport mechanisms of the collector current in SiGe HBTs operating at cryogenic temperatures and compares three technology generations of devices. Based on the experimental data, a method to differentiate direct tunneling from quasi-ballistic transport is proposed. Measurements indicate that direct tunneling becomes more significant at cryogenic temperatures. The effects of technology scaling on the direct tunneling were investigated using TCAD. Direct tunneling was found to be sensitive to the base width and the Ge profile. It is predicted that without an increase in the Ge content, direct tunneling may dominate over quasi-ballistic transport at the limits of technology scaling.

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.

In quantum computing, metrology, single-photon counting, and nanomechanics, weak electronic signals at extremely low temperatures need amplification. To this end, the authors build and study silicon-germanium heterojunction bipolar transistors, which offer excellent amplifier characteristics, integrability with silicon quantum electronics, low cost, and manufacturability. Their research at the junction of physics and electrical engineering is a step toward next-generation integrated circuits at the 90-nm scale for this temperature regime.

Cryogenic preamplification using silicon–germanium heterojunction bipolar transistors has proven to be effective in increasing the signal-to-noise ratio of the tunnel magnetoresistance of high resistance magnetic tunnel junctions at 8 K. The magnetic tunnel junctions used have resistances greater than 1 MΩ, and the cryogenic measurement system still has sufficient bandwidth for the 1/f noise to roll off. A noise model for the system has been proposed and evaluated experimentally. The noise temperature and minimum noise temperature of the transistor used in the experiment are calculated and compared. The signal-to-noise ratio of the junction alone and the transistor-junction system is derived from the sample and circuit parameters and compared. Experimental data show a signal-to-noise ratio increase by a factor of 6.62 after adding in the cryogenic preamplifier. An increase in 1/f noise in the antiparallel state of the tunneling junction as opposed to the parallel state is also observed giving evidence of 1/f noise dependence on the magnetic state of the junction.