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Quantum transport measurements in advanced Silicon-Germanium Heterojunction Bipolar Transistors (SiGe HBTs) are presented and analyzed, including tunneling spectroscopy of discrete impurity levels localized within the transistor and the dependence on an applied magnetic field. The collector current at mK temperatures is well accounted for by ideal electron tunneling throughout the entire base. The amplification principle at mK temperatures is fundamentally quantum mechanical in nature: an increase in base voltage, requiring a moderate base current, creates an equal and opposite decrease in the tunneling barrier seen by the electrons in the emitter, thereby increasing the collector current significantly more than the base current, producing current gain.Highly-scaled SiGe HBTs operate predictably at mK temperatures, thus opening the possibility of viable SiGe mK circuitry.

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.