Cryogenic Characterization of the High-Frequency and Noise Performance of SiGe HBTs from DC to 70 GHz and Down to 2 K

The high frequency and noise performance (TMIN, NFMIN, Rn, Zsopt) of SiGe HBTs is characterized for the first time from dc and S-parameter measurements up to 70 GHz and from 2 K to 400 K. Significantly improved current gain ~10,000, minimum noise temperature, TMIN, (<1 K below 8.5 GHz) MAG, fT (458 GHz) and fMAX (534 GHz) is observed at 2 K compared to 300 K, with no evidence of impurity de-ionization. It is found that the optimum noise figure current density, JOPT, increases with temperature, following the crossover between shot noise and thermal noise. In contrast, the peak-fT and peak-fMAX current densities increase by more than 50% at 2 K, likely due to the higher vsat. A decrease in BVCEO, expected due to the higher current gain, and negative output conductance are observed in the 2-200 K in the dc output characteristics at large currents above the peak-fT current.