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 ( $T_{\mathrm {MIN}}$ , NF _MIN , $R_{n}$ , and $Z_{\mathrm {sopt}}$ ) of SiGe heterojunction bipolar transistors (HBTs) are characterized for the first time from dc and $S$ -parameter measurements up to 70 GHz and from 2 to 400 K. Significantly improved current gain ~10 000, minimum noise temperature, $T_{\mathrm {MIN}}$ (< 1 K below 8.5 GHz), MAG, $f_{T}$ (458 GHz), and $f_{\mathrm {MAX}}$ (534 GHz) are observed at 2 K compared to 300 K, with no evidence of impurity deionization. It is found that the optimum noise figure current density, $J_{\mathrm {OPT}}$ , increases with temperature, following the crossover between shot noise and thermal noise. In contrast, the peak- $f_{T}$ and peak- $f_{\mathrm {MAX}}$ current densities increase by more than 50% at 2 K, likely due to the higher $v_{\mathrm {sat}}$ . A decrease in BV _CEO , expected due to the higher current gain, and negative output conductance are observed in the 2–200 K range in the dc output characteristics at large currents above the peak- $f_{T}$ current.