On the Cryogenic RF Linearity of SiGe HBTs in a Fourth-Generation 90-nm SiGe BiCMOS Technology

Large-signal (P
_{1 dB}
) and small-signal (OIP3) radio frequency (RF) linearities of silicon-germanium (SiGe) heterojunction bipolar transistors (HBTs) fabricated in a new fourth-generation 90-nm SiGe BiCMOS technology operating at cryogenic temperatures are investigated. The SiGe BiCMOS process technology has an f
_T
/f
_max
of 300/350 GHz. SiGe HBTs with two different layout configurations, collector-base-emitter (CBE) and CBE-base-collector (CBEBC), were characterized over temperature. Both dc and ac figures-of-merit are presented to aid in understanding the linearity, and to provide an overall performance comparison between the two layout configurations. The extracted peak fT/fmax for CBE and CBEBC at 78 K are 387/350 and 420/410 GHz, respectively. The P
_{1 dB}
and OIP3 linearity metrics for both configurations are comparable. Source- and load-pull measurements were performed at each temperature at 8 and 18 GHz, with the devices biased at a J
_C
of 18 mA/μm
²
. Two-tone measurements over bias were also performed at 300 and 78 K with 50-Ω terminations for the source and load impedances. The 50 Ω results follow a similar response to the source-and load-pull measurements at 300 and 78 K, and demonstrate that the small-signal linearity of the SiGe HBTs is not adversely impacted by operation at cryogenic temperatures. The CBEBC configuration demonstrated the most consistent RF linearity performance at cryogenic temperature out of the two layout options.