What is the noise performance advantage of a SiGe HBT over a CMOS transistor at the same frequency?

The noise performance advantage of a SiGe HBT (heterojunction bipolar transistor) over a CMOS transistor at the same frequency comes from its superior high-frequency transconductance, lower base resistance noise, and higher f_T at a given bias current, enabling SiGe HBTs to achieve 2-5 dB lower minimum noise figure than CMOS at frequencies above 10 GHz. The advantage arises because: the SiGe HBT has exponential transconductance scaling with current (gm = I_C / V_T, where V_T = 26 mV at room temperature), providing very high gm per unit current (for example: at 1 mA collector current: gm = 38.5 mS/mA; a CMOS device would need significantly more current to achieve the same gm due to its weaker, square-law I-V relationship), the SiGe HBT has lower parasitic gate resistance (the base resistance of a SiGe HBT is 5-20 ohms, contributing directly to thermal noise; CMOS has a distributed gate resistance that can be higher, especially for long-channel or narrow-finger devices), the SiGe HBT has higher f_T at practical bias currents (a modern 0.13 um SiGe BiCMOS process achieves f_T > 200 GHz at a few mA of collector current; a 65nm CMOS achieves similar f_T but at a higher current, increasing the shot noise and power consumption), the SiGe HBT has a lower noise figure optimization (the NF_min of an HBT is approximately: NF_min approximately 1 + (f/f_T) x sqrt(2 x I_C x R_B / V_T), which is lower than CMOS NF_min at the same frequency because of the higher gm/I_C ratio). At 10 GHz: a SiGe HBT achieves NF_min of 0.5-1.0 dB, while 65nm CMOS achieves NF_min of 1.0-2.0 dB. At 60 GHz: SiGe HBT achieves NF_min of 3-5 dB, while CMOS achieves 4-7 dB.