Si/SiGe heterojunction transistors with high germanium concentration i
n the base layer are capable of microwave noise figures below 1 dB at
X-band frequencies, because of their low base resistance. We review cr
itical device parameters which influence the minimum noise figure, loc
ation of the noise-optimum source reflection coefficient, and associat
ed gain at noise match, including both low-parasitic double-mesa and h
ighly manufacturable box-shaped HBT devices in our considerations. A h
igher associated gain can be obtained when the base-collector feedback
capacitance is reduced. Using larger transistors, both the magnitude
of the noise-optimum source reflection coefficient and the equivalent
noise resistance will be reduced, making input matching easier and les
s lossy. Low-frequency noise results show 1/f corner frequencies below
1 kHz in microwave devices. Two circuit examples, a d.c.-18 GHz ampli
fier and a low-noise active antenna at 5.8 GHz, demonstrate that devic
e capabilities can be translated into superior circuit performance. (C
) 1997 Elsevier Science Ltd.