SiGe heterojunction bipolar transistors - The noise perspective

Si/SiGe heterojunction transistors with high germanium concentration in the base layer are capable of microwave noise figures below 1 dB at X-band frequencies, because of their low base resistance. We review critical device parameters which influence the minimum noise figure, location of the noise-optimum source reflection coefficient, and associated gain at noise match, including both low-parasitic double-mesa and highly manufacturable box-shaped HBT devices in our considerations. A higher 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 less lossy. Low-frequency noise results show 1/f corner frequencies below 1 kHz in microwave devices. Two circuit examples, a d.c.-18 GHz amplifier and a low-noise active antenna at 5.8 GHz, demonstrate that device capabilities can be translated into superior circuit performance. (C) 1997 Elsevier Science Ltd.