2.1.1 Why and Where SiGe HBTs?

The SiGe HBTs are double heterojunction bipolar transistors (DHBTs) as the SiGe material is used as a narrow bandgap material in the p-type base. The emitter and the collector are silicon and have larger bandgap.

Since the first SiGe HBTs reported in the late eighties [1] many groups were involved in the development of these devices [2,3,4]. Transit frequencies of 116 GHz [5] and 130 GHz [6], and maximum oscillation frequencies of 160 GHz [7,8] were reported. The devices are fully compatible with the existing state-of-the-art 0.18 $\mu$m CMOS technology [9,10]. Digital application-specific integrated circuits (ASICs) are combined with SiGe HBT circuits in the so-called SiGe BICMOS technology and are in volume production [11]. The SiGe HBT is considered an essential technology for over 10 Gb/s optical communication systems [12,13]. However, a shortcome of Si-based HBTs is their comparatively lower breakdown voltage.