SiGe HBT Technology:Applications and Future Directions.

Applications and Future Directions

Bandgap-engineered SiGe HBTs have many attractive features which make them ideal candidates for a wide variety of circuit applications. For instance, Si BJTs are well known to have superior low-frequency noise properties compared with compound semiconductor technologies. Low-frequency noise is often a major limitation for RF and microwave systems because it directly limits the spectral purity of the transmitted signal. SiGe HBTs have low-frequency properties as good as or better than Si BJTs, superior to that obtained in AlGaAs/GaAs HBTs and Si CMOS [29,30]. The broadband (RF) noise in SiGe HBTs is competitive with GaAs and InP technology and superior to Si BJTs. In addition, SiGe HBTs have recently been shown to be very robust with respect to ionizing radiation, an important feature for space- based electronic systems [31,32]. Finally, cooling enhances all of the advantages of a SiGe HBT. In striking contrast to a Si BJT, which strongly degrades with cooling, the current gain, Early voltage, cutoff frequency, and maximum oscillation frequency (fmax) all improve significantly as the temperature drops [33–35]. This means that the SiGe HBT is well suited for operation in the cryogenic environment (e.g., 77 K),

historically the exclusive domain of Si CMOS and III–V compound semiconductor technologies. Cryogenic electronics is in growing use in both military and commercial applications such as space-based satellites, high-sensitivity instrumentation, high-TC superconductors, and future cryogenic computers.

Figure 4.9 summarizes many of the emerging applications of SiGe BiCMOS HBT technology, and span the range of the defense, navigation, automotive, and communications industries, from low RF (900 MHz) to high-mm wave (100 GHz), where it offers an ideal combination of high integration level, high performance, and low cost [36,37]. Figure 4.10 shows a schematic representation of an envisioned single-chip SiGe mm-wave transceiver for very high data rate (>1.0 Gb/s), short-range communications links.

One may logically wonder just how fast SiGe HBTs will be at the end of the day. Transistor-level performance in SiGe HBTs continues to rise at a truly dizzying pace. Both first- and second-generation SiGe HBT BiCMOS technology is widely available and even at the 200 GHz (third-generation) performance level, several companies already have robust commercially available technologies. At present, the most impressive new SiGe HBT result achieves 302 GHz peak fT and 306 GHz peak fmax, a record for any Si-based transistor. This level of performance was achieved at a BVCEO of 1.6 V, a BVCBO of 5.5 V, and a current gain of 660. Noise measurements on these devices yielded NFmin/Gassoc of 0.45 dB/14 dB and 1.4 dB/8 dB at 10 and 25 GHz, respectively [37]. Measurements of early (unoptimized prototypes) of fourth-generation SiGe HBTs have yielded record values of 375 GHz peak fT at 300K, and above 500 GHz at 5K. Simulations suggest that THz-level (1000 GHz) intrinsic transistor performance is a realistic goal. It seems likely that we will see SiGe HBTs above-500 GHz peak fT and fmax fully integrated with nanometer- scale (90 nm and below) Si CMOS (possibly strained Si CMOS) within the next 3–5 years.

One might ask, particularly within the confines of ultimate market relevance, why one would even attempt to build 500 GHz SiGe HBTs? If the future “killer app” turns out to be single-chip mm-wave transceiver systems with onboard DSP for broadband multimedia, radar, etc., then the ability of highly scaled, highly integrated, very-high-performance SiGe HBTs to dramatically enlarge the circuit/system design space of the requisite mm-wave building blocks may well prove to be a fruitful (and marketable) path.

Other interesting themes are emerging in the SiGe HBT BiCMOS technology space. One is the the very recent emergence of complementary SiGe (C-SiGe) HBT processes (npn + pnp SiGe HBTs). While very early pnp SiGe HBT prototypes were demonstrated in the early 1990s, only in the last few years have fully

complementary SiGe processes been developed, the most mature of which to date has 200 GHz npn SiGe HBTs and 80 GHz pnp SiGe HBTs [37]. Having very-high-speed pnp SiGe HBTs onboard presents a fascinating array of design opportunities aimed particularly at the analog/mixed-signal circuit space. In fact, an additional emerging trend in the SiGe field, particularly for companies with historical pure analog circuit roots, is to target lower peak fT , but higher breakdown voltages, while simultaneously optimizing the device for core analog applications (e.g., op amps, line drivers, and data converters), designs which might, for instance, target better noise performance, and higher current gain—Early voltage product than mainstream SiGe technologies. One might even choose to put that SiGe HBT platform on top of thick-film SOI for better isolation properties. Another interesting option is the migration of high-speed vertical SiGe HBTs with very-thin-film CMOS-compatible SOI [35]. This technology path would clearly favor the eventual integration of SiGe HBTs with strained Si CMOS, all on SOI, a seemingly natural migratory path. Clearly, SiGe is a highly dynamic field and much is on the horizon. Stay tuned!