To achieve high integration and multifunction capability, industry is pursuing the development of SiGe for range sensors, speed control, etc. In addition to integration and performance, this technology is likely to be utilized in customer products for wireless applications including communications and sensing/navigation. Comparisons made between InP-, GaN- and SiGe-based products show superior cost potential in the SiGe technology (see Fig. 5.11) and indicate device superiority due to very low 1/f noise and low phase noise. SiGe transit-time diodes in self-oscillating mixers have demonstrated frequency stability with subharmonic locking. Free running has demonstrated about -60 dBc/Hz at 100 kHz from the carrier, and with phase locking about -90 dBc/Hz at 100 kHz from the carrier. In this circuit technology, CPW has been chosen as the interconnect medium due to its superiority to thin film microstrip and its associated need of a via hole technology. CPW solves a number of problems but requires an air-bridge technology, which in terms of fabricating is easier to establish due to its requirements for wafer-surface and not wafer-bulk fabrication. Daimler Chrysler is a leader in SiGe technology and has demonstrated performance records in SiGe HBT, as shown in Fig. 5.12. A number of SiGe applications include Ka-Band CPW oscillator HBTs, a 77 GHz near-field sensor with SiGe Schottky diodes, a 77 GHz closing velocity sensor, etc. While SiGe technology is progressing fast, a number of processing issues still need to be resolved. To alleviate some of these issues, passivation of the device by Si3N4 has been adopted. Low temperature, low-power cpw-based HBT structures are routinely demonstrated (20 mW at 47 GHz) (6 emitter figure device). Presently research is focused on the development of phase resonant devices with fmax=300GHz achieved by quantum-well injection.
Fig. 5.11. Comparison of GaAs and SiGe (Daimler-Chrysler).
Fig. 5.12. CWorld record performance of SiGe HBTs (Daimler-Chrysler).
Daimler-Chrysler is inserting this technology into customer products via an extensive product development effort performed in the "Microwave Factory" owned by DASA. Sensors have been produced such as SatCom, MobilCom, Cruise control at 77 GHz and LMDS at 28 GHz, in addition to 24 GHz radar designed to measure material properties for application in steel production. Other products include a 58 GHz point-to-point link in hybrid configuration with GaAs MMICs using bonding wires for connection to MMIC chips.