Design and Optimization of Superjunction Collectors for Use in High-Speed SiGe HBTs

After reviewing the various mechanisms causing breakdown in bipolar transistors, we present a novel collector design for silicon-germanium heterojunction bipolar transistors (SiGe HBTs). The design improves the well-known speed/breakdown voltage tradeoff in SiGe HBTs for radio-frequency (RF) and millimeter-wave applications. Applying multiple alternating p- and n-type layers (a superjunction) deep in the collector-base (CB) space-charge region (SCR) alters the electric field and electron temperature in the CB junction. Consequently, impact ionization is suppressed, whereas the width of the CB SCR is not increased, and therefore, the breakdown voltages BV_CEO and BV_CEO are increased, with no degradation in the device speed or RF performance. For a fixed alternating-current performance, BV_CEO is improved by 0.33 V, producing a SiGe HBT with f_T = 101 GHz, f_max = 351 GHz, and BV_CEO = 3.0 V, as predicted by calibrated DESSIS technology computer-aided design simulations. Concerns with regard to the influence of thermal cycles associated with fabrication are considered, and a more practical doping profile is proposed to simplify the use of superjunctions. The proposed structure is also contrasted with other approaches from the literature.