An epitaxial emitter cap, SiGe-base bipolar technology with 22 ps ECL gate delay at liquid nitrogen temperature

Abstract

It is shown that a properly designed silicon bipolar technology can achieve significantly faster circuit speed at liquid-nitrogen temperature (LNT) than at room temperature (RT). Transistors were fabricated using a reduced-temperature process employing an in situ doped polysilicon emitter contact, a lightly doped epitaxial emitter cap layer, and a graded SiGe base. The short thermal cycle associated with this emitter contact technology allows the formation of an abrupt, heavily doped base nearly immune to carrier freezeout, while maintaining superior emitter-base leakage characteristics at LNT. Transistors have a current gain ( beta ) as high as 500 at 84 K with a cutoff frequency (f/sub T/) of 61 GHz, up from 43 GHz at 300 K. ECL circuits switch at a record 21.9 ps at 84 K at J/sub cs/=1.0 mA/ mu m/sup 2/ (25.4 ps at 310 K). For completeness the low-temperature properties of this technology are compared with more conventional ipi and pi SiGe-base designs. >