form openings as shown in fig6 the collector of npn 60 includes an isolated region 606 of n epi layer 42 an n buried layer 604 and an n isolation structure 602 that is formed in accordance with this invention n isolation structure 602 is contacted through an opening in oxide layer 612 and extends downward into n buried layer 604 the base of npn 60 includes a p type silicon germanium layer 608 and a p base contact region 610 a silicon germanium layer can be grown on the surface of n epi layer 42 in an opening not shown in an oxide layer after which the germanium diffuses downward into n epi layer 42 to form silicon germanium layer 608 n isolation structure 602 laterally surrounds isolated region 606 and silicon germanium layer 608 the emitter of npn 60 is formed by a polysilicon layer 614 that is heavily doped with an n type material such as arsenic phosphorus or antimony to a doping concentration in the range of 1 u00d7 10 19 to 1 u00d7 10 20 cm 3 which is the upper limit of doping in polysilicon polysilicon layer 614 is initially deposited in the opening in oxide layer 612 and overlaps the top surface of oxide layer 612 polysilicon layer 614 is then masked and etched so that polysilicon layer 614 is limited to the vicinity of the opening in oxide layer 612 as shown in fig6 npn 60 is isolated so long as the collector is biased positive relative to.