of fig 1 after customising to provide a polysilicon emitter bipolar transistor the polysilicon layer is patterned to provide a transistor emitter body 25 and a collector contact 26 using this patterned polysilicon as a mask a high dose implant is effected to provide p type base contact region 27 the collector polysilicon 26 ensures that this p type implant is separated from the n type sinker 17 it will be appreciated that the polysilicon layer 24 can be etched only in those regions where it is separated from the substrate by the oxide layer 15 as this oxide layer provides an etch stop thus protecting the silicon substrate and preventing undercutting of the polysilicon an insulating glass layer 28 is applied to the structure and is patterned with a further lithographic mask not shown to provide windows exposing the transistor emitter and the base and collector contacts finally a metallisation pattern 29 is applied to contact the transistor via the windows in the glass layer the structure of fig 3 includes both a bipolar transistor and a p channel field effect mos transistor the fabrication process is substantially as described above with reference to fig 2 with the exception that a polysilicon gate 31 is provided by the patterned polysilicon layer this gate forms part of the implantation mask for the p type implant forming the transistor base and source drain 32 and the transistor drain source 33 the bipolar collector terminal 34 also provides the field effect body contact a minor modification of the structure is shown in fig 4 where the field effect transistor gate 41 has been extended to provide a.