There is a method of forming a contact post and surrounding isolation trench in a semiconductor-on-insulator (SOI) substrate. The method comprises etching a contact hole and surrounding isolation trench from an active layer of the substrate to the insulating layer, masking the trench and further etching the contact hole to the base substrate layer, filling the trench and contact hole with undoped intrinsic polysilicon and then performing a doping process in respect of the polysilicon material filling the contact hole so as to form in situ a highly doped contact post, while the material filling the isolation trench remains non-conductive. The isolation trench and contact post are formed substantially simultaneously so as to avoid undue interference with the device fabrication process.