A novel silicon-on-insulator (SOI) semiconductor substrate has a first semiconductor wafer (21) with a front face oxide layer (23) bearing an undoped polysilicon layer (25) which is bonded to a second silicon wafer (29). A second oxide layer may be provided between the polysilicon layer (25) and the second wafer (29). Also claimed is a process for producing an SOI substrate by: (i) forming a preferably less than 3 mu m thick oxide layer (23) on a first wafer (21); (ii) applying a preferably 0.5-4 mu m thick undoped polysilicon layer (25); (iii) optionally forming a preferably less than 1.5 mu m thick second oxide layer on the polysilicon; (iv) bonding a second wafer (29) onto the polysilicon layer (25) or the second oxide layer; and (v) polishing the back face of the first wafer (21).
