The paper presents a 2-dimensional numerical model used to analyse the silicon-on-sapphire transistor under various geometries, doping levels and bias conditions. The model accounts for the thin-film structure and the finite interface charge at the silicon-sapphire interface. The model also includes the possibility of simulating nonisothermal effects, and is shown to be applicable to prediction of the transistor behaviour in the high-voltage region. The complete system of four coupled partial differential equations describing the internal behaviour of the s.o.s. transistor is solved exactly. Typical results for an n-channel double-implanted inversion-layer s.o.s. transistor are shown. The agreement between theory and experiment is found to be excellent.