Completely planar GaAs m.e.s.f.e.t.s were produced by selective open tube diffusion from tin-doped spin-on SiO2 films. After the diffusion process no change in the surface morphology was observed in the doped regions. The evaluation of saturation currents of the m.e.s.f.e.t.s show very good homogeneity. Isolation between doped areas was excellent. Sample preparation and device results are presented; possible applications are outlined.