Doped oxides have been used as solid diffusion sources and surface protection layers for diffusion into Cr-doped GaAs. Surface passivation during diffusion at 900°C for 45 min is good and nearly undamaged surfaces have been obtained. The carrier concentration profiles of such diffusions, determined by C-V-measurements, show a steep decrease at about 0.16 µm depth from the surface and can be explained by a concentration dependent diffusion coefficient. 1.3 µm gate length MESFETs produced from diffused layers show excellent I-V characteristics with transconductances up to 120 mmho per mm gate width.