Zn diffusion into 2‐in.‐diam semi‐insulating GaAs wafers has been carried out by a simple, open‐tube diffusion method using a commercially available GaAsZn solid source and without surface coating or encapsulation. High‐quality (&mgr;=75–100 cm2/V s) layers with a flat carrier concentration and sharp cutoff Zn distribution have been obtained. Thep‐type layers obtained by this method are uniform across the wafer, and the technique can be easily scaled up. Selective diffusion of Zn was also demonstrated with this technique, using a plasma‐deposited Si3N4as a diffusion mask.