Results are reported on 1000 diodes made from about 50 GaAs epitaxial layers. The layers are classified for quality according to the power output of the average device, and the correlation between quality and epitaxial layer thicknessland resistivity ρ are examined. It is shown that the quality cannot be linked to the thickness or resistivity alone, but is closely connected with the ratio of thickness to resistivity. Ninety-five percent of all slices with4.5 < l / rho < 10yield good results. It is shown that this is in good agreement with theory.