Theoretical calculations show that for maximum output power of LED’s an optimal ratio between thep‐njunction depths and the diffusion lengths of minority carriers of 2–3 is required. A method based on the photoluminescence technique is described which allows the determination of thea/Lratio. Measurements on various types of luminescence diodes were carried out. In double‐heterostructure lasers diffusion lengths in the range 1.2–4.6 &mgr;m were estimated.