The electrical and optical properties of GaAs diodes with three-layer structures are described. The center layer was either n-type (n +-n-p+ diodes), semi-insulating (n+-i-p + diodes) or p-type (n+-p-p+ diodes). The spontaneous emission lines from n+-n-p+ diodes were studied in detail. For a low doping level of the n layer they can be interpreted by assuming hole recombination on the n side of the p+-n junction. Lasing occurred in a 1.5-eV line and in second-order transverse modes. Small vertical beamspreads were observed. Optical gain factor and laser losses are discussed. Stimulated emission was not obtained when the n layer was very thick. n+-i-p+ diodes had a Cr-doped middle region and showed a negative resistance at 300°K. Lasing occurred in the 1.48-eV line. n +-p-p+ diodes showed emission lines attributed to tunneling. Lasing occurred in the 1.48-eV line. The temperature dependence of the threshold current density is discussed. © 1966 The American Institute of Physics.