Crystal growth, diffusion, and fabrication conditions which yield Ga(AsP) junction lasers with threshold current densities as low as 3000 A/cm 2 are described. A slow vapour transport method of crystal growth results in uniform quality material, while deep junctions diffused under excess As vapour pressure are adequately planar. A study of Zn diffusion into n-type Ga(As 1− x P x ) shows that p- n junction depths depend on the usual diffusion parameters and significantly on the crystal composition of Ga(As 1− x P x ) and the surface preparation of the wafers prior to diffusion. Zinc diffusion occurs via simultaneous interstitial and substitutional mechanisms with the interstitial mechanism predominating in degenerately-doped n-type material. A linearly increasing activation energy with increasing GaP composition is suggested. Radiative recombination in Ga(AsP) junction lasers which is found to occur on the p side of the junction results from electron transitions from smeared conduction band states to Zn acceptor states. Laser threshold current is found to decrease with increasing depletion region width and reverse breakdown voltage. Successful laser operation at temperatures up to 255°K is reported, with threshold currents for Ga(As 0.70P 0.30) following approximately a T 3 temperature dependence in the range between 100 and 200°K. The shift in emission energy in this range is approximately 3.1 × 10 −4 eV/°K. Variation of index of refraction with wavelength is calculated from the spacing of Fabry-Perot modes in laser spectra and yields a value of dn dλ ≈ −1.6 × 10 −4/ A ̊ . For Ga(As 1− x P x ) with x > 0.33, the proximity of the indirect 〈100〉 conduction band minima to the direct [000] minimum causes relatively deep donor states associated with the 〈100〉 minima to lie near or below the direct band edge, which in turn at reduced temperature leads to carrier freeze-out and increased threshold currents. Consistent with the donor ionization energies observed in GaP, the effective direct-indirect transition in Ga(As 1− x P x ) is shown to occur at a value of x less for the deeper donor S than that for the shallower donor Te. The peculiarities of the direct-indirect transition are used to estimate the position of the indirect donor levels of Te, Se, and S in Ga(AsP).