Several workers have shown that the quality of MBE material for AlxGa(1-x)As-GaAs semiconductor laser diodes can be improved by incorporating additional layers into the laser structures. One method is the introduction of superlattice prelayers of AlxGa(1-x)As-GaAs bcfore the growth of the active region. This has been showp to improve the optical qualitykl) and to reduce tb.e interface recombination velocities l2) and has given laser diodes of high performance 0). A related technique to using superlattice prelayers is to use extended regions of superlattices as parts of the laser structure. The superlattices can be of AlxGa(l_x)As-GaAs or cap be all-binary structures of AlAs-GaAs. These have been applied to the cladding regions (4-0), the barrier regions k5,7) and the active regionko), though only with a superlattice of AlxGa(l_x)As-GaAs. Although these structures have in many cases given rise to laser diodes having good performance, the characteristics of the superlattices and the nature of their influence on device performance are by no means well understood. In particular it is not clear whether the good performance is due simply to improvements in morphology and interface properties or whether there are modifications to the physics of the gain generating process in the device. To get a better understanding of the operation of these devices we have explored the use of all-binary superlattices for the cladding, waveguide and barrier regions of laser diodes using GaAs quantum wells and we have compared them with similar structures grown using alloys in place of superlattices. We have made alloy devices with and without an all-binary prelayer. We also report the use of binary superlattices in the active region of a conventional laser, we believe, for the first time. We have investigated the threshold current, and its temperature dependence, of 50pm wide oxide stripe laser diodes embodying these structures. We have also made broad area devices of differing lengths in order to examine the gain-current and loss characteristics. Finally we have investigated the spontaneous emission spectra of light emitted through a narrow window in the top contact of superlattice and alloy devices. These investigations have shown that all-binary superlattices give lasers as good as the best comparable conventional alloy quantum well lasers as far as the threshold current is concerned, and have a weaker temperature dependence. We have also seen that the gain-current relationship shows similar saturation behaviour to normal alloy barrier quantum well devices. The spontaneous emission spectra indicate that the description of binary superlattices in terms of the average effective alloy composition is not adequate for an accurate modelling of the device.