An analysis of the threshold behavior of long-wavelength ( lambda =1.55 mu m) multiquantum well separate-confinement lasers with InGaAs wells and quaternary ( lambda g=1.3 mu m) barriers is presented. Using the effective mass approximation and Fermi statistics for carriers, an approximately logarithmic dependence of optical gain on carrier density for quantum well lasers with one confined electron state is predicted theoretically. This prediction is verified by measured threshold currents of broad-area lasers of various cavity lengths and different numbers of quantum wells. Moreover, the characteristic parameters, such as transparency current density, gain constant, and absorption outside the active region, are determined

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Measurements of the threshold current densities of 1.35 mu m InGaAs/InGaAsP MQW separate-confinement laser structures are presented. It is shown that the lowest values for the threshold current density can be obtained with barrier thicknesses between 10 and 15 nm. The extrapolated threshold current densities of the investigated structures varied between 780 and 1120 A-cm-2. depending on the number of wells. The characteristic temperature T0 of the laser structures was about 60 K