The mass-transported buried-heterostructure lasers have been investigated in detail. Techniques have been developed for fabrication control and wafer characterization. High yield of low-threshold lasers was obtained with the lowest threshold current being 5.5 mA. Differential quantum efficiencies as high as 28 percent per facet and smooth far-field patterns were also obtained. Deep Zn-diffusion and higher p-doping have been used to improve the linearity of the light-current characteristics. Linear light output to greater than 13 mW per facet and well-behaved threshold-temperature dependence were achieved. A conformal mapping technique has been used to calculate the voltage distribution in the laser mesa. The forward-bias voltage thus obtained for the InP pn homojunctions in the transported regions is shown to be capable of resulting in the sublinear dependence of light output on current observed in lasers with lightly p-doped cap layers. The current limit within which the laser can be operated without leakage through the homojunctions has been calculated for various device geometries and doping levels.