This work present simulations of the operation of tapered semiconductor lasers with separate contacts, in comparison with the experimental results, with the goal of providing a physical understanding of the origin of this improvement. The simulation parameters related to the fabrication technology were calibrated by comparison with experiments in broad area lasers and in tapered lasers with common contacts. The results provide physical interpretation of the observed improvement of the beam quality when the bias of the ridge waveguide section is higher than that of the tapered section.