  Drift-diffusion computer simulations model available in Synopsys' Sentaurus
TCAD is used to study electrical, I-V, and optical, I-L, characteristics of
separate-confinement heterostructure laser based on $AlGaAs$. We investigate
the role of the width and depth of Quantum Well (QW) active region, below and
above the lasing threshold. The device properties depend on both, the number of
bound QW states and on closeness of the highest bound states to conduction or
valence band offset. The lasing action may not exist at certain widths or
hights of QW, and the threshold current is a discontinuous function of these
parameters. The effects are more pronounced at low temperatures.
Discontinuities in characteristics are found, at certain conditions, in
temperature dependencies as well. The carriers scattering time on QW is shown
to have the crucial role on amplitude of discontinuities.
