  Optical and electrical characteristics of AlGaAs lasers with separate
confinement heterostructures are modeled by using Synopsys's Sentaurus TCAD,
and open source software. We propose a modified exponential $I-V$ dependence to
describe electrical properties. A simple analytical, phenomenological model is
found to describe optical efficiency, $\eta$, with a high accuracy, by using
two parameters only. A link is shown between differential electrical
resistivity $r=dU/dI$ just above the lasing offset voltage, and the functional
$\eta(U)$ dependence.
