  Cubic BAs has received recent attention for its large electron and hole
mobilities and large thermal conductivity. This is a rare and much desired
combination in semiconductor industry: commercial semiconductors typically have
high electron mobilities, or hole mobilities, or large thermal conductivities,
but not all of them together. Here we report predictions from an advanced
self-consistent many body perturbative theory and show that with respect to
one-particle properties, BAs is strikingly similar to Si. There are some
important differences, notably there is an unusually small variation in the
valence band masses . With respect to two-particle properties, significant
differences with Si appear. We report the excitonic spectrum for both q=0 and
finite q, and show that while the direct gap in cubic BAs is about 4 eV, dark
excitons can be observed down to about $\sim$1.5 eV, which may play a crucial
role in application of BAs in optoelectronics.
