  We determine the fundamental electronic and optical properties of the
high-thermal-conductivity III-V semiconductor boron arsenide (BAs) using
density functional and many body perturbation theory including quasiparticle
and spin-orbit coupling corrections. We find that the fundamental band gap is
indirect with a value of 2.049 eV, while the minimum direct gap has a value of
4.135 eV. We calculate the carrier effective masses and report smaller values
for the holes than the electrons, indicating higher hole mobility and easier
p-type doping. The small difference between the static and high frequency
dielectric constants indicates that BAs is only weakly ionic. We also observe
that the imaginary part of the dielectric function exhibits a strong absorption
peak, which corresponds to parallel bands in the band structure. Our estimated
exciton binding energy of 43 meV indicates that excitons are relatively stable
against thermal dissociation at room temperature. Our work provides theoretical
insights on the fundamental electronic properties of BAs to guide experimental
characterization and device applications.
