  Some III-V digital alloy avalanche photodiodes exhibit low excess noise.
These alloys have low hole ionization coefficients due to presence of small
'minigaps', enhanced effective mass and large separation between light-hole and
split-off bands in the valence band. In this letter, an explanation for the
formation of the minigaps using a tight binding picture is provided.
Furthermore, we demonstrate that decreasing substrate lattice constant can
increase the minigap size and mass in the transport direction. This leads to
reduced quantum tunneling and phonon scattering of the holes. Finally, we
illustrate the band structure modification with substrate lattice constant for
other III-V digital alloys.
