  A series of III-V ternary and quarternary digital alloy avalanche photodiodes
(APDs) have recently been seen to exhibit very low excess noise. Using band
inversion of an environment-dependent atomistic tight binding description of
short period superlattices, we argue that a combination of increased effective
mass, minigaps and band split-off are primarily responsible for the observed
superior performance. These properties significantly limit the ionization rate
of one carrier type, either holes or electrons, making the avalanche
multiplication process unipolar in nature. The unipolar behavior in turn
reduces the stochasticity of the multiplication gain. The effects of band
folding on carrier transport are studied using the Non-Equilibrium Green's
Function Method that accounts for quantum tunneling, and Boltzmann Transport
Equation model for scattering. It is shown here that carrier transport by
intraband tunneling and optical phonon scattering are reduced in materials with
low excess noise. Based on our calculations, we propose five simple
inequalities that can be used to approximately evaluate the suitability of
digital alloys for designing low noise photodetectors. We evaluate the
performance of multiple digital alloys using these criteria and demonstrate
their validity.
