  Arsenic doped back illuminated blocked impurity band (BIBIB) silicon
detectors have advanced near and mid-IR astronomy for over thirty years; they
have high quantum efficiency (QE), especially at wavelengths longer than 10
$\mu$m, and a large spectral range. Their radiation hardness is also an asset
for space based instruments. Three examples of Si:As BIBIB arrays are used in
the Mid-InfraRed Instrument (MIRI) of the James Webb Space Telescope (JWST),
observing between 5 and 28 $\mu$m. In this paper, we analyze the parameters
leading to high quantum efficiency (up to $\sim$ 60\%) for the MIRI devices
between 5 and 10 $\mu$m. We also model the cross-shaped artifact that was first
noticed in the 5.7 and 7.8 $\mu$m Spitzer/IRAC images and has since also been
imaged at shorter wavelength ($\le 10~\mu$m) laboratory tests of the MIRI
detectors. The artifact is a result of internal reflective diffraction off the
pixel-defining metallic contacts to the readout detector circuit. The low
absorption in the arrays at the shorter wavelengths enables photons diffracted
to wide angles to cross the detectors and substrates multiple times. This is
related to similar behavior in other back illuminated solid-state detectors
with poor absorption, such as conventional CCDs operating near 1 $\mu$m. We
investigate the properties of the artifact and its dependence on the detector
architecture with a quantum-electrodynamic (QED) model of the probabilities of
various photon paths. Knowledge of the artifact properties will be especially
important for observations with the MIRI LRS and MRS spectroscopic modes.
