  The Mid-Infrared Instrument MIRI on-board the James Webb Space Telescope uses
three Si:As impurity band conduction detector arrays. MIRI medium resolution
spectroscopic measurements (R$\sim$3500-1500) in the 5~$\mu m$ to 28~$\mu m$
wavelength range show a 10-30\% modulation of the spectral baseline; coherent
reflections of infrared light within the Si:As detector arrays result in
fringing. We quantify the shape and impact of fringes on spectra of optical
sources observed with MIRI during ground testing and develop an optical model
to simulate the observed modulation. We use our optical model in conjunction
with the MIRI spectroscopic data to show that the properties of the buried
contact inside the MIRI Si:As detector have a significant effect on the
fringing behavior.
