  The Mid-Infrared Instrument (MIRI) on board the James Webb Space Telescope
(JWST) uses three Si:As impurity band conduction (IBC) detector arrays. The
output voltage level of each MIRI detector pixel is digitally recorded by
sampling-up-the-ramp. For uniform or low-contrast illumination, the pixel ramps
become non-linear in a predictable way, but in areas of high contrast, the
non-linearity curve becomes much more complex. The origin of the effect is
poorly understood and currently not calibrated. We provide observational
evidence of the Brighter-Fatter Effect (BFE) in MIRI conventional and
high-contrast coronographic imaging, low-resolution spectroscopy, and
medium-resolution spectroscopy data and investigate the physical mechanism that
gives rise to the effect on the MIRI detector pixel raw voltage integration
ramps. We use public data from the JWST MIRI commissioning and Cycle 1 phase.
We also develop a numerical electrostatic model of the MIRI detectors using a
modified version of the public Poisson_CCD code. We find that the physical
mechanism behind the BFE manifesting in MIRI data is fundamentally different to
that of CCDs and photodiode arrays such as the Hawaii-XRG (HXRG) near-infrared
detectors used by the NIRISS, NIRCam, and NIRSpec instruments on board JWST.
Observationally, the BFE makes the JWST MIRI data yield 10-25 % larger point
sources and spectral line profiles as a function of the relative level of
debiasing of neighboring detector pixels. This broadening impacts the MIRI
absolute flux calibration, time-series observations of faint companions, and
PSF modeling and subtraction. We also find that the intra-pixel 2D profile of
the shrinking Si:As IBC detector depletion region directly impacts the accuracy
of the pixel ramp non-linearity calibration model.
