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The Mid-Infrared Instrument (MIRI) on board the James Webb Space Telescope(JWST) uses three Si:As impurity band conduction (IBC) detector arrays. Theoutput voltage level of each MIRI detector pixel is digitally recorded bysampling-up-the-ramp. For uniform or low-contrast illumination, the pixel rampsbecome non-linear in a predictable way, but in areas of high contrast, thenon-linearity curve becomes much more complex. The origin of the effect ispoorly understood and currently not calibrated. We provide observationalevidence of the Brighter-Fatter Effect (BFE) in MIRI conventional andhigh-contrast coronographic imaging, low-resolution spectroscopy, andmedium-resolution spectroscopy data and investigate the physical mechanism thatgives rise to the effect on the MIRI detector pixel raw voltage integrationramps. 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 amodified version of the public Poisson_CCD code. We find that the physicalmechanism behind the BFE manifesting in MIRI data is fundamentally different tothat of CCDs and photodiode arrays such as the Hawaii-XRG (HXRG) near-infrareddetectors used by the NIRISS, NIRCam, and NIRSpec instruments on board JWST.Observationally, the BFE makes the JWST MIRI data yield 10-25 % larger pointsources and spectral line profiles as a function of the relative level ofdebiasing of neighboring detector pixels. This broadening impacts the MIRIabsolute flux calibration, time-series observations of faint companions, andPSF modeling and subtraction. We also find that the intra-pixel 2D profile ofthe shrinking Si:As IBC detector depletion region directly impacts the accuracyof the pixel ramp non-linearity calibration model.

The Brighter-Fatter Effect in the JWST MIRI Si:As IBC detectors I. Observations, impact on science, and modelling