  The Gaia satellite is a high-precision astrometry, photometry and
spectroscopic ESA cornerstone mission, currently scheduled for launch in 2012.
Its primary science drivers are the composition, formation and evolution of the
Galaxy. Gaia will achieve its unprecedented accuracy requirements with detailed
calibration and correction for CCD radiation damage and CCD geometric
distortion. In this paper, the third of the series, we present our 3D Silvaco
ATLAS model of the Gaia e2v CCD91-72 pixel. We publish e2v's design model
predictions for the capacities of one of Gaia's pixel features, the
supplementary buried channel (SBC), for the first time. Kohley et al. (2009)
measured the SBC capacities of a Gaia CCD to be an order of magnitude smaller
than e2v's design. We have found the SBC doping widths that yield these
measured SBC capacities. The widths are systematically 2 {\mu}m offset to the
nominal widths. These offsets appear to be uncalibrated systematic offsets in
e2v photolithography, which could either be due to systematic stitch alignment
offsets or lateral ABD shield doping diffusion. The range of SBC capacities
were used to derive the worst-case random stitch error between two pixel
features within a stitch block to be \pm 0.25 {\mu}m, which cannot explain the
systematic offsets. It is beyond the scope of our pixel model to provide the
manufacturing reason for the range of SBC capacities, so it does not allow us
to predict how representative the tested CCD is. This open question has
implications for Gaia's radiation damage and geometric calibration models.
