  The X-ray Silicon-On-Insulator (SOI) pixel sensor named XRPIX has been
developed for the future X-ray astronomical satellite FORCE. XRPIX is capable
of a wide-band X-ray imaging spectroscopy from below 1 keV to a few tens of keV
with a good timing resolution of a few tens of $\mu$s. However, it had a major
issue with its radiation tolerance to the total ionizing dose (TID) effect
because of its thick buried oxide layer due to the SOI structure. Although new
device structures introducing pinned depleted diodes dramatically improved
radiation tolerance, it remained unknown how radiation effects degrade the
sensor performance. Thus, this paper reports the results of a study of the
degradation mechanism of XRPIX due to radiation using device simulations. In
particular, mechanisms of increases in dark current and readout noise are
investigated by simulation, taking into account the positive charge
accumulation in the oxide layer and the increase in the surface recombination
velocity at the interface between the sensor layer and the oxide layer. As a
result, it is found that the depletion of the buried p-well at the interface
increases the dark current, and that the increase in the sense-node capacitance
increases the readout noise.
