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We investigate total ionizing dose (TID) effects in graphene field effect transistors comprised of chemical vapor deposition grown graphene transferred onto trimethylsiloxy(TMS)-passivated SiO2 Si substrates. TID exposure with a positive gate bias increases the concentration of positive oxide trapped charges near the SiO2/TMS/graphene interface making Coulomb-potential scatterer limited mobility more apparent. In particular, we observe asymmetric degradation in electron and hole mobility, the former degrading more rapidly. Consistent with the electron-hole puddle description, we observe an increase in intrinsic electron carrier density that varies linearly with the oxide trapped charge density, while the hole carrier density remains largely unaltered. These effects give rise to an increasing minimum conductivity.