This paper investigates the role of substrate carrier generation in determining the field, potential, and carrier distributions in MOS capacitors biased in the Fowler-Nordheim tunneling regime. We focus especially on the oxide electric field obtained under nonequilibrium conditions. We find that it is significantly reduced from its value in thermal equilibrium at the same gate-to-bulk voltage. The dependence of the reduction in the oxide field on the carrier lifetime in the substrate was determined from device simulations.